Generalising the mean spherical approximation as a multiscale,nonlinear boundary condition at the solute–solvent interface

ABSTRACT

In this paper, we extend the familiar continuum electrostatic model to incorporate finite-size effects in the solvation layer, by perturbing the usual macroscopic interface condition. The perturbation is based on the mean spherical approximation (MSA), to derive a multiscale solvation-layer interface condition (SLIC/MSA). We show that SLIC/MSA reproduces MSA predictions for Born ions in a variety of polar solvents, including water as well as other protic and aprotic solvents. Importantly, the SLIC/MSA model predicts not only solvation free energies accurately but also solvation entropies, which standard continuum electrostatic models fail to predict. The SLIC/MSA model depends only on the normal electric field at the dielectric boundary, similar to our recent development of a SLIC model for charge-sign hydration asymmetry, and the reformulation of the MSA as an effective boundary condition enables its straightforward application to complex molecules such as proteins, whereas traditionally it is primarily a bulk theory. This work also opens the possibility for other electrolyte models to be incorporated into fast implicit-solvent models of biomolecular electrostatics.     

Electro-Elastic Wetting: A Method to Migrate and Deform Droplets

A new wetting mechanism, termed electro-elastic wetting, and methods to exploit it for droplet manipulation are proposed and demonstrated. The system consists of a droplet of dielectric liquid, an elastic and conductive membrane as its shell, and an electrode-dielectric composite as its substrate. Activation is by an electric field applied between the membrane and the substrate. The equilibrium shape of the droplet is determined by the balance of membrane tension and electrostatic attraction. It is shown that the contact angle of the droplet is governed by a modified Young–Lipmann Equation. It is then demonstrated that it is possible to transport the droplet along a controlled direction, as well as to actively tune its shape, topography, and position by manipulating the spatial distribution of the electrical force.