Numerical investigation of dynamic effects for sliding drops on wetting defects

Sliding Behavior of Droplets on a Tilted Substrate with a Chemical Step

Controlling and predicting the motion of droplets on a heterogeneous substrate have received widespread attention. In this paper, we numerically simulate the droplet sliding through a "chemical step", that is, different wetting properties at two sides of the step, on a tilted substrate by the multiphase lattice Boltzmann method (LBM). Three kinds of equilibrium statuses are reproduced by observing the deformation of the droplet and the velocities of the front contact line. This study shows the droplet obtains a driving force to break through the step by deformation in the initial stage that the droplet is blocked. The droplet spreads to two sides along the step when the front end is blocked and is stretched after the front end is passed over the step. The lengths of the lateral spreading and the longitudinal stretching and the time required to pass over the step depend on the strength of the step. In the sliding process, the kinetic energy is converted into surface energy as the droplet is blocked, and the gravitational potential energy is converted into surface and kinetic energy following the droplet passes over the step. If the droplet can slide through the step, the more strength in the step, the more the gravitational potential energy is converted, and the more the surface energy increases. When the strength of the step is small, unbalanced Young's force hinders the contact line moving forward after the central part of the front end of the droplet breaks through the step. While the velocity of droplet sliding slows down with the increasing strength of the step, the unbalanced Young's force pushes the contact line forward against the resistance. These observations throw insight into the dynamics of the droplets sliding on a heterogeneous surface, which may facilitate potential applications like microfluidics and liquid transportation.

Thermocapillary Droplet Actuation: Effect of Solid Structure and Wettability

We examine the thermocapillary-driven flow of a droplet on a nonuniformly heated patterned surface. Using a sharp-interface scheme, capable of efficiently modeling the flow over complex surfaces, we perform 2D and 3D finite element simulations for a wide range of substrate wettabilities, i.e., from hydrophilic to superhydrophobic surfaces. Our results demonstrate that the contact angle hysteresis, due to the presence of the solid structures, is responsible for the appearance of a critical thermal gradient beyond which droplet migration is possible; the latter has been reported by experimental observations. The migration velocity as well as the direction of motion strongly depend on the combined action of the net mechanical force along the contact line and the thermocapillary induced flow at the liquid-air interface. We also show that through proper control and design of the substrate wettability, contact angle hysteresis, and induced flow field it is possible to manipulate the droplet dynamics: in particular, controlling its motion along a predefined track or entrapping by a wetting defect a droplet based on its size, as well as providing appropriate conditions for enhanced mixing inside the droplet.

Deviation of sliding drops at a chemical step

The motion of partially wetting liquid drops in contact with a solid surface is strongly affected by contact angle hysteresis and interfacial pinning. However, the majority of models proposed for drops sliding over chemical surface patterns consistently neglect the difference between advancing and receding contact angles. In this article, we present a joint experimental and numerical study of the interaction of gravity-driven drops with a chemical step formed at the junction between a hydrophilic and a hydrophobic region. It demonstrates the strong impact of a contact angle hysteresis contrast on the motion of drops at a linear chemical step. Surprisingly, the smallest driving force required to drag the drop across the step onto the lower hydrophobic surface is not observed at a right angle of incidence. Our model reveals that the non-monotonous response of this passive drop 'filter' is solely due to the higher advancing contact angle on the lower surface, and creates an instance where drop motion is affected by dissipation at the contact line rather than by surface energy.

Rapid, targeted and culture-free viral infectivity assay in drop-based microfluidics

A key viral property is infectivity, and its accurate measurement is crucial for the understanding of viral evolution, disease and treatment. Currently viral infectivity is measured using plaque assays, which involve prolonged culturing of host cells, and whose measurement is unable to differentiate between specific strains and is prone to low number fluctuation. We developed a rapid, targeted and culture-free infectivity assay using high-throughput drop-based microfluidics. Single infectious viruses are incubated in a large number of picoliter drops with host cells for one viral replication cycle followed by in-drop gene-specific amplification to detect infection events. Using murine noroviruses (MNV) as a model system, we measure their infectivity and determine the efficacy of a neutralizing antibody for different variants of MNV. Our results are comparable to traditional plaque-based assays and plaque reduction neutralization tests. However, the fast, low-cost, highly accurate genomic-based assay promises to be a superior method for drug screening and isolation of resistant viral strains. Moreover our technique can be adapted to measuring the infectivity of other pathogens, such as bacteria and fungi.