Active colloids are an emerging and promising class of colloidal particles, which are designed to perform autonomous motion by transforming chemical or other form of energies into work and propulsion.Here, we have investigated the active motion of self-propelled colloids confined at the air-water interface. The two dimensional motion of micron-sized Silica-Platinum Janus colloids has been experimentally measured by particle tracking video-microscopy under increasing concentration of the catalytic fuel, i.e. H2O2. Comparing to the motion in bulk, a dramatic enhancement of both the persistence length of trajectories and the speed has been observed. The interplay of colloid self-propulsion, due to an asymmetric catalytic reaction occurring on the colloid, and interfacial frictions controls the enhancement of the directional movement. The slowing down of the rotational diffusion at the interface, also measured experimentally, plays a pivotal role in the control and enhancement of active motion.

The talk is divided into two parts. First, I will show some experimental results obtained in my group on the Brownian dynamics of micrometric spherical silica colloids trapped at a planar air-water interface. The particle contact angle is finely tuned in the range 30°-140° by surface treatments and measured in situ at 0.5° resolution by a homemade Vertical Scanning Interferometer. Translational diffusion coefficients of colloids trapped at the water interface are obtained by particle tracking video-microscopy. Counter-intuitively, the diffusion coefficient decreases when the contact angle increases (i.e. when particles are less immersed in water and more in air). To explain the slowing down of the diffusion, I will discuss the effects of the hydrodynamic friction together with an extra friction term originating from the contact line fluctuations. The second part of the talk deals with the active motion of Janus colloids. Here, we have investigated the motion of self-propelled colloids at the air-water interface, where particles have been confined. The interplay of colloid’s self-propulsion, given by an asymmetric catalytic reaction occurring on the colloid, and interfacial frictions controls the direction and the speed of the movement. Two dimensional motion of micron-sized Silica-Platinum Janus colloids have been experimentally measured by particle tracking video-microscopy under increasing concentration of the catalytic fuel, i.e. H2O2,. Compared to previous bulk investigations, we observe a dramatic enhancement both of the length of trajectories travelled by particles and the speed. The slowing down of the rotational diffusion at the interface, also measured experimentally, plays a pivotal role in the control and enhancement of active motion.

In this work we have investigated the active motion of self-propelled colloids confined at the air-water interface. The two dimensional motion of micron-sized Silica-Platinum Janus colloids has been experimentally measured by particle tracking video-microscopy under increasing concentration of the catalytic fuel, i.e. H2O2. Comparing to the motion in bulk, a dramatic enhancement of both the persistence length of trajectories and the speed has been observed. We measured long range directional trajectories at the water surface with persistence length l =ca. 140 µm.The interplay of colloid’s self-propulsion, due to an asymmetric catalytic reaction occurring on the colloid, and interfacial frictions controls the enhancement of the directional movement. The slowing down of the rotational diffusion at the interface, also measured experimentally, plays a pivotal role in the control and enhancement of active motion.

Solid micron- and nano- particles have been extensively used to stabilize dispersions such Pickering emulsions and particle-stabilized foams. In many cases, particle's adsorption onto the fluid interfaces represents a limiting step for the stability of the dispersion since adsorption can be very slow or even hindered by interfacial potential barriers. In the first part of the talk, I will show experimental evidences on the slow dynamics of nanoparticles at the interface and the role of colloidal forces and wetting on the free energy profile across the interface. A step beyond the understanding of particle's adsorption onto the interface is the control of nanoparticle crossing of liquid-liquid interfaces. The second part of the talk discusses experiments and models describing the adsorption-desorption phenomena, which result in the phase transfer of nanoparticles from oil to water and viceversa . Gold nanoparticles coated with stimuli-responsive polymer transfer from water to oil and from oil to water across the planar interfaces when environmental parameters are slightly changed. The oil to water transfer occurs when the temperature is reduced below 5 °C, while they transfer from salty water to oil when the environmental temperature returns to room temperature. The water-to-oil particle transfer is dictated by the ionic strength of the aqueous phase. In contrast, the oil-to-water particle transfer is correlated with the hydration interaction. The transfer mechanism disparity for the two directions during NP crossing of oil-water interfaces result from an intricate interplay of interfacial interactions and transitions. Among the different experimental tools used for the investigation, multiple angle of incidence ellipsometry was applied to in situ assess the contact angle and surface coverage of nanoparticles as small as 18 nm at water-oil and water-air interfaces.

Solid micron- and nano- particles have been extensively used to stabilize dispersions such Pickering emulsions and particle-stabilized foams. In many cases, particle’s adsorption onto the fluid interfaces represents a limiting step for the stability of the dispersion since adsorption can be very slow or even hindered by interfacial potential barriers. In the first part of the talk, I will show experimental evidences on the slow dynamics of nanoparticles at the interface and the role of colloidal forces and wetting on the free energy profile across the interface. A step beyond the understanding of particle’s adsorption onto the interface is the control of nanoparticle crossing of liquid-liquid interfaces. The second part of the talk discusses experiments and models describing the adsorption-desorption phenomena, which result in the phase transfer of nanoparticles from oil to water and viceversa . Gold nanoparticles coated with stimuli-responsive polymer transfer from water to oil and from oil to water across the planar interfaces when environmental parameters are slightly changed. The oil to water transfer occurs when the temperature is reduced below 5 °C, while they transfer from salty water to oil when the environmental temperature returns to room temperature. The water-to-oil particle transfer is dictated by the ionic strength of the aqueous phase. In contrast, the oil-to-water particle transfer is correlated with the hydration interaction. The transfer mechanism disparity for the two directions during NP crossing of oil–water interfaces result from an intricate interplay of interfacial interactions and transitions. Among the different experimental tools used for the investigation, multiple angle of incidence ellipsometry was applied to in situ assess the contact angle and surface coverage of nanoparticles as small as 18 nm at water-oil and water-air interfaces.