Sliding drops across alternating hydrophobic and hydrophilic stripes

Abstract

We perform a joint numerical and experimental study to systematically characterize the motion of 30 μl drops of pure water and of ethanol in water solutions, sliding over a periodic array of alternating hydrophobic and hydrophilic stripes with a large wettability contrast and a typical width of hundreds of microns. The fraction of the hydrophobic areas has been varied from about 20% to 80%. The effects of the heterogeneous patterning can be described by a renormalized value of the critical Bond number, i.e., the critical dimensionless force needed to depin the drop before it starts to move. Close to the critical Bond number we observe a jerky motion characterized by an evident stick-slip dynamics. As a result, dissipation is strongly localized in time, and the mean velocity of the drops can easily decrease by an order of magnitude compared to the sliding on the homogeneous surface. Lattice Boltzmann numerical simulations are crucial for disclosing to what extent the sliding dynamics can be deduced from the computed balance of capillary, viscous, and body forces by varying the Bond number, the surface composition, and the liquid viscosity. Beyond the critical Bond number, we characterize both experimentally and numerically the dissipation inside the droplet by studying the relation between the average velocity and the applied volume forces.

Generation of Oil Droplets in a Non-Newtonian Liquid Using a Microfluidic T-Junction

“…For a better understanding of these phenomena, it would be of great interest to complement the experimental observations with numerical simulations that allow to evaluate both the stress tensor and the distribution of the shear rate inside the T-junction, and to monitor the momentum balance equations for non-Newtonian fluids during the process of break-up. To this aim, generalization of the investigations we have proposed for sliding droplets [41,42] are surely warranted for future investigations.…”

The equilibrium shape of fluid-fluid interfaces: Derivation and a new numerical method for Young’s and Young-Laplace equations

“…Aside from adsorbed colloidal particles, our numerical method can be applied and adapted to study a wide range of other physical problems relevant in soft matter and involving (equilibrium shapes of) fluid-fluid interfaces, e.g.,: droplets on super-hydrophobic or structured solid substrates, 68,69 fluidfluid interfaces in contact with deformable solid surfaces, [61][62][63] and fluid droplets adsorbed at a fluid-fluid interface.…”

Numerical investigation of dynamic effects for sliding drops on wetting defects

“…Different authors have analyzed the onset of motion for sessile droplets under the action of gravity [3][4][5][6], as well as the depinning transition from defects in lubrication approximation [7]. The motion of sessile drops on periodically patterned substrates [8][9][10][11][12] has attracted a particular interest, since the ability to shed droplets is one of the main design objectives for micropatterned, liquid repellent surfaces [13]. Moreover, wetting defects can find several applications in microfluidics devices as a means to steer or retain droplets in microchannels [14][15][16].…”