Hydrodynamics of capillary imbibition at nanoscale

Abstract

The reduced length scale comparable to molecular dimensions at nanoscale flows, lead to several non-intuitive and fascinating phenomena not seen in classical hydrodynamics. Water is a polar liquid and despite being the most studied liquid, its properties remain mysterious as the characteristic length scale approaches the nano and molecular scales, which introduces further complications in describing nano flow related phenomena like capillary imbibition of nanochannels and nanopores. Capillary imbibition is ubiquitous in nature and has recieved wide spread attention due to its implications in many natural and technologial processes. Capillary forces and related process of liquid imbibition that occur at nanoscale are critical for a wide range of applications like lab-on-a-chip units, precise drug delivery and inkjet printing etc. The better understanding and further development of these novel applications need a detailed knowledge of the surface effects and interfacial interactions which strongly influence the behavior of the fluids at nanoscale, and hence widely known macroscopic theories of flow and mass transport phenomena cannot capture the hydrodynamics at this scale. In relation to this, researchers lack consensus on the explanation of several fundamental aspects like the early effects during the capillary imbibition like the momentum balance of the imbibing fluid, the dynamics of the development of meniscus contact angle, and the filling kinetics during the capillary imbibition in nanoconfinement. Hence, this work aims to contribute to the better understanding and provide explanations to the fundamental aspects of the early stage of capillary filling in nanochannels, as a comprehensive understanding to these phenomena is crucial to design and improve various existing and future nano-devices. The precise control over fluid steering and retention, over these scales, can form the basis for several applications related to efficient fluid transport, mixing of species in nanoconfinement, ultra precise drug delivery and energy conversion and harvesting. Taking this into account, we study the effect of an external electric field in nanocapillary flow of water. Our results suggest that the nanocapillary flow control is possible through changes in solid-liquid interfacial friction and viscosity of water molecules under the influence of external electric field. Most of the fluid transported in biological systems and nanofluidic devices involves electrolyte solutions which adds further complication to understand the physics behind the process and exploit it for technological applications. To address the contribution of type and concentration of ions during nanocapillary imbibition, we study the capillary filling of mono and multivalent ions at different concentrations in nanochannels at zero net charge on the walls. Our results indicate that the capillary filling of electrolyte solutions inside hydrophilic silica nanochannels is governed by the enhanced viscosity of the imbibing solution due to electroviscous effect that depends directly upon the nature and valency of cations. In this study, we develop comprehensive theoretical understanding of the nanocapillary fluid dynamics in hydrophilic channels and its implications that provides a route for futuristic experimental studies. The main contribution of this work will be the better understanding of nanocapillary imbibition at molecular level which has not been possible till now and propose a possible mechanism for flow control during imbibition process.