# Self-Lubricating Drops

Huanshu Tan<EMAIL_ADDRESS>Detlef Lohse<EMAIL_ADDRESS>Xuehua
Zhang<EMAIL_ADDRESS>
###### Abstract

Over the past decade, there has been a growing interest in the study of
multicomponent drops. These drops exhibit unique phenomena, as the interplay
between hydrodynamics and the evolving physicochemical properties of the
mixture gives rise to distinct and often unregulated behaviors. Of particular
interest is the complex dynamic behavior of the drop contact line, which can
display self-lubrication effect. The presence of a slipping contact line in
self-lubricating multicomponent drops can suppress the coffee-stain effect,
conferring valuable technological applications. This review will explain the
current understanding of the self-lubrication effect of drops, and cover an
analysis of fundamental concepts and recent advances in colloidal assembly.
The potential applications of self-lubricating drops across different fields
will also be highlighted.

###### keywords:

Multicomponent drops , Self-lubrication effect , Coffee-stain effect / Coffee-
ring effect , Marangoni effect , Ouzo effect , Liquid-liquid phase separation
, Self-assembly and Supraparticles

[label1]organization=Center for Complex Flows and Soft Matter Research &
Department of Mechanics and Aerospace Engineering, Southern University of
Science and Technology,city=Shenzhen, postcode=518055, state=Guangdong,
country=P.R. China

[label2]organization=Physics of Fluids Group, Max Planck Center Twente for
Complex Fluid Dynamics, Department of Science and Technology, and J. M.
Burgers Centre for Fluid Dynamics, University of Twente, city=Enschede,
postcode=7500 AE , country=The Netherlands

[label3]organization=Department of Chemical & Materials Engineering,
University of Alberta,city=Edmonton, postcode=T6G1H9, state=Alberta,
country=Canada

##  1 Introduction

The contact line behavior of the drying drops on surfaces has garnered
substantial interest due to its significant relevance to various practical
applications, including surface patterning [1, 2], inkjet printing [3, 4],
healthcare diagnostics [5], and advanced material fabrication [6]. In a
groundbreaking study, Deegan et al. demonstrate the principle for the “coffee-
ring” or “coffee-stain” effect [1, 7], as illustrated in Figure 1a. The
contact line dynamics of multicomponent sessile drops are characterized by
complex behavior resulting from the interplay between physiochemical
properties and interaction among the different components [8, 9, 10, 11, 12].
These characteristics include changes with drop composition in surface tension
[13, 14], in gravitational effect [15], in chemical reactivity [16, 17], and
in freezing and boiling behaviors [18, 19]. Understanding the dynamics of the
contact line is crucial for a variety of applications, and comprehensive
reviews of self-cleaning surface, coating technology, and related topics can
be found in the literature [4, 10, 20, 21, 22, 23, 24, 25].

In recent years, a novel concept known as “self-lubrication” of multi-
component sessile drops [26, 27] has gained increasing attention. Referred to
as self-lubricating drops, this emerging drops exhibit a distinct behavior in
contact line dynamics, allowing the suppression of the coffee-stain effect.
Studies have demonstrated the potential of self-lubricating drops in various
state-of-art applications for fabrication of nanomaterials, especially via
colloidal assembly [27, 28, 29, 30].

In this article, we begin with an introduction to the self-lubrication of
multicomponent sessile drops and its underlying mechanism. We then delve into
the discussion of the dynamic processes involved, including liquid-liquid
phase separation, compositional evolution, Marangoni interfacial flow, and
surface property influence. Expanding on these fundamental concepts, we
present a concise overview of recent advancements in self-lubricating drops,
including supraparticle fabrication, the effects of particle properties on the
fabrication process, the application in sensing and detection, and exploration
of different liquid drop systems exhibiting self-lubricating behavior.
Finally, we conclude our review by posing several open questions and offering
our current opinions on relevant topics.

##  2 Self-lubrication of multicomponent sessile drops

Figure 1: Suppression of the “coffee-stain” effect by the “self-lubrication”
effect. Illustrations of (a) the “coffee-stain” effect, where the capillary
flow and pinned contact jointly lead to a ring shaped stain after the drop
evaporation, and (b) the “self-lubrication” effect, where an oil ring
spontaneously forms at the drop contact line and suppresses the “coffee-stain”
effect, thereby resulting in the assembling of particles, i.e. forming a
supraparticle. (c) Distinguished evolutions of the drop contact area between
the “coffee-stain” effect (CCR mode) and the “self-lubrication” effect. (d)
SEM photographs of the generated supraparticle by drying a self-lubricating
sessile drop ladened with TiO2 nanoparticles, showing the fractal-like
interior structure. Panel (d) is adapted with permission from ref [27].

###  2.1 Dynamic contact line modes

In this subsection, we will provide a brief review of the typical dynamic
modes exhibited by evaporating sessile drops, which will serve as a foundation
for introducing the concept of self-lubrication.

The evaporation of a liquid drop on a solid surface can occur within different
dynamics contact line modes [25, 31, 32, 33]. Specifically, the drop contact
line can slide freely with a constant contact angle, in the constant contact
angle (CCA) mode. Whereas, in the constant contact radius (CCR) mode, the drop
maintains a constant contact radius during the evaporation process. As
illustrated in Figure 1a, CCR mode is a characteristic of the coffee-stain
effect, essential for the development of the capillary flow that brings
ladened particles (or solid contaminants) towards the pinned contact line. The
particle accumulation consolidates the sticky contact line and CCR mode,
leaving a ring-shaped stain on the surface after the drop evaporation [1, 7].
Thus, the behavior of the contact line of the drop can significantly influence
the dynamics of the evaporation process.

However, evaporation of a multicomponent drop can exhibit enhanced mobility of
the contact line through the so-called “self-lubrication effect” [26, 27].

###  2.2 What is the “self-lubrication” effect?

The term “self-lubrication” refers to the phenomenon that occurs in
evaporating multicomponent sessile drops [27], as illustrated in Figure 1b.
The drop evaporation triggers the spontaneous formation of an immiscible
liquid ring at the drop contact line. The emerging oil ring promotes the
sliding contact line of the evaporating drop. For example, an oil ring around
a water-rich drop serves to prevent the accumulation of suspended particles
near the gas-liquid-solid three-phase contact line (surface property effects
refer to Sec. 3.4). There is no landing particles at the contact line to
consolidate CCR mode. Instead there is an effective suppression of the
“coffee-stain” effect. As illustrated in Figure 1c, the contact area size of
the residue from the evaporated drop is consequently reduced. It is remarkable
that, without presetting precursor films or utilizing a superhydrophobic
substrates, the self-lubrication effect improves the mobility of the sessile
drop, leading to a reduction in the contact area.

In presence of nanoparticles in the self-lubricating drop, the nanoparticles
self-assemble into a three dimensional macroscopic structure, known as a
supraparticle. Because of their unique structural features and high porosity
[27] (Fig. 1d), supraparticles are attractive and can facilitate a range of
applications [34, 35, 36, 37, 38], including catalysis, purification, drug
delivery, and energy storage. A brief discussion of the recent advances is
given in sections 4.1 and 4.2.

###  2.3 Spontaneous formation of the lubricating oil ring

The lubricating oil ring forms spontaneously from the liquid-liquid phase
separation, triggered by the preferential evaporation across the
multicomponent drop surface [26]. The subsequent merging of the separated oil
component at the drop contact line creates the continuous oil ring.

Typically, the self-lubricating multicomponent drops comprise a minimum of
three constituents, one cosolvent and two immiscible fluids that may undergo
phase separation from reducing the cosolvent function [39, 40, 41]. The
cosolvent augments the degree of miscibility between the two immiscible
fluids. For example, water and oil are soluble in the mixture, by introducing
ethanol in certain quantities [26, 42]. However, in situations where the
concentration of the cosolvent molecules fails to achieve thermodynamic
mediation between the two immiscible components, new liquid-liquid interfaces
starts to form, leading to liquid-liquid phase separation. This process of
phase separation can be triggered spontaneously by the evaporation
(dissolution) of the self-lubricating multicomponent drops, owing to the high
volatilities (solubility) of the cosolvent [43, 44, 45]. Furthermore, it is
important to note that the composition of the drop plays a significant role in
phase separation, which will be discussed in detail in section 3.1.

Figure 2: Phase diagram and the role of composition path line. (a) Ternary
phase diagram with ouzo and reverse-ouzo regions. (a1-a3) For the same ternary
liquid system, different path lines (dashed red lines) of drop’s compositional
evolution result in three different configurations of the separated liquid
phases. (b) Ternary phase diagrams of water, ethanol and one of the four
different oils, (b1) toluene, (b2) decade, (b3) octanol, and (b4) oleic acid.
Distinguished confocal images of the evaporation process of a C-dot ternary
drop in water-rich compositions (c1-c4) and oil-rich compositions (d1-4),
respectively corresponding to the blue dots and the orange dots in the four
different ternary liquid systems (b1-b4). The scale bars in (a1-a3), (c1-c4)
and (d1-d4) are 100 $\text{\,}\mu\mathrm{m}$. Panels (a1-a3), (b1-b4),
(c1-c4), and (d1-d4) are adapted with permission from ref [30].

The evaporation-triggered phase separation includes preferred heterogeneous
nucleation at the substrate [46] or in the bulk. The former directly leads the
formation of surface microdroplets of the separated component fluid, including
these oil microdroplets at the drop contact line, while numerous oil
microdroplets nucleate in the bulk. As illustrated in Figure 1b, the growing
and merging-up of oil microdroplets (orange dots) along the contact line form
the self-lubricating oil ring. The convective flow in the drop drives the
suspended oil microdroplets to move to the contact line contributing to the
oil ring formation.

Having introduced the concept of self-lubrication, it becomes evident that the
complex behavior of the multicomponent drop is a result of the interplay
between the physicochemical properties of the components and the micro-
hydrodynamics induced by evaporation [9, 40, 44, 47, 48]. It is the coupling
that affects the liquid-vapor equilibrium at the drop surface and the liquid-
liquid equilibrium within the solution [11].

Figure 3: Underlying fluid dynamics [1, 7, 61, 57, 26, 62, 47, 63, 45]. (a)
Three typical distributions of the diffusive (evaporation/dissolution) flux at
the surface of sessile drop (a1-a3), and the dimensional profiles of the
diffusive flux for three different contact angles. (b) Four typical flow
motions that affect the dynamic behaviors of the drop, including (b1)
capillary flow, (b2) particle (microdroplet) transport by bulk flow within the
sessile drop, (b3) Marangoni surface flow due to the compositional gradient
along the sessile drop surface, and (b4) self-migration of particle
(microdroplet) driven by the gradient inside the sessile drop. (b5)
Experimental images that present the simultaneous occurrence of the bulk-
transport and self-propulsion of microdroplets in a self-lubricating drop.
Panel (b5) is adapted with permission from ref [45].

##  3 Underlying dynamics and the impacts on self-lubricating drops

To gain insight into self-lubricating drops, the ternary phase diagram can be
utilized to quantify the liquid-liquid phase separation. This section
discusses solution disequilibrium, interfacial flows, particle interactions,
and these dynamics are crucial for understanding the behavior of self-
lubricating drops and their advancements in diverse applications.

###  3.1 Phase-diagram-quantified liquid-liquid phase separation

The ternary phase diagram, as shown in Figure 2a, is a graphical
representation of a three-component system, where each component is
represented by a corner of an equilateral triangle. The positions within the
triangle correspond to all possible mixture compositions of the three
components. The maximal fraction of a component is at its corresponding
corner, representing 100%, while the minimal fraction is at the opposite edge,
representing 0%. Using a ternary phase diagram is convenient to characterize
the compositional evolution, since a minimum of three components is necessary
for the formation of lubricating oil ring [9, 30, 43, 47, 49].

In the ternary phase diagram, where the immiscible components A and B
represent water and oil respectively, and the component C is a cosolvent such
as an alcohol. The diagram is divided into two distinct regions by the so-
called binodal curve. The region outside the binodal curve, the blue region in
Figure 2a, is the one-phase region. In this region, the concentration of the
cosolvent, i.e. the alcohol component, is sufficiently high to render a
homogenous solution of water, oil, and alcohol. However, within the binodal
curve (green region), the concentration of alcohol is insufficient to
facilitate miscibility between the water and oil components, leading to a
ternary isothermal liquid-liquid phase separation, i.e. a two-phase region.
Upon phase separation, the mixture separates into two phases, a water-rich
liquid phase and an oil-rich liquid phase. Each coexisting composition pair of
the two phases are connected by a tie line, which is governed by a
thermodynamic condition of the equality of the chemical potential [41, 50,
51]. This liquid-liquid equilibrium condition ensures that there is no net
transfer of matter or energy between two coexisting liquid phases.

The phenomenon of spontaneous emulsification can manifest when the mixture
composition lies within the region demarcated by the binodal and spinodal
curves. This zone as depicted by the grey regions in Figure 2a, is referred to
as the ouzo region [52, 53]. The spinodal curve is defined by the condition
whereby the second derivative of Gibbs free energy equals zero [54]. Within
the ouzo region, the mixture liquid state is under non-equilibrium conditions
and has a positive derivative of Gibbs free energy. Therefore, minor
concentration fluctuations can induce component nucleation, resulting in the
formation of liquid phase at a microscopic length scale. This thermodynamic
process leads to the creation of numerous sub-micro or microdroplets that are
homogeneously sized and stable, with minimal energy input required [55].

###  3.2 Compositional-evolution-determined disequilibrium

A noteworthy characteristic of self-lubricating drops is its transformation
from a miscible drop to a two-phase or even a multi-phase drop system. The
variation of the drop composition leads to solution disequilibrium and the
self-formation of an oil ring at the drop’s contact line that acts as a
lubricant [26, 27]. The path line is defined as a curve that connects a series
of composition points in the ternary diagram, each point representing the
composition of the mixture at a specific stage of the process. Thus, path
lines can visualize how the composition of the multicomponent drop changes
over time as it undergoes a specific set of conditions [56, 41].

During the process of evaporation (dissolution, or reaction), the composition
of the self-lubricating drop undergoes changes, corresponding to an
evaporative path line in the ternary phase diagram, as exemplified by the red
dashed arrows in Figures 2-(a1-a3). Li et al. reported the distinctive liquid-
liquid phase separation phenomena exhibited by self-lubricating drops through
different evaporative path lines [30]. When the path line crosses the ouzo
region, oil microdroplets form in the water, as shown in the insert of Figure
2-a1. Conversely, when the path line traverses the reverse ouzo region, water
microdroplets appears in the oil, as indicated with the orange circles in the
insert of Figure 2-a2. The control experiment confirmed that no microdroplets
form when the ternary liquid system lacks ouzo and reverse ouzo regions, as
indicated with the orange circles in the insert of Figure 2-a3. Li et al.
performed a comprehensive investigation into the self-lubricating drops those
containing carbon-nanodots in four distinct oil-water-ethanol ternary systems
[30]. The corresponding ternary phase diagrams are presented in Figures
2(b1-b4). The differing vapor pressure of toluene (3.8 kPa), decane (0.17
kPa), octane (0.0087 kPa), and oleic acid (7.2$\times 10^{-8}$ kPa) resulted
in a shift in the preference order among the oil, water (3.2 kPa), and ethanol
(7.9 kPa) components. The application of confocal imaging techniques
elucidated the influence of the physical properties of the oil component on
the self-lubrication process, as presented in Figures 2c (water-rich drops)
and 2d (oil-rich drops).

###  3.3 Diffusion-induced hydrodynamics

In a stationary and diffusion controlled evaporation, free spherical drops in
a quiescent open air have uniform diffusive fluxes at their surface, however,
for sessile drops on a substrate, the situation is different [57], as
illustrated in Figures 3a1-3a3. The diffusive flux at the drop surface is not
uniform any more, except the case that the drop contact angle is 90°. For the
sessile drops with sizes smaller than the capillary length
$\ell_{c}=\sqrt{\sigma/(\rho g)}$ (surface tension $\sigma$, density $\rho$,
and the gravitational acceleration $g$), the drops hold spherical-cap shapes,
as the surface tension dominates over the gravity. Given the additional
assumption of quasi-steady, the diffusive flux for various contact angle
$\theta$ expresses as $J(r)\sim
J_{0}(1-r/R)^{-\frac{\pi-2\theta}{2(\pi-\theta)}}$ [7, 57]. Figure 3a4
displays the profile of the dimensional diffusive flux for various contact
angles, where $\tilde{J}$ and $r$ are normalized by $J_{0}$ and $R$
respectively. $J_{0}$ indicates the diffusive flux at the apex, where the
substrate influence is negligible. For a flat sessile drop ($\theta<$90°), the
mass loss near the contact line is faster than other places, as sketched in
Figure 3b1. Thus, the pinned contact line consequently induces liquid
replenishment towards the contact line, i.e. the so-called capillary flow [1].
This rising bulk flow transports the laden particle in the drop (Fig. 3b2),
changing the particle distribution, such as the coffee-stain effect.

In the case of self-lubricating drops of multicomponent liquid, as illustrated
in Figure 3b3, the non-uniform diffusive flux of the volatile component
induces the compositional gradients $\nabla c$ and the consequent surface
tension gradients $\nabla\sigma$ at the drop surface. Thus, the interfacial
flows arise, known as the solutal Marangoni effect. The Marangoni convection
drives the laden particles and the nucleated microdroplets intensively within
the drop. To gain insight into the hydrodynamics, our theoretical model is
based on simplified Navier-Stokes equations for flow, convection-diffusion
equations for composition and temperature, Fick’s law for diffusion, and
multicomponent evaporation model for the mass transfer at the drop surface
[26, 59, 60]. These models account for the composition-dependent fluid
properties and the Marangoni stress that arises from interfacial tension
gradients, and a deeper understanding of the hydrodynamics and transport
phenomena within self-lubricating drops can be achieved. It is important to
note that while this modeling approach is valuable, it does not account for
the evolving drop configuration or liquid-liquid phase separation.

Beside the convective transport, recent researches also reported the self-
propulsion of microdroplets inside the self-lubricating drop [43, 45]. The
suspended microdroplets in the compositional gradient field also experience
the solutal Marangoni effect at the interface [61, 62, 63, 64], as sketched in
Figure 3b4. Hence, the interfacial flows propel the suspended microdroplets in
the opposite direction, similar to that a swimmer pushes the surrounding water
backward to move ahead. May et al. investigated this self-propelling
phenomenon in a self-lubricating drop (Fig. 3b5), and provided the explanation
of the mechanism [45].

Based on the discussion of this subsection, it is evident that the above four
hydrodynamics – capillary flow, convective transport of particles, Marangoni
flow, and self-propelling of particles – coexist and interplay, affecting the
dynamic behaviors of the self-lubricating drops.

###  3.4 Surface property influencing particle interactions

In a particle-laden self-lubricating drop, the surface properties of colloidal
particle play a role in influencing inter-particle interactions and the
interactions with various interfaces, including substrates, the drop liquid-
gas surface, and the aqueous-oil interfaces. In the self-lubricating drops
with the presence of nanoparticles, pickering microdroplets (picking
emulsions) are formed resulting from the covering of the nanoparticles over
the nucleated microdroplets, and their stability and structure depend on the
particle-microdroplet interaction [65, 66]. Manipulating these interactions is
essential in governing the self-lubricating process and supraparticle
fabrication [28, 29].

Nanoparticle assembly in the self-lubricating drop results from the
competition between a variety of intermolecular and interparticle forces,
including Van der Waals forces, magnetic and electrostatic forces, steric
forces, depletion forces, hydrodynamic forces [28, 29, 67]. Van der Waals
forces contribute to the overall interaction energy, while external forces,
whether repulsive or attractive forces, allows for delicate manipulation of
the assembly and stability of laden particles [67]. Additionally, capillary
forces, convective forces, friction and lubrication forces arising from fluid
flow and shear have a notable impact on the dynamics of the laden particles
[8, 1, 2].

Understanding and manipulating these interactions enable control over the
inner structure of particle assemblies, stabilization of pickering
microdroplets, and optimize the properties of the generated supraparticles.
This opens opportunities, by applying self-lubricating drops, for tailoring
and engineering systems with desired properties and functionalities.

Figure 4: Effects of particle surface property on the assembly process. (a)
Side view of submillimeter-sized supraparticles with different shapes obtained
by drying self-lubricating colloidal drops containing silica particles with
different sizes. (b) Varying the fraction of two types of particles with
different sizes changes the final shape of the fabricated supraparticle. Panel
(a) and (b) are adapted with permission from ref [28]. (c) Silk fibroin
supraparticle fabricated by self-lubricating drops. Panel (c) is adapted with
permission from ref [58]. (d) Four different supraparticle shapes (d1-d4)
obtained by utilizing four silica particles with different surface properties,
as the interplay between the particles (red) and the separated oil component
(yellow) affects the particle assembly process (d1-d4). Panel (d) is adapted
with permission from ref [29].  Figure 5: Application of self-lubricating
drops for sensing and detection. (a) A schematic representation of applying
self-lubricating drops to generate porous Ag-analyte supraparticles, which are
used for surface-enhanced Raman spectroscopy detection. The result shows that
the surface-enhanced Raman spectroscopy spectra can achieve a low detection
limit of (b) $10^{-16}$$\text{\,}\mathrm{M}$ for R6G detection, and (c)
$10^{-6}$$\text{\,}\mathrm{M}$ for triclosan detection. Pannels (a)-(c) are
adapted with permission from ref [6]. (d) Nano-extraction enables fluorescence
detection of Nile Blue at a concentration that is below the fluorescence
detection limit. (e) Fluorescence images of octanol nanodroplets labeled with
different initial concentrations of Nile Blue from
$10^{-9}$$\text{\,}\mathrm{M}$ to $10^{-7}$$\text{\,}\mathrm{M}$ in water.
Pannels (d) and (e) are adapted with permission from ref [76].

##  4 Potential applications of the self-lubricating drops

The self-lubricating drops have drawn people’s attention and being applied in
different contexts, particularly in particle assembly. This section presents a
review of the recent advances of these approaches, through which we intend to
highlight the diversity of the self-lubricating drops and point out open
questions in the following section.

###  4.1 Particle assembly by self-lubricating drops

The assembly of colloidal particles by drying sessile drops of colloidal
suspension is a flexible approach for the fabrication of supraparticles.
However, the coffee-stain effect impedes the assembly of the particles. Super
liquid-repellant substrates have been used to reduce the initial contact area
[70, 71, 72], thereby weakening the coffee-stain effect. But these substrates
are fragile, especially in the procedure of supraparticle detachment. Drying
colloidal drop above immiscible liquids can avoid these problems to certain
extent, but either the cheerios effect [73] or the liquid film replacement
cause other issues [74, 75]. Therefore, attention is paid to the self-
lubricating drops for supraparticle fabrication [27].

The supraparticles fabricated by self-lubricating drops have high porosities
and tunable shapes. The liquid-liquid phase separation from numerous oil
microdroplets suspending within the colloidal sessile drop. During the
particle assembly, the nucleated oil microdroplets act as templates, resulting
in multi-scale, fractal-like structures inside the supraparticle (Fig. 1d),
and the porosity up to 90% [27]. Either spherical or non-spherical shaped
supraparticles can be fabricated by adjusting the initial fraction of oil
component and the particle of the drop solution. In the case of the self-
lubricating drop having a relatively high oil-to-particle ratio, the particle
assembly process in the later stage involves the suspension water drop
completely floating on the oil phase. This leads to the formation of a
spherical shaped supraparticle. However, if the oil content is not high enough
to completely levitate the aqueous phase, the developing suspension drop
undergoes deformation and takes a non-spherical shape [27]. These unique
features of the supraparticles make the particle assembly by self-lubricating
drop as an attractive and potential technique.

Raju et al. investigated the particle size effects on the non-spherical shape
of the fabricated supraparticles in self-lubricating ternary drops [28]. By
using spherical silica particles from 20 to 1000 nm, the authors fabricated
pancake-like and American-football-like supraparticles, as displayed in Figure
4a. The in situ measurements reveal that the supraparticle formation proceeds
via the formation of a flexible silica-shell at the contracting oil ring. The
particle size affects the time when the shell-like structure ceases to shrink
and loses its flexibility. With the big particles, the shell ceases deforming
by the oil ring at a later ceasing time than the small particles, resulting
the American-football-like supraparticles. The same tendency maintains by
increasing the large particle fraction in the mixtures of small and large
particles, as presented in Figure 4b.

Since the hydrophobicity of particles and electrostatic effects can affect the
interaction between the oil microdroplets and the colloidal particles,
Koshkina et al. used silica particles as a model system to study the particle
surface property effect [29]. Four different silica particles were synthesized
to proceed particle assembly in self-lubricating ternary drops, as shown in
Figure 4. They found the hydrated negatively charged silica particle and
statically stabilized silica particles can form supraparticles, as presented
in Figures 4 d1 and d2, while dehydrated negatively charged silica particles
and positively charged amine-coated particles form flat film-like deposits, as
displayed in Figures 4 d3 and d4. Due to the effects of surface modification
on particle, negatively charged PEGylated silica particles, hydrophobicity-
enhanced dehydrated silica particles, and positively charged but
hydrophobicity-enhanced amine-coated silica particles can adsorb onto the
water-oil interface, including the oil ring interface and the oil
microdroplets. As shown in Figures 4d2-ab, d3-ab, and d4-ab, particle-coated
oil microdroplets, the so-called pickering microdroplets, are observed in suit
(confocal microscopy: red for particles, and yellow for oil).

It is notable that supraparticles, being micron-scaled assemblies composed of
nano-scaled particles, possess a high area-to-volume ratio and are easy to
handle at the micro scale. This combination makes them ideal for catalytic
applications, as they offer enhanced catalytic efficiency and practicality in
terms of operation and integration.

Figure 6: Self-lubricating drops of diverse liquid systems. (a) Evaporating
sessile drop of liquid crystalline solution. Evaporation-trigged liquid-liquid
crystalline phase separation facilitates the de-pinning behavior of
evaporating amyloid fibril-water drop. Pannel (a) is adapted with permission
from ref [68]. (b) Evaporating sessile drop of aqueous two-phase systems. The
evaporation of Polyethylene Glycol(PEG)/dextran sessile drops induces the non-
associative phase separation and the self-lubricating behaviors. The scale bar
is $500\text{\,}\mathrm{\SIUnitSymbolMicro m}$. Pannel (b) is adapted with
permission from ref [43]. (c) Reaction between Polymethylhydrosiloxane (PMH)
drop and surrounding aqueous media forms H2 nanobubbles at the drop rim. The
collapse and coalescence of the bubbles affect the contact line behavior of
the drops. The scale bar is $1\text{\,}\mathrm{\SIUnitSymbolMicro m}$. Pannel
(c) is adapted with permission from ref [69].

###  4.2 Self-lubricating drops for sensing and detection

Self-lubricating drops enable a novel approach developed for surface-enhanced
Raman spectroscopy (SERS) detection, as recently proposed by Dabodiy et al.
[6]. The components in the self-lubricating drop include Ag nanoparticles and
analytes. Facilitated by the self-lubrication effect, analyte-absorbed metal
supraparticles can form from the drop evaporation (Fig. 5a), and perform as
sensitive SERS sites. These hot spots intensify the SERS signals, leading to
an ultra-sensitive detection. The quantitative analysis of rhodamine 6G is
possible with its low concentrations from $10^{-5}$$\text{\,}\mathrm{M}$ to
$10^{-11}$$\text{\,}\mathrm{M}$ (Fig. 5b). For a model micro-pollutant in
water triclosan, the detection limit is down to $10^{-6}$$\text{\,}\mathrm{M}$
(Fig. 5c). The ultra sensitive detection mechanism is due to the liquid-liquid
extraction during the evaporation of self-lubricating drops. As demonstrated
by Qian et al. [76], nano-extraction enables fluorescence detection limit of
nano-molar to pico-molar for fluorescent model compounds in evaporating
ternary liquids. As shown in Figures 6d and e, although Nile Blue is at a
concentration below the detection limit in the initial water-octanol-ethanol
mixture, its partitioning into the separated oil (octanol) nano- and micro-
droplets gives rise to a fluorescence signal that strong enough to be detected
and quantified.

Having observed the impressive enhancements in SERS (Surface-Enhanced Raman
Scattering) detection and understanding the underlying principles of
extraction, we are convinced that self-lubricating drops have considerable
potential for use in sensing and detection across various fields. These
potential applications include but are not limited to environmental
monitoring, food safety, and early diagnosis [77, 78, 79, 80]. Given micro-
extraction’s broad applicability to diverse multicomponent liquid systems,
investigating self-lubricating drops with complex compositions thus becomes
particularly significant for practical use.

###  4.3 Self-lubricating drops with complex composition

The self-lubrication phenomenon occurs not only in the sessile drops of water-
alcohol-oil liquid system. Recent advances have demonstrated the self-
lubrication phenomenon happens also with other multicomponent liquids,
including liquid crystalline solutions, aqueous polymer solutions, and
chemical reaction solutions.

Almohammadi et al. investigated the evaporation-triggered liquid-liquid
crystalline phase separation in drops of amyloid fibril-water suspension [68].
The phase separation induces the formation of liquid crystalline tactoids with
different shapes and internal structures, as sketched in the inserts of Figure
6a. However, they found that the presence of the amyloid fibrils and tactoids
resulted in improving the de-pinning behavior of evaporating drop, rather than
enhancing the coffee-stain effect. Complex dynamics occur in this self-
lubricating drop of liquid crystalline solution. As illustrated in Figure 6a,
the rapid evaporation further causes liquid crystalline phase transition in
the drop from the isotropic (I) phase, isotropic+nematic (I+N) phase, to
nematic (N) phase.

Aqueous two-phase systems (ATPS) are organic-free and environment-friendly
extraction liquid system. Guo et al. reported the non-associative phase
separation triggered by the evaporation of Polyethylene Glycol (PEG)/dextran
ATPS sessile drops [43]. The sessile drops with different initial ATPS
compositions experience distinctive evaporation processes, as shown in Figure
6b. Their initial compositional points (star symbols) correspond to different
evaporative path lines, affecting the phase separation and the consequent drop
behaviors. Nevertheless, the PEG/dextran ATPS sessile drops also exhibit the
self-lubricating phenomenon, having smoothly reducing contact area.

In addition to the evaporation process, recent research by Dyett et al. has
explored the potential of using chemical reactions to alter the composition
and change contact line behaviors [69]. The specific reaction investigated
involves the interaction between Polymethylhydrosiloxane (PMH) and water,
resulting in the generation of hydrogen. The experimental results, illustrated
in Figure 6c, revealed that when a PMH sessile drop is immersed in an aqueous
medium, nanobubbles of H2 form along the rim of the drop as a byproduct of the
reaction. The collapse and merging of these nanobubbles subsequently impact
the behavior of the drop contact line. This approach indicates possibilities
for the formation of a self-lubricating ring through chemical reactions.

##  5 Current opinions and open questions

The self-lubrication effect occurs in various multicomponent liquid systems
with potential applications in extraction and concentration, sensitive
sensing, and fabrication of functional materials. However, precise control of
this effect remains a challenge. The multi-physic dynamics involved, as well
as their coupling interactions, are complex and fascinating.

To conclude this review, we summarize the most prominent outstanding questions
that remain to be explored.

(1) Interaction between particles and microdroplets: Spontaneous nucleated
microdroplets can act as templates for advanced regulation. Manipulating the
interaction between laden colloidal particles and the nucleated component
microdroplets provides paths for the bottom-to-up fabrication of the multi-
scale and functional structures inside the supraparticles, such as tuning the
porosity of supraparticles [81, 82]. These interactions includes capillary
interaction and electrostatic interaction in diverse contexts [85, 83, 84, 87,
86]. However, the continuously changing of composition of the self-lubricating
drop affects the interaction process, which challenges the manipulation.

(2) Hydrodynamic effect on particle assembly: The self-lubricating drops
involve different kinds of flow, including capillary flow [1, 7], solutal
Marangoni flow [26, 89, 88], and thermal Mrangoni flow [90]. These flows
interact and cause a chaotic flow field [90, 64, 88, 91], and the flow field
is further coupled with the evolving compositional fields [9, 92]. Thus,
understanding the impact of this physicochemical hydrodynamic process on the
particle assembly is essential to the fabrication of the functional
micro-/nanostructures [93, 94].

(3) A set of hidden variables from chemical reaction:  Multicomponent fluids
are inevitably accompanied with chemical reactions in many situations [95, 96,
97]. The reaction may create some new component, either miscible or immiscible
with the original mixture, which can alter the self-lubricating drop system
[69]. The chemical reaction variables [97] are quite diverse and, thereby,
provide self-lubricating drops a broad design space for specified
fabrications. However, since the design options are so huge, constructing a
design frame is necessary.

(4) Broad applications from complex composition: Aqueous two-phase systems,
known as the clean alternatives for traditional organic-water solvent
extraction, make self-lubricating drops suitable for bio-engineering processes
or environment managements [34, 99, 98]. It is also notable that liquid
crystalline solutions can broaden the applied range for fabrication of
semiconductors, photo-alignment [6, 100, 101, 95],

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported
in this paper.

## Data availability

No data was used for the research described in the article.

## Acknowledgments

H.T. acknowledges the supports from the Natural Science Foundation of China
(No. 12102171) and the Natural Science Foundation of Shenzhen (No.
20220814180959001). D.L. acknowledges supports from the EU (ERC-Advanced Grant
Project DDD No. 740479, and ERC-Proof-of-Concept Grant Project No. 862032).
X.Z. acknowledges supports from the Natural Science and Engineering Council of
Canada (NSERC) and from the Canada Research Chairs program.

## References

  * [1] Deegan, R., Bakajin, O., Dupont, T., Huber, G., Nagel, S. & Witten, T. Capillary flow as the cause of ring stains from dried liquid drops. Nature. 389(6653), 827-829 (1997)
  * [2] Yunker, P., Still, T., Lohr, M. & Yodh, A. Suppression of the coffee-ring effect by shape-dependent capillary interactions. Nature. 476(7360), 308-311 (2011)
  * [3] De Gans, B., Duineveld, P. & Schubert, U. Inkjet printing of polymers: state of the art and future developments. Advanced Materials. 16(3), 203-213 (2004)
  * [4] Lohse, D. Fundamental fluid dynamics challenges in inkjet printing. Annual Review Of Fluid Mechanics. 54, 349-382 (2022)
  * [5] Sobac, B. & Brutin, D. Structural and evaporative evolutions in desiccating sessile drops of blood. Physical Review E. 84(1), 011603 (2011)
  * [6] Dabodiya, T., Sontti, S., Wei, Z., Lu, Q., Billet, R., Murugan, A. & Zhang, X. Ultrasensitive surface-enhanced Raman spectroscopy detection by porous silver supraparticles from self–lubricating drop evaporation. Advanced Materials Interfaces. 9(35), 2201998 (2022) **Using self-lubricating drops to develop a novel approach for surface-enhanced Raman spectroscopy (SERS) detection.
  * [7] Deegan, R., Bakajin, O., Dupont, T., Huber, G., Nagel, S. & Witten, T. Contact line deposits in an evaporating drop. Physical Review E. 62(1), 756-765 (2000)
  * [8] Marin, A., Gelderblom, H., Lohse, D. & Snoeijer, J. Order-to-disorder transition in ring-shaped colloidal stains. Physical Review Letters. 107, 085502 (2011)
  * [9] Lohse, D. & Zhang, X. Physicochemical hydrodynamics of droplets out of equilibrium. Nature Reviews Physics. 2(8), 426-443 (2020)
  * [10] Wang, Z., Orejon, D., Takata, Y. & Sefiane, K. Wetting and evaporation of multicomponent droplets. Physics Reports. 960, 1-37 (2022)
  * [11] Tan, H., Diddens, C., Zhang, X. & Lohse, Chapter: Evaporation of ternary sessile drops. In Brutin D. & Seffiane K. (Ed.), Drying of Complex Fluid Drops: Fundamentals and Applications. (Royal Society of Chemistry, 2022)
  * [12] Al Balushi, K.M., Duursma, G., Valluri, P., Sefiane, K. & Orejon, D. Binary mixture droplet evaporation on microstructured decorated surfaces and the mixed stick–slip modes. Langmuir. 39(23), 8323-8338 (2023)
  * [13] Sefiane, K., Tadrist, L. & Douglas, M. Experimental study of evaporating water–ethanol mixture sessile drop: influence of concentration. International Journal Of Heat And Mass Transfer. 46(23), 4527-4534 (2003)
  * [14] Karpitschka, S., Liebig, F. & Riegler, H. Marangoni contraction of evaporating sessile droplets of binary mixtures. Langmuir. 33(19), 4682-4687 (2017)
  * [15] Li, Y., Diddens, C., Lv, P., Wijshoff, H., Versluis, M. & Lohse, D. Gravitational effect in evaporating binary microdroplets. Physical Review Letters. 122(11), 114501 (2019)
  * [16] Kumar, G. & Prabhu, K. Review of non-reactive and reactive wetting of liquids on surfaces. Advances In Colloid And Interface Science. 133(2), 61-89 (2007)
  * [17] Eustathopoulos, N. & Voytovych, R. The role of reactivity in wetting by liquid metals: a review. Journal Of Materials Science. 51, 425-437 (2016)
  * [18] Marin, A., Enriquez, O., Brunet, P., Colinet, P. & Snoeijer, J. Universality of tip singularity formation in freezing water drops. Physical Review Letters. 113(5), 054301 (2014)
  * [19] Lyu, S., Tan, H., Wakata, Y., Yang, X., Law, C., Lohse, D. & Sun, C. On explosive boiling of a multicomponent Leidenfrost drop. Proceedings Of The National Academy Of Sciences. 118(2), e2016107118 (2021)
  * [20] Blossey, R. Self-cleaning surfaces–virtual realities. Nature Materials. 2(5), 301-306 (2003)
  * [21] Ganesh, V., Raut, H., Nair, A. & Ramakrishna, S. A review on self-cleaning coatings. Journal Of Materials Chemistry. 21(41), 16304-16322 (2011)
  * [22] Giorgiutti-Dauphiné, F. & Pauchard, L. Drying drops: drying drops containing solutes: from hydrodynamical to mechanical instabilities. The European Physical Journal E. 41(32), 1-15 (2018)
  * [23] Dalawai, S., Aly, M., Latthe, S., Xing, R., Sutar, R., Nagappan, S., Ha, C., Sadasivuni, K. & Liu, S. Recent advances in durability of superhydrophobic self-cleaning technology: a critical review. Progress In Organic Coatings. 138, 105381 (2020)
  * [24] Sarkın, A., Ekren, N. & Sağlam, Ş. A review of anti-reflection and self-cleaning coatings on photovoltaic panels. Solar Energy. 199, 63-73 (2020)
  * [25] Gelderblom, H., Diddens, C. & Marin, A. Evaporation-driven liquid flow in sessile droplets. Soft Matter. 18, 8535-8553 (2022)
  * [26] Tan, H., Diddens, C., Lv, P., Kuerten, J., Zhang, X. & Lohse, D. Evaporation-triggered microdroplet nucleation and the four life phases of an evaporating Ouzo drop. Proceedings Of The National Academy Of Sciences. 113(31), 8642-8647 (2016)
  * [27] Tan, H., Wooh, S., Butt, H., Zhang, X. & Lohse, D. Porous supraparticle assembly through self-lubricating evaporating colloidal ouzo drops. Nature Communications. 10(1), 478 (2019)
  * [28] Thayyil Raju, L., Koshkina, O., Tan, H., Riedinger, A., Landfester, K., Lohse, D. & Zhang, X. Particle size determines the shape of supraparticles in self-lubricating ternary droplets. ACS Nano. 15(3), 4256-4267 (2021) **Investigating the particle size effects on the shape of the fabricated supraparticles by self-lubricating drops.
  * [29] Koshkina, O., Raju, L., Kaltbeitzel, A., Riedinger, A., Lohse, D., Zhang, X. & Landfester, K. Surface properties of colloidal particles affect colloidal self-assembly in evaporating self-lubricating ternary droplets. ACS Applied Materials & Interfaces. 14(1), 2275-2290 (2021) **Investigating the particle surface property effect on the particle assembly by self-lubricating drops.
  * [30] Li, M., Wei, X. & Bao, L. Self-assembly of carbon nanodots induced by liquid–liquid phase separation in a surface microdroplet. Soft Matter. 18(35), 6517-6528 (2022) **Demonstrating the distinctive liquid-liquid phase separation phenomena exhibited by self-lubricating drops through different evaporative path line.
  * [31] Picknett, R. & Bexon, R. The evaporation of sessile or pendant drops in still air. Journal Of Colloid And Interface Science. 61(2), 336-350 (1977)
  * [32] Bonn, D., Eggers, J., Indekeu, J., Meunier, J. & Rolley, E. Wetting and spreading. Reviews Of Modern Physics. 81(2), 739 (2009)
  * [33] Dietrich, E., Kooij, E., Zhang, X., Zandvliet, H. & Lohse, D. Stick-jump mode in surface droplet dissolution. Langmuir. 31(16), 4696-4703 (2015)
  * [34] Wintzheimer, S., Reichstein, J., Groppe, P., Wolf, A., Fett, B., Zhou, H., Pujales-Paradela, R., Miller, F., Müssig, S., Wenderoth, S. & Mandel K. Supraparticles for sustainability. Advanced Functional Materials. 31(11), 2011089 (2021)
  * [35] Guo, X., Xue, N., Zhang, M., Ettelaie, R. & Yang, H. A supraparticle-based biomimetic cascade catalyst for continuous flow reaction. Nature Communications. 13(1), 5935 (2022)
  * [36] Hu, M., Reichholf, N., Xia, Y., Alvarez, L., Cao, X., Ma, S., DeMello, A.J. & Isa, L. Multi-compartment supracapsules made from nano-containers towards programmable release. Materials Horizons. 9(6), 1641–1648 (2022)
  * [37] Li, W., Zhou, H., Zhao, K., Jin, J., Chen, X., Shahzad, M.W., Jiang, Y., Matar, O.K., Dai, S., Xu, Ben Bin & Sun, L. Conformally anodizing hierarchical structure in a deformed tube towards energy-saving liquid transportation. Chemical Engineering Journal. 431, 133746 (2022)
  * [38] Jo, S., Kim, J., Lee, J.E., Wurm, F.R., Landfester, K. & Wooh, S. Multimodal enzyme-carrying suprastructures for rapid and sensitive biocatalytic cascade reactions. Advanced science. 9(10), 2104884 (2022)
  * [39] Lu, Z., Rezk, A., Jativa, F., Yeo, L. & Zhang, X. Dissolution dynamics of a suspension droplet in a binary solution for controlled nanoparticle assembly. Nanoscale. 9(36), 13441-13448 (2017)
  * [40] Moerman, P., Hohenberg, P., Vanden-Eijnden, E. & Brujic, J. Emulsion patterns in the wake of a liquid–liquid phase separation front. Proceedings Of The National Academy Of Sciences. 115(14), 3599-3604 (2018)
  * [41] Tan, H., Diddens, C., Mohammed, A., Li, J., Versluis, M., Zhang, X. & Lohse, D. Microdroplet nucleation by dissolution of a multicomponent drop in a host liquid. Journal Of Fluid Mechanics. 870, 217-246 (2019)
  * [42] Tan, H., Diddens, C., Versluis, M., Butt, H., Lohse, D. & Zhang, X. Self-wrapping of an ouzo drop induced by evaporation on a superamphiphobic surface. Soft Matter. 13(15), 2749-2759 (2017)
  * [43] Guo, W., Kinghorn, A., Zhang, Y., Li, Q., Poonam, A., Tanner, J. & Shum, H. Non-associative phase separation in an evaporating droplet as a model for prebiotic compartmentalization. Nature Communications. 12, 3194 (2021) **Demonstating the non-associative phase separation triggered by the evaporation of PEG/dextran ATPS sessile drops.
  * [44] Chao, Y., Ramírez-Soto, O., Bahr, C. & Karpitschka, S. How liquid–liquid phase separation induces active spreading. Proceedings Of The National Academy Of Sciences. 119(30), e2203510119 (2022) **Investigating the active spreading caused by liquid-liquid phase separation of evaporating multicomponent sessile drops.
  * [45] May, A., Hartmann, J. & Hardt, S. Phase separation in evaporating all-aqueous sessile drops. Soft Matter. 18, 6313-6317 (2022) **Elucidating the self-propelling phenomenon of nucleated microdropets in self-lubricating ATPS drops.
  * [46] Zhang, X., Lu, Z., Tan, H., Bao, L., He, Y., Sun, C. & Lohse, D. Formation of surface nanodroplets under controlled flow conditions. Proceedings Of The National Academy Of Sciences. 112(30), 9253-9257 (2015)
  * [47] Tan, H. Evaporation and dissolution of droplets in ternary systems. (Ph.D. Thesis, University of Twente, 2018)
  * [48] Baumgartner, D., Shiri, S., Sinha, S., Karpitschka, S. & Cira, N. Marangoni spreading and contracting three-component droplets on completely wetting surfaces. Proceedings Of The National Academy Of Sciences. 119(19), e2120432119 (2022)
  * [49] Lu, Z., Xu, H., Zeng, H. & Zhang, X. Solvent effects on the formation of surface nanodroplets by solvent exchange. Langmuir. 31(44), 12120-12125 (2015)
  * [50] Cussler, E. Diffusion: mass transfer in fluid systems. (Cambridge university press,2009)
  * [51] Chu, S. & Prosperetti, A. Dissolution and growth of a multicomponent drop in an immiscible liquid. Journal Of Fluid Mechanics. 798, 787-811 (2016)
  * [52] Vitale, S.A. & Katz, J.L. Liquid droplet dispersions formed by homogeneous liquid-liquid nucleation:“the Ouzo effect”. Langmuir. 19(10), 4105-4110 (2003)
  * [53] Ganachaud, François & Katz, J.L. Nanoparticles and nanocapsules created using the ouzo effect: spontaneous emulsification as an alternative to ultrasonic and high-shear devices. ChemPhysChem. 6(2), 209-216 (2005)
  * [54] Solans, C., Morales, D. & Homs, M. Spontaneous emulsification. Current Opinion In Colloid & Interface Science. 22, 88-93 (2016)
  * [55] Vratsanos, M., Xue, W., Rosenmann, N., Zarzar, L. & Gianneschi, N. Ouzo effect examined at the nanoscale via direct observation of droplet nucleation and morphology. ACS Central Science. 9(3), 457-465 (2023)
  * [56] Ruschak, K.J. & Miller, C.A. Spontaneous Emulsification in Ternary Systems with Mass Transfer. Industrial & Engineering Chemistry Fundamentals. 11(4), 534-540 (1972)
  * [57] Cazabat, A. & Guéna, G. Evaporation of macroscopic sessile droplets. Soft Matter. 6, 2591-2612 (2010)
  * [58] Lamb, A., He, F., Zhai, S. & Zhao, H. Silk fibroin supraparticles created by the evaporation of colloidal Ouzo droplets. AIP Advances. 11(8), 085125 (2021) **Using self-lubricating drops to fabricate silk fibroin supraparticles.
  * [59] Diddens, C., Tan, H., Lv, P., Versluis, M., Kuerten, J.G.M., Zhang, X., & Lohse, D. Evaporating pure, binary and ternary droplets: thermal effects and axial symmetry breaking. Journal of Fluid Mechanics. 823, 470-497 (2017)
  * [60] Diddens, C. Detailed finite element method modeling of evaporating multi-component droplets. Journal of Computational Physics. 340, 67-0687 (2017)
  * [61] Leal, L.G. Advanced transport phenomena: fluid mechanics and convective transport processes. (Cambridge University Press, 2007)
  * [62] Maass, C., Krüger, C., Herminghaus, S. & Bahr, C. Swimming droplets. Annual Review Of Condensed Matter Physics. 7, 171-193 (2016)
  * [63] Tan, H., Banerjee, A., Shi, N., Tang, X., Abdel-Fattah, A. & Squires, T. A two-step strategy for delivering particles to targets hidden within microfabricated porous media. Science Advances. 7(33), eabh0638 (2021)
  * [64] Zeng, B., Chong, K., Wang, Y., Diddens, C., Li, X., Detert, M., Zandvliet, H. & Lohse, D. Periodic bouncing of a plasmonic bubble in a binary liquid by competing solutal and thermal Marangoni forces. Proceedings Of The National Academy Of Sciences. 118(23), e2103215118 (2021)
  * [65] Binks, B.P. & Lumsdon, S.O. Influence of particle wettability on the type and stability of surfactant-free emulsions. Langmuir. 16(23), 8622-8631 (2000)
  * [66] Goubault, C., Iglicki, D., Swain, R.A., McVey, B.F.P., Lefeuvre, B., Rault, L., Nayral, C., Delpech, F., Kahn, M.L. Chevance, S. & Gaffer F. Effect of nanoparticles on spontaneous Ouzo emulsification. Journal of Colloid and Interface Science. 603, 572-581 (2021)
  * [67] Min, Y.,Akbulut, M., Kristiansen, K., Golan, Y. & Israelachvili, J. The role of interparticle and external forces in nanoparticle assembly. Nature materials. 7(7), 527-538 (2008)
  * [68] Almohammadi, H., Fu, Y. & Mezzenga, R. Evaporation-driven liquid–liquid crystalline phase separation in droplets of anisotropic colloids. ACS Nano. 17(3), 3098-3106 (2023) **Investigating the evaporation-triggered liquid-liquid crystalline phase separation in drops of amyloid fibril-water suspension.
  * [69] Dyett, B. & Zhang, X. Accelerated formation of H2 nanobubbles from a surface nanodroplet reaction. ACS Nano. 14(9), 10944-10953 (2020)
  * [70] Rastogi, V., Melle, S., Calderón, O., García, A., Marquez, M. & Velev, O. Synthesis of light-diffracting assemblies from microspheres and nanoparticles in droplets on a superhydrophobic surface. Advanced Materials. 20(22), 4263-4268 (2008)
  * [71] Marín, Á., Gelderblom, H., Susarrey-Arce, A., Houselt, A., Lefferts, L., Gardeniers, J., Lohse, D. & Snoeijer, J. Building microscopic soccer balls with evaporating colloidal fakir drops. Proceedings Of The National Academy Of Sciences. 109(41), 16455-16458 (2012)
  * [72] Wooh, S., Huesmann, H., Tahir, M., Paven, M., Wichmann, K., Vollmer, D., Tremel, W., Papadopoulos, P. & Butt, H. Synthesis of mesoporous supraparticles on superamphiphobic surfaces. Advanced Materials. 27(45), 7338-7343 (2015)
  * [73] Vella, D. & Mahadevan, L. The “cheerios effect”. American Journal Of Physics. 73(9), 817-825 (2005)
  * [74] Millman, J., Bhatt, K., Prevo, B. & Velev, O. Anisotropic particle synthesis in dielectrophoretically controlled microdroplet reactors. Nature Materials. 4(1), 98-102 (2005)
  * [75] Jiao, L., Tong, J., Wu, Y., Hu, Y., Wu, H., Li, D. & Chen, R. Self-assembly of supraparticles on a lubricated-superamphiphobic patterned surface. Applied Surface Science. 576, 151684 (2022)
  * [76] Qian, J., Yamada, D., Wei, Z., Yukisada, R., Tagawa, Y., Shaw, J. & Zhang, X. One-step nanoextraction and ultrafast microanalysis based on nanodroplet formation in an evaporating ternary liquid microfilm. Advanced Materials Technologies. 5(2), 1900740 (2020)
  * [77] Perumal, J., Wang, Y., Attia, A., Dinish, U. & Olivo, M. Towards a point-of-care SERS sensor for biomedical and agri-food analysis applications: A review of recent advancements. Nanoscale. 13(2), 553-580 (2021)
  * [78] Huang, Y., Liu, W., Gong, Z., Wu, W., Fan, M., Wang, D. & Brolo, A. Detection of buried explosives using a surface-enhanced Raman scattering (SERS) substrate tailored for miniaturized spectrometers. ACS Sensors. 5(9), 2933-2939 (2020)
  * [79] Bharati, M. & Soma, V. Flexible SERS substrates for hazardous materials detection: recent advances. Opto-Electronic Advances. 4(11), 210048 (2021)
  * [80] Li, S., Cui, Q., Wang, C., Zhang, M., Zheng, L. & Chen, M. Laser-induced assembly of Au nano-polyhedron clusters as stable 3D superstructures with ultrabroadband plasmonic resonance for promoting multi-band SERS. Optical Materials Express. 12(12), 4643–4659 (2022)
  * [81] Liu, W., Kappl, M. & Butt, H. Tuning the porosity of supraparticles. ACS Nano. 13(12), 13949-13956 (2019)
  * [82] Liu, W., Kappl, M., Steffen, W. & Butt, H. Controlling supraparticle shape and structure by tuning colloidal interactions. Journal Of Colloid And Interface Science. 607, 1661-1670 (2022)
  * [83] Spasowka, E. Chemistry of functional materials surfaces and interfaces: fundamentals and applications. Polimery. 66(10), 555-556 (2021)
  * [84] Cui, L., Zhang, X., Wang, J. & Jiang, L. Functional colloidal assemblies based on superwettable Substrates. Particle & Particle Systems Characterization. 39(1), 2100196 (2022)
  * [85] Banerjee, A., Tan, H. & Squires, T.M. Drop-in additives for suspension manipulation: Colloidal motion induced by sedimenting soluto-inertial beacons. Physical Review Fluids. 5(7), 073701 (2020)
  * [86] Shim, S. Diffusiophoresis, diffusioosmosis, and microfluidics: surface-flow-driven phenomena in the presence of flow. Chemical Reviews. 122(7), 6986–7009 (2022)
  * [87] Shah, P.R., Tan, H., Taylor, D., Tang, X., Shi, N., Mashat, A., Abdel-Fattah, A. & Squires, T.M. emperature dependence of diffusiophoresis via a novel microfluidic approach. Lab on a Chip. 22(10), 1980–1988 (2022)
  * [88] Li, Y., Pahlavan, A., Chen, Y., Liu, S., Li, Y., Stone, H. & Granick, S. Oil-on-water droplets faceted and stabilized by vortex halos in the subphase. Proceedings Of The National Academy Of Sciences. 120(4), e2214657120 (2023)
  * [89] Li, Y., Lv, P., Diddens, C. & Lohse, D. Multimodal enzyme-carrying suprastructures for rapid and sensitive biocatalytic cascade reactions. Journal of Fluid Mechanics. 946, A37 (2022)
  * [90] Diddens, C., Tan, H., Lv, P., Versluis, M., Kuerten, J., Zhang, X. & Lohse, D. Evaporating pure, binary and ternary droplets: thermal effects and axial symmetry breaking. Journal Of Fluid Mechanics. 823, 470-497 (2017)
  * [91] Zeng, H., Wakata, Y., Chao, X., Li, M. & Sun, C. On evaporation dynamics of an acoustically levitated multicomponent droplet: Evaporation-triggered phase transition and freezing. Journal of Colloid and Interface Science. 648, 736-744 (2023)
  * [92] Manikantan, H. & Squires, T. Surfactant dynamics: hidden variables controlling fluid flows. Journal Of Fluid Mechanics. 892, P1 (2020)
  * [93] Feng, J., Qiu, Y., Jiang, L. & Wu, Y. Long-range-ordered assembly of micro-/nanostructures at superwetting interfaces. Advanced Materials. 34(9), 2106857 (2022)
  * [94] Bindgen, S., Allard, J., Koos, E. The behavior of capillary suspensions at diverse length scales: From single capillary bridges to bulk. Current Opinion in Colloid & Interface Science. 58, 101557 (2022)
  * [95] Mathijssen, A.J.T.M., Lisicki, M., Prakash, V.N. & Mossige, E.J.L. Culinary fluid mechanics and other currents in food science. Reviews of Modern Physics. 95(2), 025004 (2023)
  * [96] Wei, Z., Dabodiya, Tulsi S., Chen, J., Lu, Q., Qian, J., Meng, J., Zeng, H., Qian, H. & Zhang, X. In-situ fabrication of metal oxide nanocaps based on biphasic reactions with surface nanodroplets. Journal of colloid and interface science. 608, 2235–2245 (2022)
  * [97] Neto, B.A.D., Rocha, R.O. & Lapis, A.A.M. What do we know about the ionic liquid effect in catalyzed multicomponent reactions?: A critical review. Current Opinion in Green and Sustainable Chemistry. 35, 100608 (2022)
  * [98] Cao, Y., Tian, J., Lin, H., Li, Q., Xiao, Y., Cui, H. & Shum, H. Partitioning-induced isolation of analyte and analysis via multiscaled aqueous two-phase system. Analytical Chemistry. 95(10), 4644-4652 (2023)
  * [99] Wu, H., Kanike, C., Atta, A. & Zhang, X. Nanoextraction based on surface nanodroplets for chemical preconcentration and determination. Biomicrofluidics. 16(5), 051502 (2022)
  * [100] Bisoyi, H. & Li, Q. Light-driven liquid crystalline materials: from photo-induced phase transitions and property modulations to applications. Chemical Reviews. 116(24), 15089-15166 (2016)
  * [101] Abbasi Moud, A. Chiral liquid crystalline properties of cellulose nanocrystals: fundamentals and applications. ACS omega. 7(35), 30673–30699 (2022)