We analyze the rotational dynamics of spherical colloidal Janus particles made from silica (SiO2) with a hemispherical gold/palladium (Au/Pd) cap. Since the refractive index difference between the surrounding fluid and a two-faced, optically anisotropic Janus microsphere is a function of the particle's orientation, it is possible to observe its rotational dynamics with bright-field optical microscopy. We investigate rotational diffusion and constant rotation of single Janus microspheres which are partially tethered to a solid surface so they are free to rotate but show little or no translational motion. Also, since the metal cap is a powerful catalyst in the breakdown of hydrogen peroxide, H2O2, the particles can be activated chemically. In this case, we analyze the motion of coupled Janus dimers which undergo a stable rotary motion about a mutual center. The analysis of both experimental and simulation data, which are microscopy and computer-generated videos, respectively, is based upon individual particle tracking and differential dynamic microscopy (DDM). DDM, which typically requires ensemble averages to extract meaningful information for colloidal dynamics, can be effective in certain situations for systems consisting of single entities. In particular, when translational motion is suppressed, both rotational diffusion and constant rotation can be probed.

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.

COMMUNICATION (1 of 7)Colloidal particles suspended in a fluid may become self-propelling when a catalyzed chemical reaction takes place at the interface between the particle and the fluid through which motion occurs. [1][2][3][4][5][6][7][8][9] If this reaction generates a local concentration gradient, [10] a chemiosmotic flow [11] over the particle's surface may arise. Such fluid flow is usually responsible for self-propulsion. This way of achieving active nano-and microscale autonomous motion is attractive since external fields, such as electric, [12] magnetic, [13][14][15][16][17][18] light, [19] or acoustic, [20,21] do not need to be applied, and moreover, nonequilibrium self-propelled systems

Self-propelling, light-activated colloidal particles can be actuated in water alone, but their performance is material-dependent.