We have studied the combined effects of
vertical and horizontal vibrations which can lead to controlled
displacement of droplets.
We focused on the motion of a drop lying on a vibrated plate simultaneously submitted to horizontal and vertical harmonic vibrations. The two driving vibrations are adjusted to the same frequency, but according to their relative amplitude and phase difference ΔΦ the drop experiences a controlled directed motion with a tunable velocity. We present a simple model enlightening the underlying mechanism leading to a net motion of a drop. The particular case ΔΦ = π corresponds to the climbing of a drop on a vertically vibrated inclined substrate as recently observed by Brunet et al. Our study gives insights in the fundamental study of wetting dynamics and offers new possibilities of controlled motion in droplet microfluidics application.
When a water jet impinges upon a solid surface it produces a so called
hydraulic jump that everyone can observe in the sink of its kitchen. It
is characterized by a thin liquid sheet bounded by a circular rise of
the surface due to capillary and gravitational forces. In this
phenomenon, the impact induces a geometrical transition, from the
cylindrical one of the jet to the bi-dimensional one of the film. A
true jet rebound on a solid surface, for which the cylindrical geometry
is preserved, has never been yet observed. We have experimentally
demonstrated that a water jet can impact a solid surface without being
destabilized. Depending on the incident angle of the impinging jet, its
velocity and the degree of hydrophobicity of the substrate, the jet
can: i) bounce on the surface with a fixed reflected angle, ii) land on
it and give rise to a supported jet or iii) be destabilized, emitting
drops. Capillary forces are predominant at the sub-millimetric jet
scale considered in this work, along with the hydrophobic nature of the
substrat to explain why such capillary hydraulic jump gives rise to
this unexpected jet rebound phenomenon.
have studied the effect of vertical vibrations of sessile drops and
puddle (frequency fe). Above a first threshold in amplitude, we observe
the depinning of the line and the radius of the puddle starts to
oscillates. Above a second threshold, we observe an instability of the
contour at frequency fe/2, parametrically excited by the variations of
the drop radius. See my Ph.D thesis
for more details.
We have studied the fast dewetting of a water film floating on a non
miscible liquid substrate, denser and non miscible. We have measured
the dewetting velocity V versus the film thickness e. When V is larger
than the velocity of surface waves, we observe a cascade of shocks
propagating ahead or behind the rim collecting the water. See my Ph.D thesis for more details.