Grants and Contributions:

Grant or Award spanning more than one fiscal year. (2017-2018 to 2022-2023)

The flow of microfilms and liquids in microdevices, with thickness on the order of microns or smaller, is strongly influenced by the hydrodynamic characteristics at the solid-liquid interface. The aim of our research is to clarify the connection between the hydrophobicity (lack of wettability) of the solid surface and microfluidic flow, and elucidate the mechanism of drag reduction resulting from slip. Three different configurations will be examined theoretically, with close guidance from experiment. Given its fundamental importance and close connection to thin films, general boundary-layer (BL) flow will be explored first in some depth. Contrary to adhering liquid flow, the BL flow of slipping liquid is non-similar in character, and therefore much more difficult to treat. The flow of a jet impinging on a horizontal plate and hydraulic jump will be studied next, as extensive experimental work has been performed for smooth and corrugated plates. Finally, microchannel flow and micro-jet flow will be examined near the channel exit.

As microfluidic devices are widely used, there is growing need to understand the intricate interaction between the solid surface and the flowing fluid. Fluid-surface parings are developed that can achieve slip lengths on the order of micrometers rather than nanometers. The volume flow rate can be significantly enhanced if the slip length is on the order of the channel gap width, leading to significant reduction in drag. Recent studies have focused on quantifying the magnitude of the slip length and its dependence on parameters such as wettability and surface roughness. One of our main objectives is to assess the influence of slip on drag reduction in BL and channel flows.

We will examine how Superhydrophobic surfaces (SHSs) can be used to reduce drag in laminar flows. The hydrophobicity of the microscale surface roughness prevents the liquid from moving into the space between the peaks of the surface roughness, resulting in a gas-liquid interface supported by the posts. Consequently, in flows over SHSs, the fluid in contact with the solid posts experiences no slip, but the gas-liquid interfaces supported between the micro- or nanofeatures are essentially shear-free. We intend to adopt a two-phase (gas-liquid) model to mimic the flow over such SHSs.

Much effort is invested towards the use of SHSs to engineer large slip to reduce drag. These surfaces enhance the mobility of drops by reducing their contact-angle hysteresis by supporting a shear-free gas-liquid interface over which liquid slips. In laminar flows, the use of SHSs represents one of the first technologies capable of reducing drag in devices that are larger than the molecular scale. The development of these surfaces could profoundly affect a variety of important existing technologies, from microfluidic devices to marine vessels. We study the flow on SHSs over a broad range of fluid applications.