Self-Phoretic Microswimmers Propel at Speeds Dependent upon an Adjacent Surface’s Physicochemical Properties

Abstract

Self-phoretic colloids are emerging as critical components of programmable nano- and microscale active matter and may usher in a new area of complex, small-scale machinery. To date, most studies have focused upon active particles confined by gravity to a plane located just above a solid/liquid interface. Despite this ubiquity, little attention has been directed at how the physicochemical qualities of this interface might affect motion. Here, we show that both the chemical and physical properties of the solid, above which motion takes place, significantly influence the behavior of particles propelled by self-generated concentration gradients. More specifically, titania/silica (TiO/SiO) photoactive microswimmers move faster when the local osmotic flow over the stationary solid is diminished, which we demonstrate by reducing the magnitude of the surface's zeta potential or by increasing surface roughness. Our results suggest that consideration of surface properties is crucial for modeling self-phoretic active matter while simultaneously offering a new avenue for engineering the kinematic behavior of such systems.

Theory of light-activated catalytic Janus particles

“…In analogy with biological systems, this directed spatial migra- * Electronic address: uspal@hawaii.edu tion can be considered a form of "taxis". Recent studies have investigated the rheotactic response of chemically active particles to hydrodynamic flow, [17][18][19][20] gravitactic response to the earth's gravitational field, [4,21,22] "viscotactic" response to viscosity gradients, [23] chemotactic response to gradients in the concentration of chemical "fuel", [24][25][26][27][28][29][30][31][32] and "thigmotactic" response to gradients in the material composition of bounding surfaces [33][34][35]. These various forms of taxis can be understood on the basis of the microscopic physics of how the ambient field couples to the activity and motion of the particle.…”

Slip Length Dependent Propulsion Speed of Catalytic Colloidal Swimmers near Walls

“…To quantitatively unravel the origin of our observations, we consider substrate properties that may influence colloid motion. The fluid flow generated by the anisotropic catalytic reaction on the swimmer surface [28], and hence the swimming velocity [29], has been predicted to be affected by the swimmer-wall distance [9][10][11][12][13][14][15], wall zeta potential [11,20] and wall surface inhomogeneities [20]. Surprisingly, little consideration has been given until now on whether slip on the substrate impacts swimming velocities, even though slip on the colloid has already been shown to do so [27].…”

“…Based on the low velocity on hydrophilic PDMS, we conclude that the substrate zeta potential is, surprisingly, not the dominant effect. Secondly, an increase in the substrate roughness was shown to increase the velocity [20]. We thus performed Atomic Force Microscopy (AFM) measurements, see SI Section I F. The average substrate roughness Ra, is 1.5 and 5 nm, for glass and PDMS, respectively; with Ra denoting the arithmetic mean of the deviations in height from the roughness mean value.…”

Engineering the Dynamics of Active Colloids by Targeted Design of Metal–Semiconductor Heterojunctions

“…TiO 2 is an attractive material for the construction of catalytic micromotors due to stability and resistance to catalyst poisoning. Potentially more importantly, TiO 2 is a photocatalyst, and so self‐propulsion can be switched on‐and‐off, and the strength of activity can be modulated in real‐time by adjusting the intensity of the activating light source .…”