Acoustic streaming near a sharp structure and its mixing performance characterization, Microfluidics and Nanofluidics

Wet-on-wet printing is frequently used in inkjet printing for graphical and industrial applications, where substrates can be coated with a thin liquid film prior to ink drop deposition. Two drops placed close together are expected to interact via deformations of the thin viscous film, but the nature of these capillary interactions is unknown. Here we show that the interaction can be attractive or repulsive depending on the distance separating the two drops. The distance at which the interaction changes from attraction to repulsion is found to depend on the thickness of the film, and increases over time. We reveal the origin of the non-monotonic interactions, which lies in the appearance of a visco-capillary wave on the thin film induced by the drops. Using the thin-film equation we identify the scaling law for the spreading of the waves, and demonstrate that this governs the range over which interaction is observed.

The directional motion of sessile drops can be induced by slanted mechanical vibrations of the substrate. As previously evidenced [13][14][15], the mechanical vibrations induce drop deformations which combine axisymmetric and antisymmetric modes. In this paper, we establish quantitative trends from experiments conducted within a large range of parameters, namely the amplitude A and frequency f of the forcing, the liquid viscosity η and the angle between the substrate and the forcing axis α. These experiments are carried out on weak-pinning substrates. For most parameters sets, the averaged velocity < v > grows linearly with A. We extract the mobility, defined as s = ∆ ∆A . It is found that s can show a sharp maximal value close to the resonance frequency of the first axisymmetric mode fp. The value of s tends to be almost independent on η below 50 cSt, while s decreases significantly for higher η. Also, it is found that for peculiar sets of parameters, particularly with f far enough from fp, the drop moves in the reverse direction. Finally, we draw a relationship between < v > and the averaged values of the dynamical contact angles at both sides of the drop over one period of oscillation.