Motion of chemically active micro-objects in confined geometries
Final Report Abstract
Micron sized particles can self-propel through liquid-filled spaces due to self-generated gradients in solute concentrations when parts of their surface catalyze a chemical reaction in a surrounding environment. Such active particles are envisioned as a key component of future advanced lab-on-achip devices providing, for example, novel methods for targeted delivery of microcargo. However, the limited understanding of their dynamics in confined geometries as well as inability to efficiently control the direction of motion without the use of external fields hinders the development of advanced microfluidic applications. Far-reaching potential applications of active particles will also require that they sense and respond to their local environment in a robust manner. Focusing on a catalytically active Janus particle as a paradigmatic example, we have investigated in this project the influence of well-controlled geometric confinement on the motion of such particles. The most important results which have been obtained are detailed below. We have demonstrated that upon approaching a hard planar wall, catalytically active Janus particles can exhibit several scenarios of motion: reflection from the wall, motion at a steady-state orientation and height above the wall, or motionless, steady hovering. Concerning the steady states, the height and the orientation are determined both by the proportion of catalyst coverage and the interactions of the solutes with the different faces of the Janus particle. Accordingly, a desired behavior can be selected by tuning these parameters via a judicious design of the particle surface chemistry. Building upon these results, and in collaboration with our experimental colleagues, we have shown that step-like submicrometre topographical features can be used as reliable docking and guiding platforms for chemically active spherical Janus colloids. For various topographic features (stripes, squares or circular posts), docking of the colloid at the feature edge is robust and reliable. Furthermore, the colloids move along the edges for significantly long times, which systematically increase with fuel concentration. We have explained the observed phenomenology qualitatively in terms of a generic continuum model of self-diffusiophoresis near confining boundaries, indicating that the chemical activity and associated hydrodynamic interactions with the nearby topography are the main physical ingredients behind the observed behavior. Employing an approximate point-particle analysis, we have demonstrated analytically that the motion of a catalytic Janus particle can be controlled via chemical patterning of a confining wall. The pattern shapes the chemi-osmotic flows on the wall induced by particle’s activity. In turn, these flows drive translation and rotation of the particle with respect to the pattern-defined direction. The interplay of chemi-osmosis and self-diffusiophoresis induces two classes of behavior which depend, generically, on whether self-diffusiophoretic motion is catalyst- or inert-forward. Catalyst-forward particles can stably follow a chemical stripe, while inert-forward particles can dock at the chemical step between two substrate materials. The analytical predictions have been fully confirmed by numerical calculations. For a broad class of spherical active particles exposed to an ambient linear shear flow it has been shown that rheotactic behavior, i.e. the alignment of the direction of motion with that of the flow, may emerge. The mechanism for this involves self-trapping near a hard wall owing to the active propulsion of the particles, combined with their rotation, alignment, and locking of the direction of motion into the shear plane. This is the first evidence that rheotaxis is possible even for spherical particles, for which a weather vane mechanism, which is responsible for, e.g., rheotaxis of sperm or other elongated bodies, is excluded due to the symmetry of the particle shape.
Publications
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Rheotaxis of a microswimmer near a plane wall. Soft Matter 11, 6613 (2015)
W. E. Uspal, M. N. Popescu, S. Dietrich, and M. Tasinkevych
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Self-propulsion of a catalytically active particle near a planar wall: from bouncing back to sliding and hovering. Soft Matter 11, 434-438 (2015)
W. E. Uspal, M. N. Popescu, S. Dietrich, and M. Tasinkevych
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Guiding catalytically active particles with chemically patterned surface. Phys. Rev. Lett. 117, 048002 (2016)
W. E. Uspal, M. N. Popescu, S. Dietrich, and M. Tasinkevych
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Self-diffusiophoresis of chemically active colloids. Eur. Phys. J. Special Topics 225, 2189–2206 (2016)
M. N. Popescu, W. E. Uspal, and S. Dietrich
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Topographical pathways guide chemical microswimmers. Nat. Commun. 7, 10598 (2016)
J. Simmchen, J. Katuri, W. E. Uspal, M. N. Popescu, M. Tasinkevych, and S. Sanchez
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Chemically active colloids near osmotic-responsive walls with surface-chemistry gradients. J. Phys. Condens. Matter. 29, 134001 (2017)
M. N. Popescu, W. E. Uspal, and S. Dietrich
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Perils of ad hoc approximations for the activity function of chemically powered colloids. Eur. Phys. J. E 40, 42 (2017)
M. N. Popescu, W. E. Uspal, M. Tasinkevych, and S. Dietrich
