Active motion of living or artificial systems (active colloids) may occur in strong confinement or in complex environments such membranes. In confinement, the propulsion mechanism and motion features may strongly differ from the ones observed in the bulk of a simple fluid. As a primary form of confinement, we consider a single interface. Recently, we show that the interface between a liquid and a fluid dramatically affects the lateral frictions of partially wetted colloidal particles [1]. In this contribution, the impact of partial wetting dynamics on the motion of active Janus colloids will be presented. Spherical catalytic Janus colloids have been prepared by coating half surface of silica particles with a thin platinum layer. Immersion depth of the Janus colloids as well as their orientation with respect to the water surface reveal complex and rich wetting properties of Janus particles at the liquid-gas interface [2]. Active motion of Janus colloids at the interface in the presence of various concentrations of hydrogen peroxide, H2O2, has been studied. The types of observed trajectories, directional and circular ones, revealed the effective force and torque induced by the catalytic decomposition of H2O2 fuel. At the water surface, the persistence of directional active motion is greatly enhanced when compared to the motion in bulk [3]. This result has been explained in terms of loss of one rotational degree of freedom at the interface and also to a new contribution to the rotational friction due to partial wetting [1].[1] G. Boniello, C. Blanc, D. Fedorenko, M. Medfai, NB. Mbarek, M. In, M. Gross, A. Stocco, M. Nobili, Nature Materials 14, 908 (2015).[2] X. Wang, M. In, C. Blanc, P. Malgaretti, M. Nobili, A. Stocco, Faraday Discussions 191, 305 (2016).[3] X. Wang, M. In, C. Blanc, M. Nobili, A. Stocco, Soft Matter 11, 7376 (2015).

Lignin is one of the most abundant bio-polymers present in nature and its valorization is still a challenge to an economically feasible biorefinery concept. The heterogeneity and scarce reactivity of lignin are significant hurdles to the development of new valorization processes. We propose an in situ extraction electrochemical depolymerization system. Kraft lignin dissolved in a deep eutectic solvent together with an extractant phase is able to form a stable emulsion. Electro-oxidation of the emulsion using a nickel or graphite electrode results in successful depolymerization and simultaneous fractionation of the products. Over-oxidation of the depolymerization products is anticipated to be suppressed upon swift extraction from the lignin phase into the extraction phase.

In this critical review we compare two geometries in partial wetting: a liquid drop on a planar substrate and a spherical particle at a planar liquid interface. We show that this comparison is far from being trivial even if the same physical interactions are at play in both geometries. Similarities and differences in terms of free energies and frictions will be discussed. Contact angle hysteresis, the impact of surface roughness and line pinning on wetting will be described and compared to selected experimental findings.