# Light-switchable propulsion of active particles with reversible interactions

Hanumantha Rao Vutukuri,1,∗ Maciej Lisicki,2 Eric Lauga,3 Jan Vermant1,∗

1Soft Materials, Department of Materials, ETH Zürich, 8093 Zürich, Switzerland
2Faculty of Physics, University of Warsaw, 02-093 Warsaw, Poland
3Department of Applied Mathematics and Theoretical Physics
University of Cambridge, CB3 0WA Cambridge, United Kingdom

∗To whom correspondence should be addressed:
E-mail<EMAIL_ADDRESS><EMAIL_ADDRESS>
> Active systems such as microorganisms and self-propelled particles show a
> plethora of collective phenomena, including swarming, clustering, and phase
> separation. Control over the propulsion direction and switchability of the
> interactions between the individual self-propelled units may open new
> avenues in designing of materials from within. Here, we present a self-
> propelled particle system, consisting of half-gold coated titania
> ($\mathrm{TiO}_{2}$) particles, in which we can fast and on-demand reverse
> the propulsion direction, by exploiting the different photocatalytic
> activities on both sides. We demonstrate that the reversal in propulsion
> direction changes the nature of the hydrodynamic interaction from attractive
> to repulsive and can drive the particle assemblies to undergo both fusion
> and fission transitions. Moreover, we show these active colloids can act as
> nucleation sites, and switch rapidly the interactions between active and
> passive particles, leading to reconfigurable assembly and disassembly. Our
> experiments are qualitatively described by a minimal hydrodynamic model.

Designing novel artificial microswimmers or self-propelled particle systems is
currently a subject of vast interest in active matter for a variety of
reasons. First, they provide us with model systems to study the collective
behaviour of their more complex natural counterparts [1, 2, 3, 4]. Second,
they display a variety of non-equilibrium phenomena, such as active
clustering, segregation, and anomalous density fluctuations [1, 2, 3, 4].
Several experimental studies have been reported on artificial microswimmers,
invoking and using different swimming strategies [4, 1, 2, 3, 5, 6, 7, 8, 9,
10]. One class consists of internally driven systems of which Janus particles
are an example, inducing motion by converting chemical energy on one side of
the particle from its local environment [1, 2, 3, 11, 4]. In these synthetic
systems, rotational diffusion of the particle randomizes the propulsion
direction, but its directionality i.e., the velocity vector with respect to
the metal-coated hemisphere, remains constant [11, 4].

On the other hand, the position and/or the orientation of driven particles can
be controlled using external stimuli such as magnetic, and electric fields [7,
12, 6]. However, unavoidable field-induced (magnetic/electric) long-range
dipole-dipole interactions between the particles by external fields [13, 14,
15] usually preclude studies of collective behavior controlling solely the
activity [12]. We are only aware of recent proof-of-principle studies that
considered a single internally-driven particle achieving propulsion direction
reversal using a wettability contrast on both sides of Janus particles at
different temperatures [8, 16], but as these are limited by heat transfer
rates, they are inherently slow. To study the effect on collective behaviour,
the dynamics of switching need to be faster than the intrinsic timescale of
the systems, e.g., rotational diffusion. In nature, some microorganisms do
indeed display rather fast periodic reversals in the direction of motion,
examples including Myxococcus xanthus [17], Pseudoalteromonas haloplanktis
[18], and Vibrio alginolyticus [19], using complex molecular mechanisms.
Interestingly, these marine microorganisms exhibit a rich collective
behaviour, examples including accordion wave patterns [20], and high-
performance chemotaxis [21].

Controlling and switching the nature of interactions (e.g., attractive to
repulsive, and vice-versa) between individual units without changing the
chemistry of starting building blocks is a major challenge in soft condensed
matter [22]. Yuan et al. [22] have recently achieved this by exploiting the
difference in the elastic response of a complex liquid crystal matrix to
different light modulations. In this work, we use a simple approach to design
novel synthetic active particles capable of performing fast propulsion
direction reversals in combination with the switchable nature of interactions
by simply changing the wavelength of light. In order to achieve such a model
system, we take advantage of the different photocatalytic activity of a single
Janus particle, specifically a half-gold coated anatase $\mathrm{TiO}_{2}$
particle, under distinct wavelengths of light. Control over the surface
chemistry enables a rapid reversal of propulsion direction. An advantage of
our nearly two-dimensional (2D) system is the ease of imaging which makes it
ideally suited to address the question: how does the reversal of propulsion
direction of the individual propelling unit affect the collective behaviour?
As an example we exploit these particles to drive the passive particles into
dynamic and reversible 2D assemblies via light-switchable propulsion, which
changes the interaction between the active and passive particles from
attractive to repulsive. The photocatalytic decomposition of hydrogen peroxide
produces concentration gradients which depend on which side is being
activated. As the particles are force-free, the phoretic forces resulting from
the concentration imbalance around the particles, need to be compensated by a
directed motion subject to fluid drag. However, as the gold cap is heavier
than the other side, this biases the quiescent orientation of the cap relative
to the wall. As a consequence, the symmetry of the resulting disturbance
hydrodynamic flow fields of the activated particles depends on the cap
orientation and regulates interactions between active and passive particles.
As a proof of principle, we use additional external stimuli to control the cap
orientation even more, to show that the interaction between active and passive
can be switched between attractive to repulsive and vice-versa by changing the
cap orientation. To elucidate the mechanism further, we study phoretic flow
fields created by active particles attached to a wall with the cap orientation
being fixed. These experiments are also used to better understand the flow
fields around individual particles. For dilute suspensions of active
particles, even when mixed with passive ones, the complex phoretic effects can
be captured well using simple hydrodynamic scaling models. The use of low
concentrations of directionally controlled and switchable particles in the sea
of passive ones, enables therefore new routes to design materials, sculpting
them from within.

## Results

#### Light-switchable propulsion direction reversal.

We use a Janus particle with different photocatalysts on each side, activated
simply by changing the wavelength and the intensity of light to control over
the propulsion direction and strength. Our experimental system consists of
half-gold coated anatase $\mathrm{TiO}_{2}$ particles with a diameter of 3.5
$\mu$m (see scanning electron microscopy image, Figure 1a, inset) in a
$\mathrm{H}_{2}\mathrm{O}_{2}$ solution. These Janus particles are heavier
than the dispersing medium, thus they sediment to the bottom of the
observation chamber. The gravitational length, i.e., the height to which the
particles can be pushed up by thermal fluctuations, is only 30 nm, which makes
the system essentially two dimensional. Under bright-field illumination the
particles undergo Brownian motion near a wall, commensurate with their size
and the viscosity of the solution (Figure 1a-c). When UV light ($\lambda\sim
370-385\ \mathrm{nm}$) is sent through the microscope objective lens, the
particles start propelling in the direction of the $\mathrm{TiO}_{2}$ side,
the bright side of the particle as shown in Figure 1d-f. For the particle
shown in Figure 1d-f, this implies motion to the right as indicated by the red
trajectory. At this particular fuel concentration (12 vol%
$\mathrm{H}_{2}\mathrm{O}_{2}$ solution) and light intensity (3.5 mW cm-2),
the particles are propelled with a speed of 16-18 $\mu$m s-1. Conversely, when
light is switched from UV to green light (3.5 mW cm-2, $\lambda\sim$ 535-565
$\mathrm{nm}$) the particle instantaneously changes its propulsion direction
and moves in the opposite direction (the dark side of the particle), however,
with lower speeds of 4–5 $\mu$m s-1, as shown in Figure 1g-i. We note that the
swimming speed of the particles that is triggered by the UV light is 3-4 times
higher than that of green light illumination (Figure 1d-i). This can be
attributed to the fact that the anatase $\mathrm{TiO}_{2}$ particles
(Supplementary Figure 1 confirms the XRD spectrum for the anatase phase) are
photocatalytically more efficient under UV than green light [23]. We note that
a high intensity mercury white light source (25 mW cm-2) also induces the same
propulsion speed and direction as the green light.

In order to quantify the propulsion strength, we measured particle
trajectories by extracting the particles centroid from the time-lapsed images
using a particle-tracking algorithm [24] (see Supplementary Note3). The
overlaid time-lapse bright field image sequences show the typical trajectories
and the direction of propulsion. For clarity, we overlay trajectories with
different colours to distinguish each different mode of propulsion: dark gold
for Brownian motion (Figure 1a-c), red for the forward motion driven by the UV
illumination (Figure 1d-f), and green for the backward motion triggered by the
green light (Figure 1g-i). Additionally, the light-switchable propulsion
directional reversal can be repeatedly achieved as many times as needed (see
Supplementary Movie1) as long as the sufficient fuel is present. To the best
of our knowledge, we are not aware of any internally driven synthetic
experimental system with such an exquisite control over the propulsion
directional reversal and strength controlled by the intensity of light (See
Supplementary Figure 2). Strikingly, the trajectories of the particles (see
Supplementary Movie1) resemble the motion of run-and-reverse behaviour of some
marine microorganisms [17, 18, 19].

The exact description of the photocatalytic decomposition reactions [25, 26,
27] and their underlying mechanisms involved in the particle’s propulsion
direction reversal are chemically complex in nature, but a qualitative
explanation is provided. Under the UV irradiation, the decomposition of
$\mathrm{H}_{2}\mathrm{O}_{2}$ is triggered by the photoinduced electron-hole
pair generation[28, 23] on the surface of anatase $\mathrm{TiO}_{2}$. The
resulting holes oxidise the $\mathrm{H}_{2}\mathrm{O}_{2}$ thereby creating
the protons on the $\mathrm{TiO}_{2}$ side of the Janus particle. The
generated protons are subsequently involved in the reduction reaction at the
Au-coated side of the particle (see Supplementary Figure 3). The flux of freed
protons generates a fluid flow towards the Au hemisphere that induces the
self-electrophoretic motion on the other side of particle, i.e., along the
$\mathrm{TiO}_{2}$ side which is the bright side of the particle (Figure
1e-h). Under green or high intensity visible light illumination, the Au-coated
side of the particle is involved in the decomposition reaction, which in
combination with the weak photocatalytic nature of anatase $\mathrm{TiO}_{2}$
at this wavelength results in an opposite concentration gradient (see
Supplementary Figure 3) that induces a self-diffusiophoretic motion in the
direction of the Au hemisphere (see Supplementary Figure 3). It has been
reported that Au-coated Janus particles (e.g., Au-coated Janus silica, and Au-
coated Janus polystyrene) show self-propulsion behaviour induced by self-
thermophoresis [29] under green light illumination. However, this occurs in
the direction of the un-coated side of the particle, which is the opposite of
what we observe here. In order to verify our hypothesis whether gold can
decompose $\mathrm{H}_{2}\mathrm{O}_{2}$ or not, we repeated the experiments
with half Au-coated silica as well as Au-coated polystyrene particles and
maintain the similar experimental conditions. We measured the propulsion speed
(2-3 $\mu$m s-1 in 15 vol% $\mathrm{H}_{2}\mathrm{O}_{2}$ solution) in the
direction of the un-coated side, which is consistent with a previous study
[29]. However, our self-propelled particles show self-propulsion in the
opposite direction (see Figure 1g-i) suggesting that the photocatalytic
decomposition of $\mathrm{H}_{2}\mathrm{O}_{2}$ on the surface of anatase
$\mathrm{TiO}_{2}$ is the dominant contribution. For what follows it suffices
to note that the spatial dependency of the concentration fields and the
subsequent motions are controlled by the wavelength of the light used.

#### Collective motion - Fusion and fission.

To date, mainly simulation and theoretical studies have predicted that
motility alone is sufficient to drive dynamic clustering followed by a phase
separation in active systems [1, 2, 3, 4, 30]. However, most of the
experimentally observed clustering and phase separation in phoretic systems
results from a combination of motility and the presence of short-range
attractions that are mediated by the asymmetric distribution of the chemical
fuel species around the Janus particle[1, 2, 3, 4, 30, 31, 32, 33]. In the
present experimental system, because of the presence of a wall, the
disturbance velocity flow fields generated by the phoretic effects are
asymmetric and directional. The resulting hydrodynamic interactions strongly
affect the collective dynamics. In this regard, we increased the particle
concentration to an area fraction $\phi_{a}=Na_{p}/A\approx 0.06$, $N$ being
the total number of particles, $A$ the surface area of the system, and $a_{p}$
the surface area of a single particle. Upon illumination with green light, the
particles spontaneously start clustering and show long-ranged attractions
between the particles. As a consequence, small clusters (2-4 particles) of
nuclei are formed in different locations, which then evolve into a few medium-
sized (8-10 particles) clusters by coalescence of the smaller ones. Next,
these medium-sized clusters further combine into a single defect-free large
cluster at the expense of all available small clusters of particles as shown
in Figure 2a-f. This phenomenon resembles fusion, leading to crystallisation
of the particles (see Supplementary Movie 2). We measure the inter-particle
distance as a function of time to quantify how fast the particles are approach
each other and form clusters. The inter-particle approach velocity shows a
long-range spatial dependency ($1/r^{1.25}$) (Supplementary Figure 4) and
these long-range attractions result from the solute-mediated diffusiophoresis
[25, 34, 26].

To probe the dynamic response of the particle’s propulsion reversal, we switch
from green to UV illumination, which cause the cluster to explode and break up
into an array of small clusters as well as individual particles that swim
outwards as shown in Figure 2g-l. The whole process of clustering (Figure
2a-f) and exploding (Figure 2g-l) resembles fusion and fission phenomena (see
Supplementary Movie 2), which occur over a wide-range of length scales
spanning from subnuclear [35] to colloidal [36] to cells [37] to macroscopic
scales [38]. During the disassembly of the cluster we observe two distinct
time-dependent regimes. Immediately after switching the propulsion direction,
the particles at the edge of the cluster that have an outward propulsion
direction escape first and move ballistically with velocities as high as
$\sim$ 40-50 $\mu$m s-1. Meanwhile, particles at the centre of the crystal are
unable to escape because they block each other’s path.

To elucidate the role of the cap orientation of the individual particles on
the observed fusion-fission behaviour, we analyse images of projected
orientations of the caps within a cluster (Figure 2m-t). To this end, we
exploit the intensity difference between the two sides of the particle to
identify the cap orinetation. The bright side is the $\mathrm{TiO}_{2}$
hemisphere, while the dark side is the half gold-coated cap. Figure 2m-p shows
the majority of the particles swim inward along the periphery of the cluster,
while the particles with outward propulsion direction leave the cluster and
swim away (right top-corner in Figure 2m-p). When the propulsion direction of
the individual particles is reversed by switching light from UV to green
light, the cluster subsequently melts and all particles move away (Figure
2q-t).

The fission process can be easily understood by realising that a swimmer
surrounded by neighbouring ones in a fused cluster is unable to propel in the
outward direction as it is trapped by neighbours (see Figure 2m-p). Therefore,
the particle is continually exerting a propulsion stress on neighbouring
swimmers which generates a force imbalance from the centre to the outward
layer of the cluster due to different orientations of the swimmers, which
leads to an explosive disintegration. This observed fission process indicates
that upon switching to UV light the velocity field around each particle is
repulsive (Figure 2g-k). The presence of these extra active stresses has been
modelled as a swim pressure for situations where this stress is isotropic
[39]. During the disintegration process some clusters having anisotropic
shapes show a pronounced rotational motion (see Supplementary Movie 2). Due to
the distribution of cap orientations of particles within a cluster (Figure
2m-p), a few small clusters live longer than the other ones. We expect that
the clusters would lose their identity when the propulsion direction of all
the particles is directed outward. When we reverse the sequence of light
irradiation (from UV to green light), we observe similar trends, but the
explosion of clustered particles is not as strong as in the case of vice-versa
illumination (Figure 2g-k). This can be attributed to fact that the propulsion
force is 3 times higher under UV light compared to the green light, as can be
inferred from the propulsion velocity of individual particles.

To better understand the underlying mechanism responsible for the repulsion of
active particles under UV illumination, we repeated the experiments with the
isotropic anatase $\mathrm{TiO}_{2}$ particles while maintaining same
intensity (3.5 mW cm-2) of light and the fuel concentration. Previous
theoretical studies have predicted the hydrodynamically-mediated
diffusiophoretic attraction and repulsion between the pairs of isotropic
particles would lead to clustering and self-propulsion of their assemblies
[40]. In our experiments, when a cluster of isotropic $\mathrm{TiO}_{2}$
particles is exposed to UV illumination, surprisingly, particles start to move
outward radially. The over-laid trajectories depict the motion of isotropic
particles in 12 vol% of $\mathrm{H}_{2}\mathrm{O}_{2}$ solution as shown in
Figure 3a-h. In our experiments, the average velocity of the particles versus
time shows a power-law dependence $V_{p}\propto t^{-0.69\pm 0.04}$. We can
explain this behaviour using a scaling argument by modelling the particles as
sources of the solute. The gradient of this solute along the surfaces of the
particles gives rise to a phoretic slip[41, 11, 42], which induces a flow of
the solution. In the far field limit, an isotropic sphere produces a chemical
concentration field which dies out with distance as $1/r$. If another sphere,
placed at a distance $l$, moves in this concentration field, we find that its
leading order velocity should be $V\sim l^{-2}$. Since
$V=\mathrm{d}l/\mathrm{d}t$, integrating this relation, we find $l\sim
t^{1/3}$, and finally $V\sim t^{-2/3}\approx t^{-0.67}$, close to experimental
observations. To gain further insight into the observed dynamics, we performed
numerical simulations taking into account pairwise interactions of particles
due to their induced phoretic flows [40] (see Methods section on phoretic
motion of isotropic active spheres). The simulation shows similar trajectories
(Supplementary Figure 5) when started from the initial positions taken from
the corresponding experiment. An empirical fit to the experimental data for
the velocity shows good agreement with simulations and the theoretical scaling
in Figure 3i. These experimental findings suggest that the observed repulsive
interactions between the Janus particles under UV light can be attributed to a
self-diffusiophoretic effect. To further confirm our hypothesis we
analytically calculated the solute concentration field around an isotropic
$\mathrm{TiO_{2}}$ particle in proximity to a wall, and the resulting
streamlines of the hydrodynamic flow field (see Supplementary Figure 6a and
the Supplementary Note5 for the full description of calculations). Our
calculations indeed confirmed the flow fields around the isotropic
$\mathrm{TiO_{2}}$ particle is repulsive. On the other hand, the Janus nature
of the particle greatly amplifies what effectively can be described as a
repulsive behaviour (Figure 2g-k).

#### Active and passive mixtures – Interplay between attractions and
repulsions.

Unlike previous studies on active and passive mixtures, such as light-
activated clustering of active and passive mixtures[43, 44] and micropumps
[45, 26], here we combine the ability to fast switchability of the direction
and the subsequent attraction/repulsion using different wavelengths of light
illumination and relatively fast switching, compared to the diffusive time
scale of the particles. To get more insight into the effect of propulsion
direction reversal on the (dis)assembling behaviour in the active-passive
mixtures, we first immobilised an active particle on a glass substrate and
added passive particles as tracers to capture the flow field around it (see
Figure 3j-o). This experiment creates a simpler situation where we can study
the phoretic flows around the particle in detail, and investigate how the flow
field can be described. Moreover, this provide insights for the more complex
case of freely moving particles. Bright field microscopy images show that the
particle is attached on the gold-coated side, visible as the dark side of the
particle. In the case of inwards flow created by the active particle, passive
particles organize into a hexagonal structure around the immobile active
particle under green light illumination (Figure 3j), which is consistent with
the behaviour of active and passive mixtures in Figure 4a-c. Under UV
illumination the flow direction is reversed, which causes the passive
particles to be radially pushed away (see Supplementary Movie3), as shown in
Figure 3j-o. As discussed before, in the case of a freely moving active
particle, the phoretic slip is towards the gold-coated side of the particle
under UV illumination, therefore, the direction of propulsion is towards the
$\mathrm{TiO}_{2}$ side. As the particle is attached to the wall, a repulsive
flow field is produced, which can be modelled by a near-wall point force (a
Stokeslet), in contrast to force-free swimmers which can be expected to give
rise to force dipole flow fields [46].

We use a minimal model to describe the experimentally observed repulsive flow,
a fixed particle (downward cap orientation) sets up an osmotic flow which can
be described by a Stokeslet near a wall [47] (see Supplementary Informations
for the detailed model). A comparison of the experimentally observed
velocities with a Stokeslet solution gives reasonable agreement (Figure 3p).
In contrast to earlier work involving attraction by an immobilized active
particle [31], the dynamics in our case are restricted to a 2D plane parallel
to the wall at a distance $h$. The velocity of a tracer depends on the
distance from the Stokeslet as $R/(R^{2}+4)^{5/2}$, where $R=r/h$ is the
rescaled distance [46]. We integrated numerically this relation to get the
velocity of passive tracer particles which is empirically fitted to the
experimental data (Figure 3p). Good agreement with the experimental far field
behaviour of the in-plane Stokeslet flow, decaying as $1/R^{4}$, is found.
However, in the near field the Stokeslet fails to describe the observed
behaviour as other factors come into play (e.g., steric, slip, lubrication,
and potentially electric interactions have a pronounced effect). Furthermore,
the change in illumination will only lead to the reversal of the direction of
the flow and the magnitude of the Stokeslet, not affecting its qualitative
characteristics.

#### Control over cap-orientation

For the freely moving particles, the disturbance velocity fields (force
dipoles) will be closely related to those inferred from the fixed particles.
To demonstrate further the role of cap orientation and the direction of
resulting flow fields, we control the orientation of the gold cap using an
external magnetic field (see Supplementary Figure 7). This strategy allows us
to switch the flow fields generated by active particles from repulsive to
attractive and back. Supplementary Figure 7 demonstrates that the role of the
cap orientation and the nature of the disturbance velocity fields are the key
for controlling attraction versus repulsion. We demonstrated two ways to
control the direction of the flow field from repulsive to attractive: i) for a
given wavelength, changing the cap orientation externally, and ii) for a given
cap orientation, changing the wavelength has the same effect. The latter
provides an elegant route to exploit these effects to direct reconfigurable
assembly and disassembly process in passive-active mixtures. We start with a
dilute dispersion ($\phi_{a}<0.5\%$, area fraction) of active particles in a
concentrated dispersion ($\phi_{p}\approx 12\%$) of passive silica particles
($\sigma_{p}=2.1\leavevmode\nobreak\ \mu$m). When green light is on, the
active particles spontaneously act as nucleation sites and they pull the
neighbouring passive particles isotropically towards them while moving (Figure
4a-c). As shown in Figure 4a-c, when the passive particles accumulate around
the active particle their propulsion speeds drastically decrease
($<1.0\leavevmode\nobreak\ \mu$m s-1). At this fuel concentration and light
intensity, the range of the attractions between the active and passive
particle is about 4-5 $\sigma$. We note that this clustering is not caused by
opposite charges on active/passive particles because both are negatively
charged (with zeta potentials of $\zeta_{\mathrm{active}}\approx-39$ mV, and
$\zeta_{\mathrm{passive}}\approx-48$ mV). When we switch the light
illumination to UV, the active particles instantaneously reverse their
propulsion direction and increase their propulsion speed (Figure 4d-f). The
passive particles are now pushed away by the active ones (Figure 4d-f), which
creates a depletion zone ($\sim 8-10\sigma$) around the active particle as
shown in the inset of the Figure 4f. Time-stamped trajectories depict the
motion of passive particles Figure 4g. The trajectories of passive particles
(cyan colour) illustrate the direction of the flow that is created by the
propelling particle, and the red colour shows its the propulsion path (the
inset of the Figure 4f). This observation of the pushing behaviour and the
creation of a depletion zone (see Supplementary Movie4) is reminiscent of
electroosmotic-flow-driven micropumps [25, 26]. To control the interplay
between attraction and repulsion, i.e., pulling and pushing behaviour of the
active particles in passive mixtures, we vary the intensity of the UV light
while the visible light intensity is kept constant. The attraction dominates
at low intensities ($<$50 % of $I_{max}$) and repulsive behaviour is only
observed at the high intensities. Next, we increase the surface area fraction
of passive particles ($\phi_{p}\approx 35\%$) while the active particle
concentration is kept constant ($\phi_{a}<0.5\%$). Under UV irradiation,
passive particles are compressed locally by the active particles (see
Supplementary Movie 5), thus inducing local crystallisation as shown in
Supplementary Figure 8. During the compression process the velocity of the
active particle is reduced by factor of 10, i.e., to 1.6 $\mu$m s-1. As we
demonstrate here, our self-propelled particle system can be used to stir
locally to drive the passive particles in dynamic assemblies which are relaxed
and expand after UV light is turned off.

We note that the rotational dynamics of an isolated self-propelled particle is
entirely diffusive, self-propulsion does not change it [1, 2, 3]. However, it
has been recently reported that multibody effects will have stronger effects
on rotational dynamics in the active and passive mixtures [48, 49]. This is
observed in our work for the case of attractive interactions. Owing to
attractions with surrounding passive particles, the active particle’s
rotational diffusion is slowed down (much smaller than the rotational
diffusion of passive particle, Dr $\approx$ 0.0305 sec-1) and the propulsion
direction is now dominated by the passive particle’s arrangement (assembly)
around the active particle as shown in the Supplementary Figure 9. However,
for the repulsive case we do not see any measurable crowding effects.

When we increase the concentration of active particles to match that of the
passive particles (here $\phi_{p}\approx 15\%$, $\phi_{s}\approx 12\%$), we
observe a dynamic phase separation in which the active particles cluster
together in the centre and the passive particles organize around the active
cluster resembling a core-shell like configuration under green light, as shown
in the Supplementary Figure 10. This can be attributed to the fact that flow-
mediated interactions between the active-active particles are stronger than
those of the active-passive particles. Next, these small clusters further
merge into a large cluster and then eventually reach a larger core-shell like
structure (Supplementary Figure 10a-f). Depending on the wavelength of light
illumination used, the active particles can induce either attractions or
repulsions with the passive particles thereby assembling or disassembling
around them (Figure 4).

In conclusion, we developed a facile route to produce switchable
photocatalytic swimmers which exhibit an almost instantaneous reversal of
propulsion direction between both ends of the half-gold coated anatase
$\mathrm{TiO}_{2}$ particles by light modulation. The fast optical switching
makes it possible to direct the assembly and disassembly by the mere control
of the wavelength and intensity of light. The typically observed growth of the
active clusters is a consequence of the solute mediated long-range attractions
to induce a fusion process, while the breakage process result of the
propulsion reversal coupled with the propulsion strength and the gold cap
orientation from the particle that gives some force imbalance across the
fusion cluster that can trigger the fission process. Moreover, the small
clusters surviving the fission process can be attributed to the different
orientations of the active particles. These active particles can act as seed
particles in active passive mixtures, and conveniently drive the assembly and
disassembly of passive particles by light-modulation. Additionally, we are
able to qualitatively explain the experimental results using a simple
hydrodynamic scaling. We believe that the process of forced disintegration, as
opposed to Brownian spreading, has new dynamical features which allow greater
control over the collective dynamics of phoretic active matter. Our system
paves a new way to realize fast switching between nature of interactions from
attractive to repulsive, and back enable the design of reconfigurable
materials by sculpting them from within. Moreover, the presented experimental
system opens a new avenue for developing a new theoretical framework of active
systems where the propulsion velocity becomes a vector quantity, instead of a
scalar parameter of the model.

### Methods

Fabrication of self-propelled particles. $\mathrm{TiO}_{2}$ particles were
synthesized following the procedure of He et al. [50] (see Supplementary
Note1). We used thermal sintering to convert amorphous $\mathrm{TiO}_{2}$
particles to anatase phase. To obtain half-Au coated anatase
$\mathrm{TiO}_{2}$ particles, a monolayer of particles was first deposited on
a glass slide. Gold was then sputter-coated (Safematic, CCU-010 HV Compact
Coating) over the monolayers to get a final thickness of $\sim$40 nm. Next,
particles were detached from the glass slide by sonication (Bandelin, Sonorex)
for 4-5 min and collected them by sedimentation. Finally, hybrid
$\mathrm{TiO}_{2}$-Ni-Au particles were used to control the cap orientation
using external magnetic field, in this case, a Ni layer was deposited and
sandwiched between the $\mathrm{TiO}_{2}$ and gold (see Supplementary Note2 in
more details).

Imaging and data analysis. We followed the particle dynamics using an inverted
microscope (Zeiss 200M) equipped with a high numerical aperture oil objective
lens (60x 1.25NA, 100x 1.3NA, Oil, EC Plan NeoFluar) and with a mercury lamp
(X-Cite 200 DC, Excelitas Technologies, USA), and a Nikon inverted
fluorescence microscope equipped with a CCD camera (Olympus CKX41). Light that
was sent through the microscope objective lens and the intensity of light was
uniform across the area of interest. Moreover, the light source is equipped
with a manual knob to control the intensity of light. Different bandpass
filters were used to restrict light emission to 370-385 nm (UV) and 535-565 nm
(green). With our current experimental setup, we found that the time required
to switch between two wavelengths is about 0.6 s. This is fast enough for the
current study, as the time scale of switching needed to be smaller than the
persistence time ($\approx$ 35 sec, $1/D_{r}$, rotational diffusion of the
self-propelled particle) for the self-propulsion. The intensity of light
emission was measured with a photo detector (Thorlabs, PM 100D). We acquired
images with frame rates ranging between 5-50 $fps$. The brightness and
contrast of the images were adjusted using ImageJ. The particle’s centroid was
extracted from time-lapsed images using ImageJ plugin, Trackmate [24].

Mean squared displacement. Using time-lapsed particle coordinates we measured
the mean squared displacement (MSD) thereby the propulsion velocity of the
SPPs using home-written Matlab programme. The mean squared displacement of
SPPs differs significantly from the passive particles [51]. By fitting the
mean squared displacement curve with $\langle\Delta
r^{2}\rangle=\langle[r(t+\Delta t)-r(t)]^{2}\rangle=v^{2}t^{2}+4D_{t}t$ for
time scales much smaller than the rotational diffusion of the particle [51],
we measured the propulsion speed of the particles for a known
$\mathrm{H}_{2}\mathrm{O}_{2}$ (BASF, Germany) concentration solution. A
typical MSD curve for forward and backward motion of particles is shown
Supplementary Figure 11. The forward propulsion speed of SPPs is
$v_{f}=16.0\pm 2.0\,$ $\mu$m s-1, and backward motion is $v_{b}=4.8\pm 0.6\,$
$\mu$m s-1 in 12 vol $\%$ $\mathrm{H}_{2}\mathrm{O}_{2}$ solution. We used the
high fuel concentration to achieve higher propulsion speeds with a low
intensity of light illumination (3.5 mW cm-2) to avoid any thermal heating
effects in our study.

Phoretic motion of isotropic active spheres. A system of two isotropically
active spheres has been analysed by Michelin and Lauga[52] in detail for
arbitrary separations. In the far field limit, it can be regarded as one
sphere moving in the concentration gradient created by the other. The leading
order concentration field produced by a uniformly active sphere of radius $R$
is given by

$c(r)=\frac{\mathcal{A}}{\kappa}\frac{R^{2}}{r},$ (1)

where $r$ is the distance from the sphere, $\mathcal{A}$ is the chemical
activity of the surface, and $\kappa$ is the diffusivity of the solute. For
two spheres separated by a distance $d$, each particle is exposed to two
concentration gradients: one stemming from its isotropic emission, and another
generated by its neighbor. Since the latter is asymmetric, it leads to
propulsion with a velocity $V$ which for uniform surface mobility
$\mathcal{M}$ to leading order reads

$\mathbf{V}=\frac{\mathcal{A}\mathcal{M}R^{2}}{\kappa d^{2}}\mathbf{\hat{d}},$
(2)

where $\mathbf{\hat{d}}$ is a unit vector pointing along the symmetry axis of
the system.

This calculation enables us to simulate the far-field dynamics in systems of
many spheres by superposition of pair-wise interactions. Within this
framework, in free space, the velocity of the particle $i$ is given by

$\mathbf{V}_{i}=\frac{\mathcal{A}\mathcal{M}R^{2}}{\kappa}\sum_{j\neq
i}\frac{\mathbf{r}_{j}-\mathbf{r}_{i}}{|\mathbf{r}_{j}-\mathbf{r}_{i}|^{3}}.$
(3)

This equation is the basis for our simulations of spreading of a cluster of
active particles. To this end, we consider a cluster of $N=21$ particles with
their initial positions $\\{\mathbf{r}_{i}\\}_{i=1,\ldots,N}$ taken from the
experimental configuration as in Figure 3a. The positions of the particles are
then evolved using a standard Euler scheme in Matlab. Then the average
velocity is calculated as a function of time, as depicted in Figure 3i.

For this type of interaction, a scaling argument can be made to understand
spreading of particles under repulsive interactions. Since the velocity scales
with the distance as $V\sim r^{-2}$, integration gives $r\sim t^{1/3}$, and
thus the scaling $V\sim t^{-2/3}$, which is very close to the experimentally
measured exponent (0.69$\pm$0.04), as described in the paper.

Phoretic flow due to a fixed sphere. A phoretic particle can propel itself by
generating gradients of physical quantities such as temperature, electric
charge, concentration of solute and exploiting the flow generated by these
inhomogeneous fields. The gradients are generated close to the particle
surface, thus leading to local slip flows. Similar to swimming microorganisms,
the flow signature in the far-field is that of a force dipole $\bm{v}(\bm{r})$

$\bm{v}(\bm{r})=\frac{p}{8\pi\eta
r^{2}}\left[3(\hat{\bm{r}}\cdot\hat{\bm{p}})-1\right]\hat{\bm{r}},$ (4)

where $p$ is the dipole strength, $\hat{\bm{r}}=\bm{r}/r$, and the unit vector
$\hat{\bm{p}}$ describes the orientation of the dipole. The flow field decays
with distance like $1/r^{2}$, that is faster than the point force Stokeslet
solution which decays as $1/r$. This is due to the force- and torque-free
character of motion of a swimmer in a viscous fluid. Depending on the sign of
the dipole moment $p$, we discern pushers ($p>0$) which typically generate
their propulsion from the back and push the fluid along the direction of
motion, dragging it in from the sides, and pushers which have the same flow
field streamlines with reversed direction of flow.

However, when such a particle is confined next to a surface and immobilized,
it is still creating flow and stirring the surrounding fluid. In this case,
the net thrust generated by the phoretic flow is not balanced by the viscous
drag force. Consequently, there is a net force pointing in the normal
direction. Thus the flow field around can be modelled as one generated by a
point force singularity acting close to a planar no-slip wall. The effect of
the wall is accounted for by considering image singularity solutions, and the
resulting flow field is given by the Blake tensor [47] and for the case of
normal force is sketched in Supplementary Figure 6b.

Assuming the point force to be at a distance $h$ to the wall, the flow due to
a force $F$ in the plane $z=h$ is given in terms of the rescaled distance
$r=x/h$ and dimensionless time $t$ scaled by $2\pi\eta h^{2}/3F$ as

$\frac{\mathrm{d}r}{\mathrm{d}t}=\frac{r}{(r^{2}+4)^{5/2}}.$ (5)

The flow in the direction normal to the wall is smaller in magnitude.
Henceforth we consider lateral motion of the due to a fixed particle.

In the far field, the following scaling dictates the relation: the velocity
field scales as $V\sim r^{-4}$, which is integrated to give $t\sim r^{5}$ or
$r\sim t^{1/5}$. Combining these, we obtain $V\sim r^{-4/5}$, which is slower
than the experimentally observed spreading. However, since the particles
involved have a finite size and are in a crowded environment, the far-field
effects in the flow are expected to be seen at long times. At short times, the
finite size of the tracers results in complex many-body hydrodynamic effects
and thus no simple scaling can be found. To elucidate these effects, we have
integrated Equation (5) numerically. In Supplementary Figure 12, we present a
comparison of two test trajectories of tracers starting at $r_{0}=0.1$ which
are advected in the flow due to a Stokeslet and subsequently decay as
$r^{-4}$, far-field component.

In Figure 3p we present an empirical fit of the data to the theoretical curves
obtained above. There are two fitting parameters in the problem: the strength
of the source Stokeslet and the unit of time by which we rescale to make time
dimensionless. These two factors come in when rescaling the experimental data
to compare with simulations given in dimensionless units. Here they were
chosen in such a way that the theoretical curve passes through most
experimental points.

## Data availability

The data that support the plots within this paper and other findings of this
study are available from the corresponding authors upon reasonable request.
The numerical programs that are involved in this study are available from the
corresponding authors upon reasonable request.

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Figure 1: Reversing the propulsion direction using different wavelengths of
light. a-i Time-lapsed bright field microscopy images are showing the mode of
the motion and the overlaid trajectories revealing the typical trajectories,
respectively. The half-Au coated anatase $\mathrm{TiO}_{2}$ particles are
dispersed in 12 vol% $\mathrm{H}_{2}\mathrm{O}_{2}$ solution. Note the bright
side is the $\mathrm{TiO}_{2}$ side while the dark side is the half gold-
coated cap. In the schematic, the gold colour corresponds to the gold-coated
side of the particle, whereas the light brown represents the sintered
$\mathrm{TiO}_{2}$. Color of lightning-bolt illustrates the wavelength of
light used in the experiment. a-c Brownian motion of the particle. Inset shows
a scanning electron microscopy (SEM) image of the Au-coated $\mathrm{TiO}_{2}$
Janus particle. d-f The swimmer propels in the forward direction under
illumination UV-light. g-i The swimming direction of the particle is reversed
after a switch to green light. The arrow depicts the direction of the
propulsion. The brightness and contrast of the images are adjusted using
ImageJ. Scale bar is 5.0 $\mu$m. Scale bar in the inset is 2.0 $\mu$m. Figure
2: Fusion and fission dynamics of switchable photoresponsive colloids. a-f
Fusion: Time-evolution and dynamic growth show the fusion of the active
particles into a single cluster. g-k Fission: Time-evolution of the fission
dynamics shows the big cluster under the UV illumination. This single cluster
explodes into small clusters when the direction of propulsion of the particle
is reversed. l Small clusters dissolve when the illumination switches to green
light. Colour of lightning-bolt shows the wavelength of light used in the
experiment. m-t Projected orientations of the caps reveal the self-trapping of
the particles in a cluster, and the melting of the cluster when the propulsion
direction is reversed. m-p Red colour arrow points the propulsion direction
along the $\mathrm{TiO}_{2}$ side, bright side of the particle. q-t Magenta
colour arrow depicts the propulsion direction reversal under green or high
intensity white light illumination. Scale bar is 5.0 $\mu$m. Figure 3:
Response of the isotropic $\mathrm{TiO}_{2}$ particles under the illumination
of UV-light. a-h Time-lapsed bright field micrographs show the pushing
behaviour of individual swimmers against the neighbouring particles. These
repulsions are isotropic. Overlaid trajectories reveal the dynamic response of
the particles. i The average velocity over all particles decays with time as
$V\propto t^{-0.69\pm 0.04}$ and follows the predicted far-field scaling
($t^{-0.67}$). The blue line is the prediction of the theoretical model
obtained by numerical simulations. The error bars in i represent the standard
deviation of the average velocity of over all particles. j-o Dynamic response
of the passive particles to an immobile active particle under the illumination
of UV-light. Time-lapsed overlaid trajectories show the tracer passive
particles experience isotropic repulsions and push the tracers radially. p, An
empirical fit of the experimental velocity data shows the predictions of the
theoretical model for the dynamics of tracer particles (solid line). The
experimental average velocity over the all particles decays with time and
follows the numerically-obtained flow field produced by an immobilized active
particle, modelled as a near-wall Stokeslet. Broken blue line is the far field
prediction which is expected to be valid at much longer times. The error bars
in p represent the standard deviation of the average velocity over all
particles. Scale bar is 5.0 $\mu$m. Figure 4: Switchable photoresponsive
colloidal particles in a sea of passive particles: from pulling to pushing.
a-c Assembly: Sequence of time-lapsed images show first the dynamic clustering
of passive particles around the active particle under the green light
illumination. d-f Disassembly: Sequence of time-lapsed images obtained upon
the switching of the illumination to UV light. Overlaid trajectories reveal
the outward motion of the passive particles. Inset f, overlaid trajectory
shows the active particle motion. Cyan colour corresponds to the trajectory of
the passive particles the red colour represents the active one. The black
arrow depicts the position of the active particle. Colour of the lightning-
bolt represents the wavelength of light used in the experiment, respectively.
g Time-stamped trajectories show motion of passive particles under UV light
illumination. Scale bar is 5.0 $\mu$m. Scale bar in the inset is 10.0 $\mu$m.

### Acknowledgments

H. R. V. acknowledges financial support through a Marie Skłodowska-Curie Intra
European Individual Fellowship (G. A. No. 708349- SPCOLPS) within Horizon
2020. E. L. has received funding from the European Research Council (ERC)
under the European Union’s Horizon 2020 research and innovation programme
(grant agreement 682754). We would like to thank Prof. Jan Dhont for useful
discussions, and Tian Liu is acknowledged for help in measuring XRD. We also
thank ScopeM-the microscopy center, and FIRST-the cleanroom facility at ETH
Zürich.

### Author contributions

H. R. V., conceived and designed the project. H. R. V., performed the
experimental work and analysed the data. M. L., and E. L., performed
theoretical calculations. H. R. V., M. L., E. L., and J. V., participated in
the discussions, and all authors wrote the manuscript.

### Additional information

Supplementary Information accompanies this paper at https://
Competing interest. The authors declare no competing interest.