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.

We investigate electro-optical properties of suspensions of gold anisometric particles. Gold nanoparticles exhibit localized surface plasmon properties with a transverse mode and longitudinal mode in case of ellipsoidal shape. The transverse mode remains maximum in the visible range but the longitudinal one is red shifted and its resonance strongly depend on the aspect ratio of the nanoparticles. Among others, we thus expect highly tunable properties at optical frequencies under the external electric field. In our experimental setup, the electrodes are placed outside the sample. In this configuration, they do not interact with the suspension. We work with two kinds of particles: gold nanorods (GNRs) dispersed in water with an aspect ratio AR ≈10 (Φ=10nm) and gold nanowires (GNWs) dispersed in hexane with an AR≈1000 (Φ=1.5nm). We observed alignment of both GNRs and GNWs along the applied electric field due the intrinsic conductivity of the particles. Time responses of GNRs to the electric field are of the order of the millisecond and of more than 10 ms for GNWs. The saturated voltage is 4 times smaller in this last case compared to GNRs. Those results are in agreement with a change in the aspect ratio. More surprising, dichroism and spectroscopy measurements reveal, in both cases, a positive dichroism in the optical visible range. On the contrary, we expect a negative one (ie absorption higher for polarization perpendicular to the electric field) while in this frequency range, we are sensitive to the transverse mode of the plasmon band. This suggests that this mode is less sensitive to polarization than the longitudinal one . Concerning GNWs, a longitudinal plasmon band is not yet evidenced. To the best of our knowledge, numerical simulations have only predicted their presence for wire of similar length but larger diameter. Similarly to carbon nanotubes, we observe their progressive assembly into fibers strictly oriented along the electric field. Those fibers appear highly birefringent between crossed polarisers. Their properties and the way to control their growth are under investigation.

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.