Drop race: How electrostatic forces influence drop motion

Water drops sliding down inclined planes are an everyday phenomenon and are important in many technical applications. Previous understanding is that the motion is mainly dictated by viscous and capillary forces. Here we demonstrate that, in addition to these forces, drops on hydrophobic surfaces are affected by self-generated electrostatic forces. In a novel approach to determine forces on moving drops we imaged their trajectory when sliding down a tilted surface and apply the equation of motion. We found that drop motion on low-permittivity substrates is significantly influenced by electrostatic forces. Sliding drops deposit a negative charge on the surface, which interact with the positively charged drops. We derive an analytical model to describe the force and validate it by numerical computations. The results indicate how to describe and facilitate drop motion in applications, such as in microfluidics, water management on car surfaces, and the creation of sliding drop electrical generators.

Morphological Evolution and Interfacial Effects Analysis of Drop Motion in Transverse Vibration of Inclined Plate

Based on experimental and simulation research, analysis of the morphological evolution and interfacial effects of drop motion in the transverse vibration of inclined micro-textured plate are studied. Experimental results show the morphological evolution of drop involves an oscillation stage, spreading and migration stage, and infiltration stage. The spread diameter increases from the initial 3.02 to 5.12 mm. Meanwhile, based on the real experimental morphology of the drop dynamic wettability, a two-phase flow theoretical model of motion evolution of forced vibration drop was established to simulate the drop spreading process. The analysis result shows the calculated results are close to the experimental results, and the on micro-textured surface is faster spreading coefficient is S-shaped and increases with the increase of time. The spreading velocity than the smooth one, and there is low-speed rotating airflow in the micro-textured pit. The vortex cushion effect and vortex wheel effect are the main reasons for the acceleration of drop motion. Two interfacial effects reduce the friction resistance and impel fluid movement.

Simulating collisions of charged cloud drops in an ABC flow

<p>Calculating the electric force between cloud drops is not straightforward. Since water drops are conducting, the electric force is not just simply the force between point charges, but instead the charge in each drop induces an infinite number of image charges in the other drop. The effect of these image charges can cause the electric force between two like charged cloud drops to become attractive on very short distances, when only applying Coulomb's law would result in a repulsive force. The attractive effect of image charges could potentially increase the collision rate of cloud drops. Within the United Arab Emirates Rain Enhancement Program (UAE REP) we are investigating the potential for rain enhancement by charging clouds.</p><p>Simulating the behaviour of cloud drops is numerically very expensive. A large number of drops needs to be simulated to obtain stable collision statistics. Additionally, the drops move in a complex turbulent environment with eddies spanning several orders of magnitude in size. Simulating the turbulent flow alone is an expensive task. Because of the typical sizes of cloud drops, their motion is predominantly influenced by the smallest turbulent scales in the flow. Therefore, Direct Numerical Simulation (DNS) is necessary and used to simulate the influence of turbulent flow on drop motion. In this work, instead of using DNS, we use an ABC flow to simulate the turbulent effect on cloud drops. This simple approximation for the turbulent flow allows to simulate the drop motion using much less computational resources then needed by DNS and therefore, allows to include the very expensive effect of electrical drop charge in our simulation of colliding drops in a turbulent environment.</p><p>To investigate the effect of electrical charge on drop collisions, a Lagrangian particle code for the interaction of cloud drops is used. It calculates the motion of individual drops based on the aerodynamical force due to the ABC flow and the gravitational force and registers drop collisions from which collision statistics can be calculated. In the cloud model all drops carry positive charges. The effect of the electric force is calculated by an approximation which uses Coulomb's law for the effect of the point charges and an additional term to approximate the effect of image charges which produce an attractive force on short distance.</p><p>Results for the collision kernel with and without charge will be presented. The effect of the additional term to Coulomb's law will be shown for different drop sizes and drop charges. It will be discussed if the attractive force for like charged drops on short distances can lead to an enhancement in drop collisions and under which conditions the effect is the largest.</p>

Validation of Numerical Simulation of Drop Motion on Surfaces With Micro Patterns

Numerical simulation of drop motion on surfaces with micro patterns is conducted. The results are compared with existing experimental and analytical studies to validate the reliability of the numerical simulations. In the comparison of the liquid phase morphology on a surface with straight grooves, it is confirmed that a variety of liquid shapes, including droplets, filaments with positive/negative Laplace pressure and so on are successfully reproduced by the numerical simulation. Moreover, the numerically observed transition between these morphologies in a broad range of the groove aspect ratio and the static contact angle agrees with the morphology diagram which is obtained by a semi-analytic approach based on the surface free energy minimization. Furthermore, in the comparison of the spreading behaviors of a liquid drop on a surface with square pillars, it is shown that the numerical simulations can predict the time-dependent drop deformation during the spreading process. The comparison of the length of two spreading modes shows a quantitative agreement with the experimental results.

Electrical switching of a surfactant coated drop in Poiseuille flow

Electrical effects can impart a cross-stream component to drop motion in a pressure-driven flow, due to either an asymmetric charge distribution or shape deformation. However, surfactant-mediated alterations in such migration characteristics remain unexplored. By accounting for three-dimensionality in the drop motion, we analytically demonstrate here a non-trivial switching of drop migration with the aid of a surfactant coating on its surface. We establish this phenomenon as controllable by exploiting an interconnected interplay between the hydrodynamic stress, electrical stress and Marangoni stress, manifested so as to achieve a net interfacial force balance. Our results reveal that under different combinations of electrical conductivity and permittivity ratios, the relative strength of the electric stress with respect to the hydrodynamic stress, the applied electric field direction and the surfactants alter the longitudinal and cross-stream velocity components of the droplets differently. The effect of drop deformation on its speed is found to be altered with the increased sensitivity of the surface tension to the surfactant concentration, depending on the competing effects of the electrohydrodynamic flow modification and the tip stretching phenomenon. Further, with a suitable choice of electrical property ratios, the Marangoni effects can be exploited to direct the drop in reaching a final transverse position towards or away from the channel centreline. These results may turn out to be of immense consequence in providing an insight to the underlying complex physical mechanisms dictating an intricate control on the drop motion in different directions.