Here multiple angle of incidence ellipsometry was successfully applied to in situ assess the contact angle and surface coverage of gold nanoparticles as small as 18 nm, coated with stimuli-responsive polymers, at water-oil and water-air interfaces in the presence of NaCl and NaOH, respectively. The interfacial adsorption of the nanoparticles was found very slow and took days to reach a fairly low surface coverage. For water-oil interfaces, in-situ nanoparticle contact angles agree with the macroscopic equilibrium contact angles of planar gold surfaces with the same polymer coatings; whilst for water-air interfaces, significant differences have been observed.

Dynamics of partially wetted colloids at the interface between two fluids is a crucial and timeliness topic: for instance, it governs the behavior of kinetically arrested colloidal gels, the formation of bacteria based biofilm and the cellular signaling via membrane proteins. Moreover, this subject is also challenging from a theoretical point of view because of the complexity of hydrodynamics at the interface and of the unexplored role of the contact line [1]. Despite this great interest, the behavior of a single particle at a fluid interface was never directly characterized. In this contribution, the dynamics of micrometric spherical silica particles trapped at a flat air-water interface is fully addressed. The particle contact angle θ 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 [2]. The particle dynamics and translational diffusion coefficients are obtained by particle tracking. Counter-intuitively, and against all hydrodynamical models [3], the diffusion is much slower than expected; the drag exerted on the particle increases when the particle is less immersed in water. To explain this extra dissipation we devised a model considering the effect related to surface waves. Such waves are generated by the thermally activated triple line jumps between hydrophilic sites of the particle surface [4]. The total friction is thus coherent with the measured particle diffusion, and in particular with its slowing down at large contact angles. The jump lengths λ are consistent with the Cassie - Baxter model of an heterogeneous surface characterized by an intrinsic roughness and by hydrophobic and hydrophilic patches. [1] D. M. Kaz, R. McGorty, M. Mani, M. P. Brenner, V. N. Manoharan, Nature materials, 11.2, 138-142 (2011). [2] C. Blanc et al., Phys.Rev. Lett.,111, 058302 (2013). [3] T. M. Fischer, P. Dhar, P. Heinig, T, J.Fluid. Mech., 558, 451-475 (2006). [4] T. Blake, J.Colloid Interface Sci., 299, 1-13 (2006).

The dynamics of colloids at the interface between two fluids is a crucial and timeliness research topic as it governs the behavior of kinetically arrested colloidal gels in interfacial microrheology, the formation of bacteria based biofilm and the cellular signaling via membrane proteins. Moreover, this subject is also challenging from a theoretical point of view because of the complexity of hydrodynamics at the interface and of the unexplored role of the contact line. Despite this great interest, the behavior of a single particle at a fluid interface was never directly characterized. In this contribution, the Brownian motion of micrometric spherical silica beads at a flat air-water interface is addressed. We fully characterize and control all the experimentally relevant parameters. The particle contact angle is finely tuned in the range 30-140° by surface treatments and measured in situ at 1° resolution by a homemade Vertical Scanning Interferometer. The particle dynamics and translational diffusion coefficients are obtained by particle tracking. Counter-intuitively, and against all hydrodynamical models, the diffusion is much slower than expected; the drag exerted on the particle increases when the particle is less immersed in water. To explain this extra dissipation we devised a model considering the effect related to thermally activated fluctuations of the interface at the triple line. Such fluctuations couple with the lateral movement of the particle via tiny random forces that add to the ones due to the shocks of surroundings molecules. Fluctuation-dissipation theorem allows obtaining the extra friction associated to this additional mechanism. The obtained total friction is thus in agreement with the measured particle diffusion, and in particular with its slowing down at large contact angles. The physical origin of the fluctuations is discussed in the two extreme limits: of a moving line using the generally accepted Blake molecular kinetic theory and of a pinned line taking into account the role of capillary fluctuation at the interface. Both of them lead to the right order of magnitude for the extra dissipation and can capture the measured dynamics.

Gold nanoparticles (NPs) 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 after warming is dictated by the ionic strength of the aqueous phase, which is directly associated with the ionic strength response of the polymer brushes anchored on the NPs. In contrast, the temperature onset for the oil-to-water NP transfer after cooling is weakly correlated with the transition temperature of the polymer brushes in either of the bulk phases. 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. In this talk, experimental investigations and some modeling will be presented in order to elucidate the NP transfer across fluid interfaces. References Bidirectional Nanoparticle Crossing of Oil-Water Interfaces Induced by Different Stimuli: Insight into Phase Transfer. Stocco Antonio, Chanana Munish, Su Ge, Cernoch Peter, Binks Bernard, Wang Dayang: Angewandte Chemie / Angew Chem Int Ed, vol. 51 p.9647 (2012) Acknowledgements Special thanks go to Helmuth Möhwald, Munish Chanana (MPIKG), and all colleagues involved in this project.

At the interface, as in the bulk, the Brownian motion rules the diffusion of colloids. The motor of the motion is the thermal energy kT; and the friction coefficient accounts for viscous dissipations at the interface. These physical conditions constrain the colloid to move following random walk trajectories. To observe directional trajectories, overcoming the limitation of Brownian motion, we have investigated Janus Silica-Platinum active colloids. The catalytic motor of the motion in our case is provided by the Platinum face, on which H2O2 (the fuel) decomposes into oxygen gas and water. To study the motion at the interface we have used a particle-tracking video-microscopy method. We start presenting the results on the motion of Silica colloids (Radius = 1 µm) at air-water interface. Then we describe the fabrication of Janus Silica-Platinum colloids. Presenting the results on the active motion, we discuss the effect of H2O2 fuel concentration and the competition between directional and Brownian motion.