Momentum and Heat Transport at Solid-Liquid Interfaces

he combination of micro-channels for fluid delivery and micro-electronic devices for flow control has led to the creation of highly sensible measuring devices, such as lab-on-a-chip for the examination of minuscule particles contained in a flow stream. The biomedical community has taken advantage of these technologies in applications such as DNA sampling, high precision drug delivery, and low-cost cell analysis for disease diagnostics. In addition to biomedical applications, understanding of nanoconfined flows is critical to bring improvement in many applications such as electro-chemical energy conversion in batteries, energy harvesting from salinity gradients, selective filtration and separation using thin barrier films, and nanofluidics diodes and transistors.

The dynamics of thin confined fluids are important in many industrial processes as the tolerances of machine parts are reduced and operating speeds increase as appropriate lubrication of small gaps is needed. Highly confined liquids are likely to experience high shear rates causing overheating and temperature increase of the system. A good understanding of thermal energy transport at solid-liquid interfaces is required for systems where heat transfer at interfaces is critical for system performance, e.g., evaporation, condensation, etc.

Surface effects, such as wettability and roughness, govern the transport in nano-confined liquids. The wettability of solid surfaces, usually characterized by the static contact angle, has been used to describe the hydrodynamic boundary condition and thermal transport at solid-liquid interfaces. A correlation between contact angle and the thermal boundary conductance (TBC) has been consistently reported in various experimental and numerical investigations. The number of experimental reports on solid-liquid thermal transport are limited due to the difficulties in measuring the TBC. The high conductivity of the interface and the difficulty in fabricating samples, characterizing interface structures, and preparing experimental set-up to measure TBC at solid-liquid interfaces are the reasons for limited experimental studies and also for the acceptance of correlation between contact angle and the TBC.

Recently, Bladimir Ramos-Alvarado a doctoral candidate, Satish Kumar an Associate professor in the George W. Woodruff School, and President G. P. Peterson, reported on a new perspective for characterizing the thermal transport at solid-liquid interfaces using atomistic models for thin water film confined between Si slabs. In this investigation, atomistic modeling and theoretical analyses were conducted in an effort to unravel the nature of the momentum and heat transport at solid-liquid interfaces. Momentum and thermal transport at the interface of nano-confined liquids are affected by the solid-liquid affinity. Likewise, the affinity between both phases modifies the structure, or layering of liquids near the solid surface. Therefore, the efficiency of the momentum and thermal transport is strongly determined by the interfacial liquid structure and the depletion of liquid particles. The researchers reported that the contact angle, a macroscopic property, cannot be used to completely describe the hydrodynamic boundary condition and thermal transport for nanoscale interfaces. They reported for the first time the anisotropy of momentum and interfacial heat transfer in silicon-water systems and the breakdown of the conventional relationships between the slip length and TBC with the contact angle. Bladimir Ramos-Alvarado mentioned that a more universal description of the interfacial transport characteristics was attained when the concentration and depletion of liquid particles at the interface were properly quantified and correlated with a solid theoretical background. The findings were reported on August 19 in the Journal of Physical Chemistry Letters.

The characterization of interfacial liquid depletion allowed to uniformly describe thermal transport in different silicon planes in contact with water, as well as graphene-coated silicon surfaces having the same wettability as the bare silicon surfaces. “We reconciled interfacial transport property calculations with a conservative definition of the liquid density depletion at the interface, thus, we are providing a set of scaling laws that could allow us to tailor the performance of micro- and nano-fluidic devices”, Satish Kumar, and G. P. Peterson remarked.

These groundbreaking results provide the foundations to theoretically tackle the challenging task of describing interfacial transport in nanoconfined liquids; likewise, these results suggest the possibility of developing highly efficient nanofluidics devices with applications in the biomedical sciences, the energy sector, ultrafast flow delivery systems, condensation/evaporation, thermal treatment of cancer, etc., by means of tailoring the properties of the interfaces in order to achieve low friction flows or highly conductive solid-liquid interfaces. Citation: Bladimir Ramos-Alvarado, Satish Kumar, and G. P. Peterson, Solid–Liquid Thermal Transport and Its Relationship with Wettability and the Interfacial Liquid Structure, J. Phys. Chem. Lett., 2016, 7, pp 3497–3501. DOI: 10.1021/acs.jpclett.6b01605