Slip Length Dependent Propulsion Speed of Catalytic Colloidal Swimmers near Walls

Abstract

Catalytic colloidal swimmers that propel due to self-generated fluid flows exhibit strong affinity for surfaces. We here report experimental measurements of significantly different velocities of such microswimmers in the vicinity of substrates made from different materials. We find that velocities scale with the solution contact angle θ on the substrate, which in turn relates to the associated hydrodynamic substrate slip length, as V ∝ (cos θ + 1) −3/2 . We show that such dependence can be attributed to osmotic coupling between swimmers and substrate. Our work points out that hydrodynamic slip at the wall, though often unconsidered, can significantly impact the self-propulsion of catalytic swimmers. arXiv:1812.08631v3 [cond-mat.soft]

Catalytically propelled 3D printed colloidal microswimmers

“…From the MSD, we also determined the average velocity of our sphere-like microswimmers and found it to be 0.5 AE 0.1 mm s À1 . The value that we measure here is slightly lower than values previously measured in the same hydrogen peroxide concentration above similar glass substrates with smooth 2.7 mm TPM (3-(trimethoxysilyl)propyl methacrylate) spheres, 45 in line with the inverse speed-size scaling found by Ebbens et al 46 The 2.7 mm TPM particles exhibited a speed of 1.05 AE 0.09 mm s À1 . We would, therefore, expect 4 mm spheres to swim with a speed of 0.7 mm s À1 in comparison to 0.5 AE 0.1 mm s À1 which we measured here.…”

“…We speculate that the spread observed in the measured translational velocities of individual particles, see also Fig. 5b, is due to either local variations in the substrate 45,53,54 or slight differences in the Pt coating. [48][49][50] The average diffusion coefficient was D = 0.012 AE 0.011 mm 2 s À1 .…”

“…We speculate that this circular motion occurs as a result of the interaction of one side of an anisotropic helix with the underlying substrate. The substrate, parallel to which active particles swim, has been shown to have a strong effect on active motion, 45,53,54 and might induce a torque in opposite directions for the different chiralities.…”

Impact of surface charge on the motion of light-activated Janus micromotors

“…However, a less well studied but key piece to the control puzzle is how artificial micro-objects interact with their microenvironment [20][21][22], both in terms of nearby boundaries [21,23,24] and the presence of other (biological) objects [22,[25][26][27]. The surface properties of both the micromotors and the substrate have been shown to play an important role in determining the motion of self-phoretic micromotors [24,[28][29][30]. This is to be expected, as their propulsion depends crucially on the local asymmetry in the distribution of the chemical reaction products, the electrical potential, and the fluid flow, all of which can be modified by a nearby boundary.…”

Lift induced by slip inhomogeneities in lubricated contacts

“…Such slippage inhomogeneities may arise from small variations in surface preparation, localized defects, or cleaning processes, and this slippage has the potential to modify the observed wallnormal force. For example, in the experiments of a traveling Janus particle near a wall [36], variations of contact angle up to 10 • on the same substrate were reported. Finally, our numerical results for both gravity-driven and other type of configurations show how nontrivial particle trajectories can spontaneously emerge from slippage inhomogeneities.…”