# Interfacial assembly of anisotropic core-shell and hollow microgels

Anne C. Nickel  [  Andrey A. Rudov  [  Igor I. Potemkin<EMAIL_ADDRESS>aachen.de [  Jérôme J. Crassous<EMAIL_ADDRESS>[  Walter
Richtering<EMAIL_ADDRESS>[

###### Abstract

Microgels, cross-linked polymers with submicrometer size, are ideal soft model
systems. While spherical microgels have been studied extensively, anisotropic
microgels have been hardly investigated. In this study, we compare the
interfacial deformation and assembly of anisotropic core-shell and hollow
microgels. The core-shell microgel consists of an elliptical core of hematite
covered with a thin silica layer and a thin shell made of
poly(N-isopropylacrylamide) (PNiPAM). The hollow microgels were obtained after
a two-step etching procedure of the inorganic core. The behavior of these
microgels at the oil-water interface was investigated in a Langmuir Blodgett
trough combined with ex-situ atomic force microscopy. First, the influence of
the architecture of anisotropic microgels on their spreading at the interface
was investigated experimentally and by dissipative particle dynamic
simulations. Hereby, the importance of the local shell thickness on the
lateral and longitudinal interfacial deformation was highlighted as well as
the differences between the core-shell and hollow architectures. The shape of
the compression isotherms as well as the dimensions, ordering and orientation
of the microgels at the different compressions were analysed. Due to their
anisotropic shape and stiffness, both anisotropic microgels were found to
exhibit significant capillary interactions with a preferential side-to-side
assembly leading to stable microgel clusters at low interfacial coverage. Such
capillary interactions were found to decrease in the case of the more
deformable hollow anisotropic microgels. Consequently, anisotropic hollow
microgels were found to distribute more evenly at high surface pressure
compared to stiffer core-shell microgels. Our findings emphasize the complex
interplay between the colloid design, anisotropy and softness on the
interfacial assembly and the opportunities it thus offers to create more
complex ordered interfaces.

RWTH Aachen University] Institute of Physical Chemistry, RWTH Aachen
University, 52056 Aachen, Germany, European Union DWI] DWI - Leibniz-Institute
for Interactive Materials, 52056 Aachen, Germany, European Union DWI] DWI -
Leibniz-Institute for Interactive Materials, 52056 Aachen, Germany, European
Union RWTH Aachen University] Institute of Physical Chemistry, RWTH Aachen
University, 52056 Aachen, Germany, European Union RWTH Aachen University]
Institute of Physical Chemistry, RWTH Aachen University, 52056 Aachen,
Germany, European Union

##  1 Introduction

Colloids, particles in the size range of 1 nm to 1 µm dispersed in a
continuous phase1, are ideal model systems to investigate properties found in
nature.2, 3, 4, 5, 6, 7 Soft anisotropically shaped objects like proteins,
bacteria or complete cells contribute within many biological phenomena.8, 9,
10, 11, 12, 13 Hence, anisotropic microgels can be ideal model systems to
obtain more information about the influence of softness and anisotropy.
Microgels based on poly(N-isopropylacrylamide) (PNiPAM) are highly interfacial
active, lower the interfacial tension and are strongly deformed at oil-water
interfaces. The interfacial phase behavior of such spherical microgels has
been studied intensively showing the effect of size and hard cores to the
spreading of spherical microgels at the interface and their response to an
increase in surface pressure within the Langmuir-Pockels trough.14, 15, 16,
17, 18, 19, 20, 21, 22, 23, 24, 25, 26
Only few studies investigate the interfacial behavior of particles with an
anisotropic shape. Due to the specific shape, different ordering phenoma
become relevant at the interface revealing tip-to-tip ($t-t$),27 side-to-side
($s-s$)27, 28, 29 or triangular28 ordering for ellipsoidal particles. This
ordering is a result of capillary interactions caused by a curved contact line
to fulfill Young’s equation.30 Hence, the ordering of anisotropically shaped
particles is influenced by multiple factors such as the particle shape and
dimensions and the interfacial properties.28, 31
The effect of softness onto the interfacial behavior of anisotropically shaped
microgels was first introduced by Honda et al.32 and further developed in our
previous study.33 Hereby, the interfacial behavior of anisotropic core-shell
microgels and more especially the influence of the microgel shell on their
deformation, ordering and orientation at the interface were discussed.33 This
led to the conclusion that both the anisotropy of the hard core and the
softness of the shell are key parameters defining the nearest neighbor
ordering, the microgel orientation in respect to the barriers and the
prominence of capillary interactions.
Many objects found in nature are soft and, hence, require a completely soft
object as model. This reduces the applicability of core-shell microgels with a
rigid core as optimal model system. Hence, a soft but still anisotropically
shaped microgel is desirable. This can be achieved by removing the hematite-
silica core of anisotropically shaped core-shell microgels leading to hollow
microgels.34 Similar to spherical microgels35, the hollow design is expected
to have a major influence on the spreading and assembly of anisotropic
microgel at the interface. In order to investigate this effect, appropriate
anisotropic hollow microgels that remain anisotropic at the interface must
therefore be designed.
In this work, we study the influence of the rigid core on the behavior of
anisotropic microgels at the fluid decane-water interface by comparing the
behavior of a core-shell microgel with a thin shell to the resulting hollow
microgel after etching the core. The shape and size of the microgels at the
interface, and their collective and individual response to compression are
investigated by combining Langmuir-Blodgett deposition and atomic force
microscopy (AFM). Dissipative particle dynamics (DPD) simulations were
performed to support the experiments and rationalize the interfacial
deformation of the individual anisotropic microgels and the importance of the
core-shell and hollow microgel design. We further analyzed and discussed the
collective assembly and ordering of both systems at the interfaces from low to
high compression.

##  2 Experimental Section

Materials All materials were used as purchased. The cross-linker BIS
(N,N’-methylene-bisacrylamide) was purchased from Sigma-Aldrich and the main
monomer NIPAm (N-isopropylacrylamide) from Acro organics. The initiator KPS
(potassium peroxydisulfate) was bought from Merck. The co-monomer BAC
(N,N’-bis(acryloyl)cystamine) was purchased from Alfa Aesar. For the Langmuir-
Blodgett trough experiments decane (Merck) , ultrapure water (Astacus2,
membraPure GmbH, Germany) and propan-2-ol (Merck) were used. The decane was
cleaned by filtering it three times over a column of basic Al2O3 (Merck). The
third filtering was done just before the measurement.
Synthesis The synthesis of the core-shell microgel ($CS-165$) is described in
detail in a former study33. Briefly, 584.4 mg of NIPAm, 39.6 mg of BIS and
14.3 mg of BAC were dissolved in 280 mL of filtered bidistilled water and 3 mL
of an ethanol solution with 1.4 wt% of ellipsoidal functionalized hematite-
silica cores were added into the three-neck flask after sonification for 30
minutes. The mixture was heated with an external oil-bath to 60 °C and degased
with nitrogen. After 45 Minutes, the oil-bath was set to 80 °C and after
another 15 Minutes, the polymerisation was started by adding the initiator
solution of 41.0 mg KPS degased in 10 mL filtered bidestilled water. dropwise.
The reaction was stopped after 4 h and the microgels were purified with three
times centrifugation at 5000 rpm for 60 Minutes and redispersing in filtered
bidestilled water. The hollow microgels ($HS-165$) are obtained after the two-
step etching of the $CS-165$. The hematite was first etched under acidic
conditions using concentrated hydrochloric acid via dialysis. Afterwards the
solvent was exchanged with ultrapure water until the pH was neutral again. The
remaining silica shell was etched under alkaline conditions using dialysis
against 0.5 molar sodium hydroxide. The solvent was exchanged to obtain a
neutral pH after the silica was etched away and the resulting microgels were
lyophilizates.
Characterization
A detailed characterization of the microgels is described in former studies34,
33. The most important microgel characteristics are summarized in Table 1.

Table 1: Characterisation of the synthesized microgels. | $CS-165$ | $HS-165$
---|---|---
dimensions of the core | (330 $\pm$ 12) nm $\times$ (75 $\pm$ 8) nm | -
polymer content | 65.0% | 100%
$R_{H}^{10~{}^{\circ}C}$ | (165 $\pm$ 2) nm | (182 $\pm$ 2) nm
$R_{H}^{50~{}^{\circ}C}$ | (122 $\pm$ 1) nm | (127 $\pm$ 1) nm

Langmuir-Blodgett technique. The isotherms and depositions were conducted
simultaneously at a decane-water interface in a customized liquid-liquid
Langmuir-Pockels trough (KSV NIMA, Biolin Scientific Oy, Finland) equipped
with two movable barriers. The exact procedure is described in Ref. 33. The
trough was cleaned with ethanol and milli-q water before a clean water-decane
interface was established. To this interface, the microgel dispersion to which
isopropanol was mixed as spreading agent were added with a syringe. The
substrates were cleaned before they were moved through the water-decane
interface. The microstructures of the microgels on the substrates were further
investigated with atomic force microscopy (AFM).
For the hollow microgel $HS-165$, it was not possible to obtain the full
compression isotherm within one single measurement with the used Langmuir-
Blodgett trough. Hence, a compression isotherm had to be measured with an
initial surface pressure ($\Pi$) larger than zero. The values for the
compression isotherm obtained at the highest $HS-165$ concentration were
rescaled by 5 mN/m each to overlap all three compression isotherms conducted
on this system. The original data can be found in the supporting information
(Figure S4).

Atomic force microscopy. Dry-state AFM was performed to visualize the
monolayers deposited on the silica substrates. The experiments were carried
out in tapping mode using a Dimension Icon AFM with a closed loop (Veeco
Instruments Inc., USA, Software: NanoScope 9.4, Bruker Co., USA). The tips
used for imaging were OTESPA tips with a resonance frequency of 300 kHz, a
nominal spring constant of 26 N$\cdot$m-1 of the cantilever and a nominal tip
radius of <7 nm (NanoAndMore GmbH, Germany). The images were recorded with the
programmed move function to scan the silica substrates. On each substrate, six
rows along the gradient of microgel concentration were captured with a step-
width of 1000 µm. Each image is 7.5 µm $\times$ 7.5 µm large with a resolution
of 512 pixels $\times$ 512 pixels. Higher resolution images were obtained by
using the same number of pixels (512 pixels $\times$ 512 pixels) for an image
size of 3 µm $\times$ 3 µm. Such micrographs were captured additional to the
programmed move.

Image Analysis The program NanoScope Analysis 1.9 was used for the acquisition
and leveling of the AFM micrographs. The processed AFM images were analysed
with an updated version of the MATLAB routine of Bochenek et al.20, 33,
details are provided in the supporting information.

Computer simulations
Method. To assess the difference between the behavior of anisotropic core-
shell (CS) and hollow-shell (HS) microgels at the decane-water interface DPD
simulations were used36. The method was successfully applied and described in
detail for the study of anisotropic hybrid core-shell microgels with a
different shell thickness33.
The modeling of the microgels, which have a size of hundreds of nanometers,
requires a proper coarse-graining procedure37 leading to the effective
reduction of the gel size. Dealing with the hollow anisotropic microgels, we
found it crucial to readjust a coarse-grained relation between the size,
softness of the microgel, and surface/volume microgel-solvent interactions.
The higher the coarse-graining degree, the higher the sensitivity of the
system to the disbalance between surface and volume terms, which is especially
noticeable for oblong or hollow objects at the same coarse-grained level. Thus
the size-limit effect (see Supporting information, Size-limit effect) must be
considered. In the case of regular or hollow spherical microgels the effect is
less visible due to the spherical symmetry of the system. The
swelling/deswelling process is accompanied by the self-similar change in the
size of the microgel and its cavity. Both, in collapsed and swollen states,
the hollow microgel and its cavity are spherically shaped 35, 38, 39. This
result is reproduced in a fairly wide range of interaction parameters as well
as the flexibility of the subchains40, 41. However, following such a strategy
one could notice, that simulation of the hollow ellipsoidal microgels failed
and could not reproduce the experimental observations34. In particular,
different trends of the shape/size evolution of the shell and the void upon
collapse above their volume phase transition temperature (VPTT) are observed
or extra flattening of the microgel at the interfaces is detected.
In this study, we extended the method used earlier and applied several
modifications to improve the modeling of the CS and HS anisotropic microgels
in bulk and at the decane-water interface. The first modification is aimed to
choose the reasonable coarse-grained level and reproduce interfacial tension
value for the pure decane/water interface without the microgel. Decane
molecule is treated as a single bead in the system similarly to Ref.42. The
DPD interaction parameters $a_{ij}$ were estimated through the Flory-Huggins
interaction parameter matching the infinite dilution activity coefficient of
liquids, where $i$ and ${j\in}$(W - water bead, D - decane bead) see
Supporting information, Decane/water interface. Such a choice of parameters at
20 ${}^{\circ}C$ provides the value of the interfacial tension of water/decane
liquids equal to 51.9 mN/m which is in good agreement with the experimental
value 52.33 mN/m.43
The second modification is related to the model of the $CS-165$ and $HS-165$
microgels. The anisotropic CS microgel is composed of a polymeric shell and a
solid core. The beads forming the core and the shell are denoted by $C$ and
$S$, respectively. The dimensions of the hematite-silica core in the
experiment were 330$\pm$12nm$\times$75$\pm$8 nm. The ellipsoidal nanoparticle
having similar aspect ratio between the long and short semi-axes 4.4$\times$1
have been created. Since the real aspect ratio of the CS-165 is unknown, we
designed three different microgels with the same shell mass and cross-linking
density but different initial aspect ratios of the shells $L/d$ = 2.8; 2.5 and
2.2 to capture the peculiarities of the shell dimensions (Figure S5, Table
S3). The microgel with a shell aspect ratio of 2.8 is featured by a thicker
shell at the top of the core compared to its side. The microgel with an aspect
ratio of 2.2 has approximately the same thickness of the shell around the
solid particle. Hollow microgels were obtained after removal of the solid core
from the corresponding core-shell microgels (see Supporting information,
Microgel description).
In our previous work 33, we assumed that independently of the shell thickness
$CS$-samples has a 5 mol% cross-linker groups homogeneously distributed within
the gels. Indeed, the molar mass composition of the cross-linker BIS in
experiments was around 4.69, 4.63 and 4.72 mol% for $CS-356$, $CS-254$ and
$CS-165$, respectively. However, related to the specificity of the synthesis
of $CS-165$, the exchange of the dye methacryloxyethyl thiocarbamoyl rhodamine
B (MRB) to BAC both introduce additional cross-links and may change the
polymerization kinetics resulting in a stiffer microgel shell and a lower
degree of swelling. The DLS measurements and results of the mass loss during
the thermogravimetric analysis for the CS-254 and CS-165 microgels with
different shell thicknesses confirmed our conclusions. The samples have a
similar amount of shell related to the core mass: 60.9% and 65.0% respectively
which explain similar hydrodynamic radii in the collapsed state33. At the same
time in the swollen state at 20 °C, $CS-254$ is around 90 nm large than
$CS-165$ (the swelling degree of CS-254 and CS-165 is equal to 2.2 and 1.4,
respectively) which indicates a higher cross-linking density of $CS-165$
sample. We found that in simulation an approximate degree of crosslinking,
which could reproduce the swelling degree of the $CS-165$ microgel in water,
is equal to 7.5%. The average subchain length, $N$, equals $6\pm 1$ (see
Supporting information, Microgel characterization in bulk).
The third modification provides the way to reproduce the swelling degree of
the microgel at chosen coarse-grained level by establishing the relation
between the number of the beads per subchain and the subchain flexibility.
Similarly to the work by Nikolov et al.44 in addition to the harmonic
potential, a bending angle potential as well as a segmented-repulsive
potential (SRP)45 between the polymer beads have been introduced. The SRP
potential prevents polymeric subchains from crossing one another. The bending
potential enables to control the Kuhn length of the subchains and thus the
flexibility of the subchains and the entire microgel. Based on the idea that
in a swollen state PNiPAM polymers exhibit flexible behavior, we determined
bending stiffness value for the subchain length N=6 (see Supporting
information, Microgel description). In order to reproduce the
swelling/deswelling behaviour of the microgels in the liquids, we alter the
polymer-liquid repulsion ${a_{Si}}$ in a range between $a_{Si}=25k_{B}T/r_{c}$
and $a_{Si}=30.2k_{B}T/r_{c}$, where $i$ ${\in(W,O)}$. Following that strategy
we found suitable parameters to reproduce both the experimental swelling
degree of the microgels in the bulk and their deformability at the interface.
Detailed information about simulation parameters, modifications and its impact
are fully described in Supporting information.

Simulations were performed using a DPD code supplied by Lammps46. Simulations
of microgel in a solution were performed in the $NVT$ ensemble. Cubic boxes of
a constant volume $V=L_{x}$ $\times$ $L_{y}$ $\times$ $L_{z}$ = 100 $\times$
100 $\times$ 100 $r_{c}^{3}$ with imposed three-dimensional periodic boundary
conditions, containing single microgel immersed into the solvent were
simulated for an equilibration phase of $10^{6}$ steps followed by a
production phase comprising of $10^{6}$ steps. Equilibrium was deemed to have
been achieved when the cumulative average temperature, pressure, gyration
radii reached a plateau. The simulations of microgel at water/decane interface
were performed in the $NVT$ ensemble in rectangular box of a constant volume
$V=L_{x}$ $\times$ $L_{y}$ $\times$ $L_{z}$ = 140 $\times$ 140 $\times$ 37
$r_{c}^{3}$ with periodic boundary conditions in all directions. The box were
fulfilled by water and decane beads. Water and decane phase-separate. Water is
at the bottom of the simulation box, decane at the top. Note that the $z$
direction of the simulation box is perpendicular to the liquid/liquid
interface. The interfacial tension between the two liquids is determined using
the pressure tensors as shown in Eq. 1. The microgel was placed close to the
water/decane interface. The equilibration run comprised $10^{6}$ steps
followed by a production phase comprising of $10^{6}$ steps. The interfacial
tension, the gyration tensor and the surface area of the microgel, were
averaged over the last $10^{6}$ steps of a total $2\cdot 10^{6}$ steps.

Computational details
Interfacial tension (IFT). The DPD IFT (${\gamma_{DPD}}$) was evaluated by
dividing the simulation box into a number of slabs parallel to the interface
and obtaining the ensemble average of pressure tensors which can be presented
by the following equation47:

$IFT_{DPD}=0.5\int[<p_{zz}(z)>-\frac{1}{2}(<p_{xx}(z)>+<p_{yy}(z))>]\,dz$ (1)

where rescaled $z=z/r_{c}$ and the angle brackets indicate the ensemble
average of local dimensionless tensors components ($p_{xx}$, $p_{yy}$ and
$p_{zz}$) of $x$, $y$ and $z$ directions. In application of Eq. 1, the
interface between two immiscible fluids is considered as a plane perpendicular
to the $z$-direction. The evaluated ${\gamma_{DPD}}$ which is in DPD units and
can be converted into conventional or physical IFT which is measured
experimentally, by using the conversion factor 47 of $k_{B}T/r_{c}^{2}$ (i.e.,
$\gamma[mN/m]=(k_{B}T/r_{c}^{2})\cdot\gamma_{DPD})$.

Gyration tensor. The gyration tensor was calculated using the followings
equation:

$\boldsymbol{S}=\frac{1}{N}\sum\nolimits_{i=1}^{N}\boldsymbol{s}_{i}\boldsymbol{s}_{i}^{T}=\overline{\boldsymbol{ss}^{T}}=\left[\begin{array}[]{rrr}\overline{x^{2}}&\overline{xy}&\overline{xz}\\\
\overline{yx}&\overline{y^{2}}&\overline{yz}\\\
\overline{zx}&\overline{zy}&\overline{z^{2}}\\\ \end{array}\right],$ (2)

where $\boldsymbol{s}_{i}=\left[\begin{array}[]{r}x_{i}\\\ y_{i}\\\ z_{i}\\\
\end{array}\right]$ is the position vector of each bead, which is considered
with respect to the center of mass of the microgel
$\sum\nolimits_{i=1}^{N}\boldsymbol{s}_{i}=0$ , and the overbars denote an
average over all the beads in the microgel, $N$. To get a complete
characterization of the elliptic microgels, we computed the eigenvalues of the
gyration tensor $\lambda_{x}=\overline{X^{2}}$,
$\lambda_{y}=\overline{Y^{2}}$, $\lambda_{z}=\overline{Z^{2}}$ as well as
eigenvectors $\overline{T_{1}}$,$\overline{T_{2}}$,$\overline{T_{3}}$. The
gyration tensor is symmetric thus, the Cartesian coordinate system can be
found in which it is diagonal. Transformation to the principal axis system
diagonalizes $S$, and we chose the principal axis system in which

$\boldsymbol{S}=diag(\lambda_{x},\lambda_{y},\lambda_{z}),$ (3)

where we assume that the eigenvalues of $\boldsymbol{S}$ are sorted in
ascending order, i.e., $\lambda_{x}\leq\lambda_{y}\leq\lambda_{z}$ and $x$,
$y$ and $z$ are the new coordinate axes. These eigenvalues are called the
principal moments of the gyration tensor. The first invariant of $S$ gives the
squared radius of gyration,

$tr(\boldsymbol{S})\equiv
I_{1}=\lambda_{x}+\lambda_{y}+\lambda_{z}=R_{g}^{2},$ (4)

a measure of the average size of the particular conformation of the microgel.
We also defined the shape anisotropy of the microgel as

$L/d=\lambda_{z}/\lambda_{x,y}$ (5)

Form factor. The form factor P(q) of microgel in bulk was calculated using the
followings equation:

$P(q)=\frac{1}{N}\sum_{ij}<e^{(-i\boldsymbol{(qr_{ij})}})>$ (6)

where the angular brackets indicate an average over different equilibrium
configurations of the same microgel and over different orientations of the
wavevector $\boldsymbol{q}$, $\boldsymbol{r}_{ij}$ is the radius vector
between the $i^{th}$ and $j^{th}$ beads. The fitting of the form factors were
computed with the program sasview v.5.0.4 with a core-shell ellipsoid model.

##  3 Results and Discussion

Compression isotherms and interfacial deposition. In this study, a hybrid
core-shell microgel ($CS-165$) and its corresponding hollow counterpart
($HS-165$), obtained after its core dissolution, were employed to investigated
the influence of microgel architecture on the interfacial deformation and
assembly of anisotropic microgels. Core-shell microgels with larger and softer
shells introduced in previous studies34, 33, did not exhibit any anisotropic
shell at the interface. Conversely, the microgel $CS-165$ with the thinnest
and stiffest shell exhibited an anisotropic shape at the interface33 and was
therefore selected for this study to investigate the differences between the
behavior of anisotropic core-shell and hollow microgels at a decane-water
interface.

The microgels were investigated at the water/decane interface using a
Langmuir-Blodgett setup. Subsequently, the dried deposited monolayer was
analyzed via atomic force microscopy (AFM) to obtain microstructural
information at the different compression states. The compression isotherms of
hollow ($HS-165$, brown) and core-shell ($CS-165$, orange) microgels in
dependence of the mass of the shell per area of the trough are shown in Figure
1. The AFM phase micrographs in Fig. 1 evidence the anisotropic shape of
hollow (left) and core-shell (right) microgels, respectively, in the
concentrated (upper row) and diluted (lower row) regions. The AFM images have
a size of 3 µm $\times$ 3 µm and demonstrate that even in the concentrated
state, the microgels appear elongated independently of the presence or absence
of the anisotropic core. Hence, the two microgel systems are suitable to
identify the effects of the internal architecture on the interfacial behavior
of anisotropic microgels. Furthermore, the hard core of core-shell microgels
makes deformations such as bending impossible and the elliptical shape is
preserved also at high surface pressures. The hollow microgels without the
core obtain the freedom to slightly deform which can be observed at high
surface coverage. This means they still have an anisotropic shape but can bend
to better fit into observed microgel clusters. The microgels form clusters
which do not show a direct contact in the dilute regime. This could be
explained by either a polymer layer which is too thin to be visualized with
dry-state AFM or by a deswelling of the outer polymer layer due to the drying
at the silicate substrate.

$\Pi$ is reported as function of mass of the shell $m_{shell}$ normalized by
the trough area, $A_{Trough}$. Considering that both microgels have the same
microgel shell, the compression isotherms are therefore compared in respect to
the number of microgels adsorbed at the interface. This representation of the
data was selected, as the clustering observed for both microgels did not allow
for an accurate normalization by the number of microgels observed within the
AFM images. For $HS-165$, the total mass of the sample was considered as the
microgel consists of the polymeric shell only, whereas for $CS-165$, 65 % of
the deposited mass corresponds to the polymeric shell as determined with
thermogravitometry (TGA) in our former study.33

Both compression isotherms exhibit five distinct regions with increasing
surface pressure (Figure 1 center). Former studies on spherical microgels
reported a similar two-step increase in surface pressure, where the first
increase was attributed to the interactions of the uncompressed adsorbed
microgel coronas and the second to the interactions between the harder
cores.35, 18, 20, 48 For hard ellipsoidal particles, compression isotherms
differ with their aspect ratio.49 While spherical hard particles and particles
with an aspect ratio of 9 showed only one increase in surface pressure,49
intermediate aspect ratios of 5.5 exhibited a two-step increase.50 A flipping
of the ellipsoidal particles at high surface pressure causes a decrease in the
occupied area and that leads to the formation of the intermediate plateau in
the compression isotherm.50

Figure 1: AFM phase images of hollow (left, brown) and core-shell (right,
orange) microgels in a concentrated region (upper row) and a diluted region
(lower row). Center: Compression isotherms of hollow (brown) and core-shell
(orange) microgels in dependence of the mass of the shell divided by the
trough area. Both compression isotherms reveal five regions. The AFM images
have a size of 3 µm $\times$ 3 µm. The images and data of the core-shell
microgels are reproduced from Nickel et al. Anisotropic Microgels show their
Soft Side. Langmuir 2021, https://doi.org/10.1021/acs.langmuir.1c01748
Copyright 2021 American Chemical Society

For the hollow and core-shell microgels, no flipping was expected due to the
spreading of the microgel shells. The two-step increases of the surface
pressure in the compression isotherms observed for both microgels are more
similar to the two-step increase observed for spherical microgels as compared
to those reported for hard ellipsoids.

Region I in Fig. 1 is characterized by a constant, zero $\Pi$ indicating that
the concentration of the microgels is too low to influence $\Pi$. In region I
I, a sharp increase in surface pressure is observed for both microgels. Region
III is ended by a sharp increase in surface pressure (region IV) followed by a
constant value for the surface pressure (region V). Even if both microgels
show the same number of regions, the shape of their isotherms differs. The
main difference lies in the width of region III and the steepness of the
surface pressure increase in region IV. The hollow microgel has a more
extended region III and only a small increase in surface pressure in region
IV. In comparison, the core-shell microgels has a much narrower region III and
presents a steeper increase in surface pressure in region IV. For spherical
microgels, an enlargement of region III is related to the softness of the
microgels that can be either achieved for core-shell microgels with a hard
silica core by decreasing the microgel shell crosslinking or by the removing
the core.35, 25. Similarly for the anisotropic microgels investigated in this
study, $HS-165$ appears softer than $CS-165$ leading to an enhanced spreading
at the interface and an enlargement of region III.

Geisel et al.35 investigated the compression modulus ($C^{-1}=-A(d\Pi/dA)$)
for spherical hard core-shell and the corresponding hollow microgels. Two
maxima were observed for the compression modulus revealing a two step-increase
within the compression isotherm. For spherical microgels, the existence of the
hard core did not influence the height of the first maxima, but rather the
second maximum.35 The same accounts for the anisotropically shaped microgels
in this study (see SI for further information). The dense anisotropic microgel
monolayers show a comparable elasticity as the one reported for spherical
microgels.
This is inferred by comparing the data of the anisotropic microgels (Figure
S1) with the data obtained for spherical core-shell microgels from Geisel et
al.35. Spherical microgels with a cross-linking density of 5 mol% have the
first maximum at $\Pi$ = 10 mN/m for $C^{-1}$= 46-47 mN/m 35. Although AFM
phase images and dynamic light scattering data of the anisotropic microgels33
pointed to a stiff microgel network, the compression modulus indicates a
similar elasticity at the interface as was found for spherical microgels with
similar cross-linker content.35

The onset of the surface pressure increase takes place at lower compression
for the hollow as compared to the core-shell microgels. Geisel et al.35
observed a similar shift when comparing the compression isotherms of spherical
core-shell and hollow microgels. This can be explained by the different size
of hollow and core-shell microgels at the interface. Height profiles and the
determination of the long and short semi-axes are shown in Figure 2 The mean
area of one microgel increases from 0.0607$\pm$0.0055 µm2 for $CS-165$ to
0.127$\pm$0.022 µm2 for $HS-165$. By removing the core, the microgel shell can
spread freely at the interface and the area covered by one microgel roughly
doubles in size. This increase in area comes from an increase in the microgel
length and diameter while the aspect ratio of the microgels stays nearly
constant (see Table 2). The length of the microgels increases by 283 nm from
$CS-165$ to $HS-165$ while the additional spreading at the sides is 160 nm
from $CS-165$ to $HS-165$.

.

Figure 2: Height profiles of the long and short axes of one microgel of
$HS-165$ (top) and $CS-165$ (bottom) obtained from AFM height micrographs.

Fig. 2 (top) illustrates an example of the rapid decrease and increase of the
height profile along the long and short axes of one ellipsoidal microgel. A
smooth profile with a nearly constant height is observed for $CS-165$. For
$HS-165$, the elliptical core was etched away, which leads to a rougher and
flattened height profile and a much lower overall height (Fig. 2 bottom).

Interfacial deformation of individual anisotropic microgels: comparison with
simulations. The conclusions about shape and spreading of individual particles
in the uncompressed region I are nicely supported by the computer simulations.
The core-shell ($CS$) and hollow ($HS$) ellipsoidal microgels were adsorbed at
the water/oil interface from the water phase. It was assumed in earlier
studies that $CS-165$ microgels have a constant shell thickness, which means
that the thickness of the shell at the top and the sides of the core are the
same.34, 33 However, this may not be the case. In order to get a deeper
understanding on the correlation between the thickness anisotropy of the shell
and spreading of the microgel at the interface, three types of core-shell
microgels with the same solid core, shell mass and cross-linking density
$CS_{2.8},CS_{2.5},CS_{2.2}$ as well as corresponding hollow microgels,
$HS_{2.8},HS_{2.5},HS_{2.2}$ have been designed in silico (Figure 3A,B). The
aspect ratio of the solid core, $L_{core}/d_{core}$ was set to $4.4\pm 0.1$ to
match the dimensions of the hematite-silica core in the experiments. The
aspect ratio of the shells was varied in such a way to preserve the overall
number of monomers and cross-links from sample to sample. After equilibration
in a swollen state, the overall aspect ratios of the shell determined from the
fitting of $P(q)$, $L/d=2.8\pm 0.1,~{}2.5\pm 0.1$ and $2.2\pm 0.1$ were
determined for the $CS_{2.8},CS_{2.5}$ and $CS_{2.2}$, respectively (see
Supporting information for further details). The sample $CS_{2.2}$ corresponds
to the case of the constant shell thickness surrounding the ellipsoidal core.
The ratio between the thicknesses at the tip and the sides, $dL/dd$ = 1.1. The
$CS_{2.5}$ and $CS_{2.8}$ samples represent the microgels with slight and
strong shell thickness anisotropy, $dL/dd$ = 1.5 and 1.9, respectively (Figure
3A). For the investigated systems, removing the core does not significantly
change the radii of the hollow microgels similarly to the observations for the
microgels with the thin shell34 (Figure 3B). The higher shell thickness at the
tip of the microgel leads to the extra swelling of the shell, Figure S8.
Slight increase of the $L/d$ value was observed for the samples $HS_{2.5}$ and
$HS_{2.8}$.

Figure 3: Top row: Equilibrium structure of the swollen $CS$ (A) and $HS$ (B)
microgels with different initial shell configuration:
$L_{frame}/d_{frame}=2.8,2.5$ or $2.2$ together with their corresponding
overall aspect ratio, $\rho$ and the ratio of the longitudinal and lateral
shell thickness, $t_{L}/t_{d}$. Middle row: Color height maps of the core-
shell, $CS_{2.8},CS_{2.5},CS_{2.2}$ (C) and the hollow,
$HS_{2.8},HS_{2.5},HS_{2.2}$ (D) anisotropic microgels at the decane/water
interface. Top view looking from the water phase. (E) The values of the major,
$L$, and minor, $d$, axes of the hollow (orange) and the core-shell (brown)
anisotropic microgel obtained by the fitting of the contact area of the
mirogels at the water/oil interface. Interfacial (F) and bulk (G) aspect
ratio, $\rho_{int}$, for different samples. Solid and dashed lines correspond
to $CS$ and $HS$ anisotropic microgels, respectively.

Equilibrium structures of the $CS$ and $HS$ anisotropic microgels at the
decane/water interface are shown on Figure 3C,D. Decane is a poor solvent and
penetration of the oil molecules into the microgels is negligible in
comparison with water molecules. The major part of the microgels is located in
the water phase, and the narrow polymer/oil interface is slightly bent. After
adsorption microgels become significantly stretched due to the minimization of
the water/decan surface energy. However, the presence of the solid core and
thin, highly cross-linked shell restricts the spreading and preserves
anisotropy. Comparing the relative size changes of the $HS$ and $CS$ microgel
summarized in Figure 3E, we found out that the effect of shell anisotropy is
also accompanied by increasing the surface area. The highest surface area
occupied by the $HS_{2.8}$ in Figure 3D and gradual decrease of the spreading
in a sequence is the following ${HS_{2.8}\rightarrow HS_{2.5}\rightarrow
HS_{2.2}\rightarrow CS_{2.8}\rightarrow CS_{2.5}\rightarrow CS_{2.2}}$. Figure
3G,F presents the simulated aspect ratio of the two microgel types in bulk and
at the water/oil interface. When both $CS$ and $HS$ microgels have almost
similar aspect ratio in bulk, the influence of the microgel design is more
striking at the interface. Interestingly, increasing of the aspect ratio of
the microgels lead to the extra stretching of the $CS$ microgels at the tip of
the solid core accompanied by the increasing of the shape anisotropy, $L/d$.
Thus, in the case of $CS_{2.8}$ microgel, $L/d$ = 3.0$\pm$0.1 which is higher
compare to the bulk. On the contrary, etching away of the core lead to the
decreasing of $L/d$. In the case of $HS_{2.8}$ microgel, $L/d$ = 2.6$\pm$0.1.
This trend is reverse when the shell thickness is comparable at the sides and
tips, $HS_{2.2}$ microgels become slighlty more anisotropic than $CS_{2.2}$
microgels.

The heights of the CS and HS anisotropic microgels for the different design
are shown in Figure 4C. The microgels show a decrease within the height after
etching away of the core. In Figure 4A,B the lateral cross-sections of
$CS_{2.2}$ and $HS_{2.2}$ are presented. The cavity is distinguishable, but
highly stretched parallel to the interface. The results are in a good
agreement with the experimental observations keeping in mind that the
experimental height profiles were captured in the dried state. Indeed, due to
the solid silica coated hematite core, the relative difference of height
between the two microgel types is much more significant in the dried state
compared to the swollen one.

Figure 4: Longitudinal and lateral cross-sections of $CS_{2.2}$ and $HS_{2.2}$
anisotropic microgels at the decane/water interface. Water and decane phases
are colored in blue and brown respectively. Maximum heights of the $CS$ (full
symbols, solid line) and $HS$ anisotropic microgels (hollow symbols, dashed
line) for the different designs are shown on the right.

Furthermore in the experiments, the length and diameter of $HS-165$ decreased
with increasing surface pressure to 456$\pm$56 $\times$ 213$\pm$12 which
differs from $CS-165$ having similar sizes in the uncompressed region I
(355$\pm$40 $\times$ 195$\pm$14) and the compressed region V (367$\pm$37
$\times$ 186$\pm$14). This observation thus supports the fact that the $HS$
microgels are more deformable at the interface. Looking back to our
simulations, $CS_{2.2}$ and $HS_{2.2}$, corresponding to microgels with
approximately the same shell thickness at the sides and tips provide the
closest description of the experiments as summarized in Table 2.

Table 2: Length, diameter and aspect ratio of the hollow and core-shell microgels obtained from the experimental AFM images and simulations. microgel |  | length | diameter | aspect ratio
---|---|---|---|---
$CS-165$ | experiment |  355$\pm$40 nm |  195$\pm$14 nm |  1.82$\pm$0.08
$HS-165$ | experiment |  573$\pm$64 nm |  290$\pm$15 nm |  1.98$\pm$0.12
$CS_{2.2}$ | simulation |  74.9$\pm$1.8$\sigma$ |  34.8$\pm$1.4 $\sigma$ |  2.15$\pm$0.1
$HS_{2.2}$ | simulation |  92.2$\pm$1.6$\sigma$ |  40.8$\pm$1.5 $\sigma$ |  2.26$\pm$0.09

The simulation results illustrate the intricate interplay between the microgel
design and its spreading at the interface. Due to the presence of the core to
which the polymeric network is grafted, the spreading of the $CS$ microgels is
strongly constrained. As a consequence, an increase of the shell thickness
results in a larger interfacial deformation. This can be observed for
$CS_{2.8}$ with a much thicker shell at the tip that becomes slightly more
anisotropic at the interface. The dimensions of the microgel play however an
important role too. Indeed, spherical microgels with similar crosslinking
degree flatten more than larger ones which can be explained by higher relative
contribution of the interfacial interactions in comparison with the volume
contribution similar to that shown in Eq. (1). Consequently, when the
thickness at the side of the anisotropic microgels is comparable to the one at
the tip as for $CS_{2.2}$, the microgels deform more laterally resulting in a
lower aspect ratio at the interface.

For $HS$ microgels, the spreading is less constrained, which can be evidenced
by the larger occupied interfacial area and decrease height. When looking to
polymeric capsules, both the thickness of the shell and dimensions of the
cavity are key parameters; a large capsule with thin shell being more
deformable. Reducing the relative thickness at the tip has therefore the
opposite effect than for the $CS$ microgel, $i.e.$ , the microgels become more
anisotropic at least compared to their equivalent $CS$ microgels as shown for
$HS_{2.2}$. Further reducing the relative thickness at the tip of our designed
$HS$ systems may promote their anisotropic spreading. Finally, we should keep
in mind that the dimensions of the cavity may also play an important role as
$HS$ microgels can also spread towards the center of the voids and close the
cavity. This adds an extra constraint to the interfacial deformation and may
explain that the lateral spreading still remains greater than the longitudinal
for all the simulated systems.
We now discuss the contact line distortion created by a single microgel on a
flat decane/water interface. We expect that both microgel design and the
presence or absence of the solid core may affect distortion and trigger the
capillary forces. We have tracked the position of the microgel perpendicular
to the interface and estimated the deformation of the contact line of the
liquids for different samples. Figure S12 provides an overview of the position
of the microgels relative to the interfaces. The figure shows 3D views of the
halves of the CS and HS microgels at the water/decane interface and the
results of the surface reconstruction of the interface as a red and white and
black gradient color code height maps. The corresponding top view and front
view two-dimensional projections can be found in Figure S13. We distinguish
reliable differences in the deformation of the contact line between the $CS$
and $HS$ samples. For microgels with a cavity the fluid interface is quite
flat. The trench depth is slightly more than 3${\sigma}$ (Figure S14B). The
difference between the surface area of the interface with (interface with
distortion) and without microgel (undisturbed interface) is shown in Figure
S14A. The distortion of the fluid interface is rather small. The microgels
with solid core create a downward distortion. The polymeric shell tends to
spread over the surface, accompanied by downward force that pushes the solid
core towards the decane. We observe significant increase both area of
deformation and trench depth compared to their counterparts with a cavity,
which resulted in the expectation of stronger capillary interactions for the
$CS$ anisotropic microgels.

Microstructural analysis of the interfacial assembly.
AFM height micrographs of the deposited monolayers were analyzed to evaluate
the different microstructures with increasing compression. Representative
micrographs in each of the five regions of the compression isotherm recorded
for the hollow and core-shell microgel are displayed in Figure 5 A-E and
Figure 5 H-K, respectively. The direction of the compression with the Langmuir
trough barriers is marked with grey arrows. Additionally, AFM images after
expansion of the monolayer at the end of the experiment are displayed for the
hollow (Fig. 5 F) and core-shell (Fig. 5 L) microgel. Images F and L exhibit
similar heterogeneous structures as observed in region I (Fig. 5 A and G)
demonstrating the reversibility of the structures upon reducing the surface
pressure. Unlike conventional spherical microgels that usually do distribute
homogeneously when the barriers are released, we ascribe this effect to the
release of elastic energy stored upon compression overcoming the capillary
interaction holding some of the particles at contact.

Figure 5: AFM height images of hollow ($HS-165$) (A-F) and core-shell
microgels $CS-165$ (G-L) at the five different regions of the compression
isotherm and after expansion of the monolayer after the compression. The
arrows indicate the direction in which the barriers were compressed in respect
to the images. AFM height images are 7.5 µm$\times$7.5 µm. The images of the
core-shell microgels are reproduced from Nickel et al. Anisotropic Microgels
show their Soft Side. Langmuir 2021,
https://doi.org/10.1021/acs.langmuir.1c01748 Copyright 2021 American Chemical
Society

Both microgels assemble in non-close-packed clusters in region I indicating
the presence of attractive capillary interactions balanced by the elasticity
of the adsorbed deformed corona independent of the existence of a rigid core
for these anisotropic microgels. Note that no clear preferential assembly
could be observed as expected for ellipsoidal particles with such lower aspect
ratio.51 Still, differences within the ordered structures of $HS-165$ and
$CS-165$ are clearly visible. The clusters of the core-shell microgels are
larger and more evenly packed as compared to the clusters of the hollow
microgels which indicates stronger capillary interactions. Spherical core-
shell microgels that have a similar diameter as the length of the anisotropic
core-shell microgels do not form clusters at low surface pressures.52 This
indicates that it is the anisotropic shape of the microgel shell that leads
attractive capillary interactions, which might be further strengthened by the
extra weight and higher rigidity of the core-shell microgels.

A similar trend is visible for the other four regions of the compression
isotherms. For $CS-165$, the presence of uncovered areas and thus of a highly
inhomogenous assembly of microgels at the interface even at the highest
surface pressure (Fig. 5 K). In contrast, the nearly homogeneous distribution
(without any uncovered areas) of the hollow microgels at high surface
pressures (Fig. 5 D and E) indicates weaker capillary interactions and larger
compressibility. This could be an additional effect of the possibility of the
soft hollow microgels to bend and therefore slightly change their shape to fit
into the microgel structures.

A closer examination of the monolayer topology is shown in Figure 6 and
illustrates height profiles of $s-s$ and $t-t$ clusters for $HS-165$ (Fig. 6A,
B) and $CS-165$ (Fig. 6C, D) within the high compressed regions IV (Fig. 6A,
C) and V (Fig. 6B, D). Even at high surface pressure, the length of the hollow
microgels is larger as compared to the core-shell microgels, similarly to what
was found at low surface pressure (see Fig. 2).

The same observation holds for the short axis ($d$) of the microgels which are
shown as dotted lines in Fig. 6. The height profiles of the short axis show
similar distinct peaks for $HS-165$ and $CS-165$ in regions IV and V. The
maximum height in Fig. 6 D for $d$ is larger compared to the other three
images. The reason is, that at such high surface coverage the normalization to
zero might not be correct when a bare substrate was not accessible. In the
case of Fig. 6 D, the chosen $s-s$ cluster of microgels lays next to a darker
and properly empty surface area. This led to a more realistic determination of
the substrate height and values for the maximum height of circa 90 nm still
smaller compared to the maximum value of $\approx$120 nm. The height profile
of the length of the hollow microgels display similar, rough and more
flattened profiles as observed in Fig. 2 (top). The lengths of each microgel
decreases as a result of the compression within the Langmuir-Blodgett trough.
For $CS-165$, the height profiles for the long axis reveal distinct peaks
which larger dimensions compared to the respective short axis and compared to
the initial height profile of the long axis in Fig. 2 (bottom). As only
exemplary height profiles are chosen, no concrete conclusion can be drawn
about the change in size between the regions IV and V as the alignment of the
different $s-s$ or $t-t$ clusters is not perfect and especially differs
between the slightly different configurations. Hence, drawing a straight line
through three microgels does not reproduce the height profile for the
individual microgels perfectly . Nevertheless, we can assert that the
dimensions of the individual microgels decreased at high $\Pi$ compared to the
individual microgels measured at low surface pressure (Fig. 2).

Figure 6: Height profiles determined along the short axis (dotted lines) and
long axis (solid line) of three microgels in a $s-s$ and $t-t$ configuration,
respectively. The larger micrographs were acquired within region IV and V of
the compression isotherm for $HS-165$ (IV: A;V: B) and $CS-165$ (IV: C; V: D)
and have a size of 7.5 µm $\times$ 7.5 µm. The smaller micrographs are cut
outs which were enlarged from the larger micrographs as highlighted in them.

The orientation of all microgels ($\beta$) compared to the direction of
compression with the Langmuir trough barriers, the nearest neighbor distances
($NND$) and the amount of $t-t$ and $s-s$ ordering were computed for both
microgels. For this $\beta$ is defined as the angle between the direction of
compression with the barriers and the long semi-axis of the microgel cores. A
sketch highlighting the definition of $\beta$ in dependency of the direction
of compression is shown in Figure 7. The obtained values of $\beta$ for the
five different regions are reported as histograms in Figure 7 (top) for
$HS-165$ and in Fig. 7 (bottom) for $CS-165$. No clear preferential direction
for any of the five regions could be identified for $HS-165$. The angles 0-60°
showed slightly higher frequency values compared to the ones above 90° for
region III - V. Conversely, $CS-165$ showed a clear preferential orientation
parallel to the direction of compression at regions IV and V.33 When compared
to the clear trend observed in Fig. 7 (bottom) for $CS-165$, $HS-165$ can
therefore be accounted as randomly orientated.

Figure 7:  Development of the orientation of the microgels ($\beta$) of
$HS-165$ (top) and $CS-165$ (bottom) for the five different regions within the
compression isotherm. Additionally, a definition of the orientation $\beta$ of
the microgels is illustrated in the right of the image. The data of the core-
shell microgels are reproduced from Nickel et al. Anisotropic Microgels show
their Soft Side. Langmuir 2021, https://doi.org/10.1021/acs.langmuir.1c01748
Copyright 2021 American Chemical Society

Hence, the presence of the hard, in-compressible core has a major influence on
the overall orientation of the core-shell microgels at higher compressions as
discussed in more detail in our former study.33 The larger stiffness of
$CS-165$ microgels was here responsible for the preferential direction
observed for the anisotropic core-shell microgels in region IV and V of the
compression isotherm. As both systems present a similar aspect ratio at the
interface, we conclude that the stiffness of the anisotropic microgels plays a
key role in their orientation under the directional compression set by the
trough barriers.

In addition, the nearest neighbor distance $NDD$ was computed within the five
regions as summarized in Fig. S3. This analysis first confirms that the
average $NDD$ decreased upon compression for the two systems. This effect is
attributed to the deformability of the microgels, but also to the promoted
$s-s$ assembly at larger compression. Indeed the $NDD$ distributions presented
in Fig. S3 show a broader distribution at low compression. This is
particularly the case for $HS-165$ for which the observed clusters do not
present a clear preferential configuration at low compression. Increasing the
surface pressure then results in a more compact preferential $s-s$ assembly
characterized by the narrowing of the distribution and its decrease to shorter
distances. The less deformable $CS-165$ present stronger capillary
interactions characterized by a preferential $s-s$ assembly already at lower
surface compression.

To obtain more detailed information about the short range ordering of the
microgels, the $s-s$ and $t-t$ ordering is determined similar to ref. 33. We
refer to this study for more details on this analysis. In a nutshell, we
considered neighboring microgels presenting the same orientation $\pm$ 15°. An
$s-s$ configuration was identified if the nearest neighbor center was located
at the side of a microgel with a tolerance of $\pm$ 30°. Similarly, a $t-t$
configuration was considered when the center of the nearest neighbor lay in
the direction of the long axis $\pm$ 25°. A random orientation is
characterized here by a value in the order of 0.167. Hence, only the
orientations of the nearest neighbors were compared and related to the
different configurations.
Figure 8 presents the compression isotherms of hollow (Fig. 8 top) and core-
shell (Fig. 8 bottom) microgels together with their respective $t-t$ (circles)
and $s-s$ (squares) ordering with increasing surface compression. For $HS-165$
(Fig. 8 top), the $s-s$ ordering increases for higher $m_{shell}/A_{area}$
while the $t-t$ ordering remains constant. This increase in $s-s$ ordering
appears from region III to region IV in the compression isotherm.

Figure 8: Compression isotherms of $HS-165$ (top) and $CS-165$ (bottom) as
function of $m_{shell}/A_{Trough}$ reported together with the amount of $t-t$
(circles) and $s-s$ ordering (squares). The continuous line at 0.167 refers to
the value for a random orientation. The dotted lines at 0.2 and 0.4 are a
guide to the eye for an easier comparison between the two systems. The data of
the core-shell microgels are reproduced from Nickel et al. Anisotropic
Microgels show their Soft Side. Langmuir 2021,
https://doi.org/10.1021/acs.langmuir.1c01748 Copyright 2021 American Chemical
Society

In contrast to $HS-165$, both the $t-t$ and $s-s$ ordering increase at higher
surface pressure for $CS-165$. Nevertheless, the increase in $s-s$ ordering is
larger compared to the increase in $t-t$ ordering leading to a higher $s-s$
ordering of $CS-165$ in region III, IV and V.
Hence, the preferential $s-s$ ordering is a result of the elliptical shape of
the microgel and only slightly influenced by the hard elliptical core. Even in
the compressed region V, the elliptical shape of the hollow microgels is
preserved leading to $s-s$ ordering indicating shape-dependent short range
capillary interactions as also observed for hard ellipsoids.31 The elliptical
core slightly enhances this trend as the $s-s$ ordering of $HS-165$ goes up to
0.4 and the $s-s$ ordering of $CS-165$ increases to slightly higher values.
Additionally, the preferred $s-s$ ordering appears already in region I I for
$CS-165$ which results in $\sim$0.4 $s-s$ ordering already in region III. This
is different for $HS-165$ where the high values of $\sim$0.4 $s-s$ ordering
are observed in region IV and V only which is the region where no prominent
clustering was observed. To conclude, the hard core has an influence on the
nearest neighbor ordering but the general type of order is determined by the
elliptical shape of the microgels in both cases.

##  4 Conclusions

In this work, we investigated the influence of a hard elliptical core on the
behavior of anisotropic microgels at a decane-water interface. For this, we
prepared first a core-shell microgel with a thin and stiff polymer shell
synthesized around an anisotropically shaped inorganic core and then prepared
a hollow, anisotropic microgel via etching the core. Comparing the interfacial
behavior of core-shell $CS$ and hollow shell $HS$ anisotropic microgels thus
having the same shell properties, the examination of the compression isotherms
confirms the promoted deformability of the $HS$ system. Both systems maintain
an anisotropic shape at the interface with an aspect ratio in the order of 2.

Supported by computer simulations, we highlight how the combination of shape
anisotropy and microgel architecture influences the microgel deformation at
the interface. We first confirmed that hollow microgels spread more at the
interface than the stiffer core-shell microgels. A closer investigation of the
interfacial deformation revealed that the local shell thickness plays a
crucial role and $CS$ microgels with a thicker shell at the tip were found to
become more anisotropic at interface than in bulk. For $HS$ microgels, similar
to capsules, both the local thickness and dimensions of the cavity are key
parameters. For our design it turns out that hollow microgels are then
conversely to core-shell microgels becoming more elongated when the lateral
shell thickness is in the order of the longitudinal one. From the trend, we
can assume that this effect will be amplified when the lateral thickness
exceeds the longitudinal one. We believe that these conclusions will
contribute to a better understanding of the interfacial deformation of soft
anisotropic systems with more complex architectures and may be relevant for
instance for biological systems such as cells or virus capsids. In addition,
our results are expected to motivate the rational design of soft anisotropic
objects with a preprogrammed interfacial deformation set by the dimensions of
the core/cavity and the softness and local thickness of the polymeric shell.

Interestingly, hollow and core-shell anisotropic microgels were both found to
exhibit capillary interactions, which appeared weaker for the more deformable
hollow microgels. In line with our former study on anisotropic core-shell
microgels,33 it illustrates the importance of the anisotropy and softness on
the interfacial assembly and may be employed to create defined nanostructured
materials. In this case, more elongated objects with a more preferential side
to side self-assembly, such as stretched core-shell microgels53, 32 might be
interesting to create more defined assemblies.

We further demonstrate the influence of monolayer compression on the
interfacial ordering. While both systems build up a preferential side to side
ordering at larger compression, as revealed by the formation of small nematic
domains, stiffer anisotropic core-shell microgels additionally tend to align
parallel to the direction of the compression set by the barriers, whereas
softer hollow microgels did not.

Both microgel monolayers are highly compressible allowing to finely tune the
nearest neighbor distance between the microgels. Similar to spherical hollow
microgels, hollow anisotropic microgels are for more compressible than their
core-shell counterparts. We believe our results will allow to extent the use
of microgel monolayers for nanolithography to more complex assemblies. Hereby,
deposited monolayers of soft microgels were used as mask for the growth of
nanowires to create photonic materials. Due to their compressibility,
microgels have the advantage to allow the fine control of the nanowire
dispacing.54, 55

The next challenge for anisotropic microgels is to promote their ordering into
large nematic, smectic or even crystalline domains at the interface. More
complex lattices may in the future be achieved that could polarize and guide
the light, which further motivates the control of the domain orientation. Our
results, as proofs of concept, assert the potential of anisotropic microgels
for such applications but also the challenges it represents: first from the
synthetic viewpoint to create highly monodisperse anisotropic microgels with a
proper design and second in respect to their interfacial assembly related to
their anisotropy and deformability to properly balance capillary interactions
and allow long-range ordering at higher compression as well as domain
alignment.

We gratefully acknowledge financial support from the Deutsche
Forschungsgemeinschaft within SFB 985 “Functional Microgels and Microgel
Systems”. We thank Timon Kratzenberg for the help with the experiments on the
Langmuir-Blodgett trough.

The following files are available free of charge.

  * •

The SI contains detailed information about the image analysis procedure of the
AFM images, the compression modulus obtained in the experiments, the nearest
neighbor distances at different compression regions in the compression
isotherm and the original data of the compression isotherms. Furthermore, the
SI contains the details of the model of the microgels used in simulation as
well as the microgels characterization in bulk, including swelling curves,
form factor profiles of the microgels P(q), their sizes and shape anisotropy
parameters in a swollen and collapsed sates.

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Figure 9: TOC Graphic