Energy dissipation of a contact line moving on a nanotopographical defect.

Understanding the origin of the dissipative mechanisms that control the dynamics of a contact line is a real challenge. In order to study the energy dissipation at the contact line when a moving meniscus encounters topographical defects, we developed atomic force microscopy (AFM) experiments using nanofibers with nanometer scale defects. These experiments realized with three liquids are performed in two AFM modes: the contact mode (C-AFM) is used to measure the energy associated with the contact angle hysteresis in the limit of a static situation, deduced from advancing and receding dipping experiments on an isolated defect; the frequency-modulation mode (FM-AFM) is performed at different amplitudes and then velocities to measure the energy dissipated as the contact line moves over the same defect.

Wetting at the Nanoscale: Molecular Mobility Induced by Contact Line Forces.

In this paper, we study the interaction of a contact line with molecules physically adsorbed on a surface. We developed specific atomic force microscopy (AFM) experiments where a nanoneedle attached at the extremity of the cantilever is dipped in a liquid droplet. The motion of the contact line at the extremity of the meniscus formed depends on the presence of topographical and chemical defects at the surface of the nanoneedle.

Air humidity effects on water-drop icing.

The canonical problem of the icing of a water drop lying on a cold substrate is revisited to take into account the effects of atmospheric conditions on the icing front kinetics and on the tip formation. Here, we demonstrate both experimentally and theoretically that the air humidity induces liquid-vapor phase change at the icing droplet interface and that the associated heat transfer has a strong influence on both the icing front kinetics and the iced drop shape. The experimental results obtained in this study, as well as results from literature, compare well to a modified Stefan model accounting for the effects of humidity, showing a good agreement with the experimental data of both the front kinetics and tip angle.

Molecular Desorption by a Moving Contact Line.

The interaction of the contact line with topographical or chemical defects at the nanometer scale sets the macroscopic wetting properties of a liquid on a solid substrate. Based on specific atomic force microscopy (AFM) experiments, we demonstrate that molecules physically sorbed on a surface are removed by a dynamic contact line. The mechanism of molecules desorption is directly determined by the capillary force exerted at the contact line on the molecules.

Long-range hydrodynamic forces in liquid FM-AFM.

We study the effects of hydrodynamic forces in frequency-modulation AFM experiments (FM-AFM) in liquid. We first establish the theoretical equations needed to derive the interaction stiffness k and the damping β due to the hydrodynamic forces from the frequency shift and the excitation amplitude. We develop specific FM-AFM experiments to measure the variation of k and β over a large range of distance in water up to 200 µm.