Active matter

Researchers: Dr. Mihail N. Popescu (contact person)
Dr. Zahra Eskandari
Dr. Paolo Malgaretti
Dr. Shang-Yik Reigh
Dr. William E. Uspal
B.Sc. Ruben Pöhnl (M.Sc. student)
Collaborators: Prof. Alvaro Dominguez (Univ. of Seville)
Prof. Gleb Oshanin (CNRS, UMR 7600, Paris)
Prof. Samuel Sanchez (IBEC Barcelona)
Dr. Juliane Simmchen (TU Dresden)
Dr. Mykola Tasinkevych (Northwestern University, US)

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.:

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 hovering.

[ W.E. Uspal, M.N. Popescu, S. Dietrich, and M. Tasinkevych, Self- propulsion of a catalytically active particle near a planar wall: from reflection to sliding and hovering, Soft Matter 11, 434 (2015) ]
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.

[ J. Simmchen, J. Katuri, W.E. Uspal, M.N. Popescu, M. Tasinkevych and S. Sánchez, Topographical pathways guide chemical microswimmers, Nat. Comm. 7, 10598 (2016) ]
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 ("trapping") and of "chemically-confined" directional motion from the interplay between self-diffusiophoresis and induced osmotic flows has been documented.

[ W.E. Uspal, M.N. Popescu, S. Dietrich, and M. Tasinkevych, Guiding catalytically active particles with chemically patterned surfaces, Phys. Rev. Lett. 117, 048002 (2016) ; M.N. Popescu, W.E. Uspal, and S. Dietrich, Chemically active colloids near osmotic-responsive walls with surface-chemistry gradients, J. Phys.: Condens. Matter 29, 134001 (2017) ]
"Rheotaxis" denotes the alignment of swimming direction along that of an external 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 rheotaxis may emerge from the interplay between wall- confined activity and hydrodynamics.

[ W.E. Uspal, M.N. Popescu, S. Dietrich, and M. Tasinkevych, Rheotaxis of spherical active particles near a planar wall, Soft Matter 11, 6613 (2015) ]
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 type.

[ P. Malgaretti and H. Stark, Model microswimmers in channels with varying cross section, J. Chem. Phys 146, 174901 (2017) ]
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.

[ A. Dominguez, P. Malgaretti, M.N. Popescu, and S. Dietrich, Effective interaction between active colloids and fluid interfaces induced by Marangoni flows, Phys. Rev. Lett. 116, 078301 (2016) ]
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 capillary interactions. 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 attraction.

[ A. Domínguez, P. Malgaretti, M.N. Popescu, and S. Dietrich, Collective dynamics of chemically active particles trapped at a fluid interface, Soft Matter 12, 8398 (2016) ]
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.

[ P. Malgaretti, M.N. Popescu, and S. Dietrich, Active colloids at fluid interfaces, Soft Matter 12, 4007 (2016) ]
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.

[ X. Ma et al, Reversed Janus Micro/Nano-Motors with Internal Chemical Engine, ACS Nano 10, 8751 (2016) ]