Dr. Mykola Tasinkevych (CFTC, Univ. of Lisbon, Portugal)
Asst. Prof. William E. Uspal (Univ. of Hawai'i)
An important class of active matter systems is that of chemically active colloids.
These particles can achieve self-propulsion via the catalytic promotion of chemical
reactions in the surrounding solution on their surface. Such systems serve as benchmark
examples for non-equilibrium processes, while from an application viewpoint they are
envisioned, e.g., to act as "carriers" in novel lab-on-a-chip devices.
Our efforts are aimed at understanding the complex behavior exhibited by chemically active
colloids when they operate nearby confining walls or soft-interfaces, or in the presence
of external flows and fields, e.g.:
Active particles near or at interfaces
The self-generated hydrodynamic and chemical fields, which induce
particle motion, probe and are modified by that very environment,
including confining hard walls. These changes couple back to the
particle motion via the phoretic slip on the particle surface and lead to
the emergence of "wall-bounded" steady-states such as sliding and
If a chemically (or thermally) active particle is close to a liquid-fluid
interface, the inhomogeneous distribution of reactant and product molecules
(or temperature) at the interface can induce local variations of the
surface tension. This gives rise to interfacial stresses and hence leads
to the onset of hydrodynamic flow (the so-called Marangoni flows). We
have shown that such flows propagate in the bulk and drive the particle
close to (or far away from) the interface. This effective interaction is
long ranged and may provide an alternative mechanism to control particle
accumulation at fluid-fluid interfaces.
We derive a mean-field model for the dynamics of the large-scale spatial
distribution of a monolayer of spherically symmetric active particles
trapped at a fluid-fluid interface.
The model accounts for direct pair interactions as well as hydrodynamic
interactions (including the Marangoni flow induced by the response of the
interface to the chemical activity).
A situation of particular interest occurs if the pair interaction includes
The model predicts that in typical experiments (i.e., the activity of the
particles leads to a decrease of surface tension) the activity-induced
Marangoni flow can prevent the clustering instability driven by capillary
A monolayer of spherically symmetric active particles, located at a fluid-fluid interface and interacting via a "soft" repulsive potential, exhibits collective dynamics driven by the long-ranged Marangoni flows due to the response of the interface to the activity of the particles. Using a mean-field model, we demonstrate that, in spite of the intrinsic out-of-equilibrium character of the system, the monolayer evolves to a "pseudoequilibrium" state in which the Marangoni flows force the coexistence of the thermodynamic phases associated to the direct interaction in a radially stratified, "onion-like" structure within the monolayer.
The conditions, under which an active, spherical Janus colloid trapped
at a liquid-fluid interface can translate along the interface, have been
established. The corresponding persistence length has been estimated and
it has been shown that for the particle trapped at the interface the
persistence length can be significantly larger than the corresponding
one in the bulk liquid, as one can infer from recent experimental results
reported by the group of A. Stocco at the Laboratoire Charles Coulomb,
University of Montpellier.
When active swimmers are confined in varying-section channels the hydrodynamic
interactions with the channel walls can induce novel dynamical regimes. We derive
analytic expressions for the lateral probability distribution of a dilute suspension
of swimmers cofined inside channels fo varying cross-section. Our results show that
the accumulation of microswimmers at channel walls is sensitive to both the
underlying swimming mechanism and the geometry of the channels. Finally, for
asymmetric channel corrugations, our model predicts a rectification of microswimmers
along the channel, the strength and direction of which strongly depends on the swimmer
In the presence of a chemically active colloidal particle, the motion
of an inert particle by diffusiophoresis is considered and the
chemical and hydrodynamic interactions between them are investigated
by continuum theory and particle-based simulations. The flow generated
by the inert particle drags the reactive particle toward it, which
leads to a self-assembled dimer that is able to self-propel itself.
For walls which are topographically patterned, novel states of "guided"
motion along the edges of the patterns emerge when the parameters of the
particles are such that a sliding state occurs in the absence of the
patterns. Such "topography-guided" states emerge from a complex interplay
between chemical activity of the particle, hydrodynamic interactions, and
the confinement of the chemical and hydrodynamic fields by the topography.
These states have been studied in collaboration with the "Lab-in-a-tube
and Nanorobotic Biosensors" group (Prof. S. Sánchez) at the MPI-IS.
Chemically active colloidal particles in the vicinity of a hard planar wall,
with which the chemical produced by the particle interacts, are also exposed
to osmotic flows driven by the phoretic slip induced at the wall. The effects
of such flows, if the wall is chemically patterned, have been investigated.
The emergence of spatially-localized-particle steady-states ("docking") and
of "chemically-confined" directional motion from the interplay between
self-diffusiophoresis and induced osmotic flows has been documented, and the
dependence of the dynamics on the shape of the particle has been explored.
"Rheotaxis" denotes the spontaneous polarization of the orientation of
a microswimmer in the presence of ambient flow.
For spherical objects the shape-symmetry rules out the "weather vane"
mechanism of rheotaxis (which is operational for elongated swimmers).
We have shown that for chemically active spherical particles in shear
flow near a planar surface a novel mechanism of upstream rheotaxis may
emerge from the interplay between wall-confined activity and
hydrodynamics. Furthermore, the additional phenomenology of
cross-stream rheotaxis has been evidenced and studied both
and experimentally in collaboration with the group "Smart
nano-bio-devices" (Prof. S. SÃ¡nchez) at the IBEC Barcelona and with
Dr. J. Simmchen at TU Dresden.
In collaboration with the group Micro, Nano, and Molecular Systems
(Prof. P. Fischer) at the MPI-IS and with Dr. L. Wilson at the Univ.
of York the self-propelled motion of photo-chemically active
titania/silica particles, which are bottom heavy and posses self-shadowing
properties, has been studied. When the particle are immersed in
peroxide solutions and UV illuminated, at low light intensities they
exhibit wall-bound states of motion while upon sufficiently increasing
the intensity of light they lift off from the wall and swim against
gravity and away from the light source. The phenomenology is captured
by a theoretical model within the framework of self-diffusiophoresis
which explicitely accounts for the shadwoing effect on the photochemical
A new design of active colloids, consisting of a hollow, spherical, porous shell decorated on the inside by a
catalyst, was developed by the group "Smart nano-bio-devices" (Prof. S. Sánchez) at the MPI-IS.
The motility of these particles with "reversed" Janus structure (the catalyst is inside the cavity) was modeled
in terms of a self-phoretic mechanism.
In agreement with the experimental observations, the model predicts phoretic motion towards the non-metallic
side, where the exterior solution is rich in product molecules.
This direction of motion presents an unexpected contrast with the case in which the catalyst is on the outside,
for which motion is directed away from high product concentrations.
The motion of a spherical particle with arbitrary catalytic patches and non-uniform
chemical activity is investigated. The model may explain a well-defined circular motion
observed in recent experiments. The resulting trajectories are in general helical, and
their pitch and radius can be controlled by adjusting the angle between the translational
and angular velocity.