Journal of Mechanical Science and Technology 29 (4) (2015) 1681~1688 www.springerlink.com/content/1738-494x DOI 10.1007/s12206-015-0340-6 Nano-scale liquid film sheared between strong wetting surfaces: effects of interface region on the flow † Truong Quoc Vo, BooSeong Park, ChoHee Park and BoHung Kim * School of Mechanical Engineering, University of Ulsan, Ulsan, 680-749, Korea (Manuscript Received September 1, 2014; Revised December 11, 2014; Accepted December 22, 2014) ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Abstract In this paper, we use molecular dynamics (MD) simulations to investigate changes in fluid flow at a solid/liquid interface. The flow is driven by shearing FCC structured solid molecular walls under isothermal conditions using previously developed interactive thermal wall models. For the nano-scale thin liquid film flows, a fluid molecular layer attached to the wall molecules behaves as an extended wall layer, which induces increased shearing in the middle of the fluid by reducing the width of the flow region. Small variations in molecular diameter length at the interface significantly affect flow characteristics. Shear locking on strong wetting surfaces caused by the dynamic structuring of fluid molecules (i.e., the fluid molecules layering on the solid surface due to the wall force field) increases the density and viscosity and decreases the shear rate and the heat dissipation ratio on the interface, which are important in nano-scale fluid flow analysis. Keywords: Nano-scale fluid flow; Solid-liquid interface; Shear locking; Liquid layering; Boundary slip ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction Nano-/micro-scale liquid flow on solid surfaces is an impor- tant part of micro/nano-fabrication processes (i.e., photo- resistant coating), liquid film lubrications, drug delivery, and selective absorption [1]. However, the mechanisms of nano- scale thin film flows and interface phenomena on the solid substrates are not fully understood. Experimental evidence [2] shows that a strong wetting surface results in dynamic struc- turing of the fluid near the solid surface as the thin liquid film flows and advances as a series of distinct molecular layers. For such sub-micro/-nano scale fluid flows, the layering of the fluid molecules on the strong wetting surface is an important factor influencing fluid properties at the solid/liquid interface, including the density, viscosity and thermal conductivity [3, 4]. For nano-scale thin film fluid flows, the surface effects domi- nate the nano-scale conduits due to their miniscule dimensions compared to the force penetration distance induced by the intermolecular force fields of the wall molecules. Investiga- tions of momentum transport at liquid-solid interfaces have shown velocity-slip and -stick conditions depending on the wall-fluid interaction strength, fluid/wall density, and shear rate [5-9]. Recent studies have shown that the thermal oscilla- tion and crystal bonding stiffness of solid molecules affect the slip length for specific surface wettabilities [10, 11]. In addi- tion, significant variations in the diffusion coefficient, shear viscosity, and thermal conductivity have been reported de- pending on the wall/fluid interaction strength [12-14]. In par- ticular, the layering of fluid molecules on strong wetting sur- faces is a well-known phenomenon. However, the influence of layered liquid molecules on flow properties of strong wetting interfaces has been less frequently studied compared to liquid slips on weak wetting interfaces. Layering (Dynamic structur- ing) of fluid molecules on the solid surface is not prominent in most macroscopic problems but is important in multi-scale methods or nano-scale fluid flow problems where the contin- uum approach breaks down and the analysis is sensitive to variations on the interface/flow boundary layer [15, 16]. This study demonstrates variations in viscosity, shear rate, and heat dissipation as a function of wettability caused by the dynamic structuring of fluid molecules on strong wetting sur- faces with shear locking (Negative slip). Our MD simulation implements coupled molecular modeling of momentum and heat transport in the flow to calculate the properties above. In order to investigate changes in fluid flow on a molecular scale, we performed molecular dynamics (MD) simulations, driving the flow by shearing FCC molecular walls under isothermal conditions using a previously developed interactive thermal wall mode [17] with various strong surface wettabilities but the same wall shearing conditions. 2. Theoretical background and model description In the case of shear driven flow, the momentum and heat * Corresponding author. Tel.: +82 52 259 2705, Fax.: +82 52 259 1680 E-mail address: bohungk@ulsan.ac.kr † Recommended by Associate Editor Suk Goo Yoon © KSME & Springer 2015