The viscosity of water under an external electric field of 0.00-0.90 V/nm was studied using both molecular dynamics simulations and atomistic modeling accounting for intermolecular potentials. For all temperatures investigated, the water viscosity becomes anisotropic under an electric field: the viscosity component parallel to the field increases monotonically with the field strength, E, while the viscosity perpendicular to the field first decreases and then increases with E. This anisotropy is believed to be mainly caused by the redistribution of hydrogen bonds under the electric field. The preferred orientation of hydrogen bonds along the field direction leads to an increase of the energy barrier of a water molecule to its neighboring site, and hence increases the viscosity in that direction. However, the probability of hydrogen bonds perpendicular to the electric field decreases with E, together with the increase of the average number of hydrogen bonds per molecule, causing the perpendicular component of water viscosity to first decrease and then increase with the electric field.

No abstract

No abstract

The wetting kinetics of droplets on lyophilic pillar-arrayed substrates is the driving mechanism of several natural phenomena (e.g., insect capturing by Nepenthes) and many industrial technologies (e.g., gas-liquid separation). For a lyophilic pillar-arrayed surface, a fringe film is formed ahead of the contact line, resulting in distinct wetting kinetics, which needs further investigation. In this study, Si(100) substrates with square micropillars were used to investigate the early spreading of droplets on lyophilic pillar-arrayed surfaces through the droplet-spreading method. A fringe film was observed ahead of the contact line for micropillar-arrayed surfaces. The spreading radius was enhanced by micropillars and mainly caused by liquid penetration into the pillar forest, resulting in alteration of the dissipation mechanism. The early spreading of droplets on lyophilic micropillar-arrayed surface was affected only by the solid fraction and independent of the pillar height. A semitheoretical model without adjustable parameters was established on the basis of the global energetic equation, considering the local dissipation, viscous dissipation, and the dissipation in the precursor film. The prediction of the model agrees with the experimental results. Our semitheoretical model may aid in predicting the wetting kinetics on lyophilic pillar-arrayed substrates and assist the design of pillar-arrayed surfaces in practical applications.

No abstract