Wetting and orientation of catalytic Janus colloids at the surface of water

Abstract

Janus colloidal particles show remarkable properties in terms of surface activity, self-assembly and wetting. Moreover they can perform autonomous motion if they can chemically react with the liquid in which they are immersed. In order to understand the self-propelled motion of catalytic Janus colloids at the air-water interface, wetting and the orientation of the catalytic surface are important properties to be investigated. Wetting plays a central role in active motion since it determines the contact between the fuel and the catalytic surface as well as the efficiency of the transduction of the chemical reaction into motion. Active motion is not expected to occur either when the catalytic face is completely out of the aqueous phase or when the Janus boundaries are parallel to the interfacial plane. The design of a Janus colloid possessing two hydrophilic faces is required to allow the catalytic face to react with the fuel (e.g. HO for platinum) in water and to permit some rotational freedom of the Janus colloid in order to generate propulsion parallel to the interfacial plane. Here, we discuss some theoretical aspects that should be accounted for when studying Janus colloids at the surface of water. The free energy of ideal Janus colloidal particles at the interface is modeled as a function of the immersion depth and the particle orientation. Analytical expressions of the energy profiles are established. Energetic aspects are then discussed in relation to the particle's ability to rotate at the interface. By introducing contact angle hysteresis we describe how the effects of contact line pinning modifies the scenario described in the ideal case. Experimental observations of the contact angle hysteresis of Janus colloids at the interface reveal the effect of pinning; and orientations of silica particles half covered with a platinum layer at the interface do not comply with the ideal scenarios. Experimental observations suggest that Janus colloids at the fluid interface behave as a kinetically driven system, where the contact line motion over the defects decorating the Janus faces rules the orientation and rotational diffusion of the particle.

Two-dimensional nature of the active Brownian motion of catalytic microswimmers at solid and liquid interfaces

“…The behavior of self-propelling silica/Pt particles at air-water interfaces has been recently investigated by Wang et al [28,29], showing both analogies and striking differences when compared with our observations. These authors similarly reported larger values of some rotational diffusion times and a consequent enhancement of the persistence length of the corresponding active trajectories.…”

Rotational diffusion of partially wetted colloids at fluid interfaces

“…We have revealed an enhancement of their two dimensional active motions compared to the bulk [28] [29]. In this paper, we report new experimental results on their rotational Brownian motion extending our first observations on the slowing down of the perpendicular and parallel rotational diffusion coefficients, Dr, and Dr,|| [27,29]. A model for the rotational friction is also developed in order to describe our experimental findings.…”

“…Rotational Brownian diffusion is also a key parameter that sets the persistence of active motion of self-propelled colloids [26]. Janus spherical silica particles half coated with a thin platinum layer are such particles, self-propelling in presence of hydrogen peroxide [27]. Recently, we have studied their behaviour at the air/water interface.…”

Active Atoms and Interstitials in Two-Dimensional Colloidal Crystals

“…In fact, when the microswimmer approaches a passive particle ( Fig. 2(g) and Video S7 [22]), its direction of motion (blue arrow) changes from the one observed before the collision and no longer corresponds to the 2D projection of the particle asymmetry (green arrow, note that in 2(g) a 20 nm Pt-layer is used to visualize the cap and this choice leads to a metastable tilted orientation [27]). Eventually, the microswimmer escapes the passive obstacle in a random direction (inset to the bottom-right panel in Fig.…”