Superhydrophobicity, drag reduction and microfluidic flow

在厚度在微米量级或更小的微器件中，微薄膜和液体的流动受到固液界面流体动力学特性的强烈影响。本研究的目的是阐明固体表面的疏水性(缺乏润湿性)与微流体流之间的联系，并阐明滑移减阻的机理。在实验的密切指导下，我们将从理论上检验三种不同的结构。鉴于一般边界层流动的重要性和与薄膜的密切联系，我们将首先深入探讨一般边界层流动。与粘性液体流动相反，滑移液体的BL流在性质上是不相似的，因此处理起来困难得多。接下来将研究射流撞击水平板和水跃的流动，因为已经对光滑和波纹平板进行了大量的实验工作。最后，对通道出口附近的微通道流动和微喷流进行了研究。随着微流控器件的广泛应用，人们越来越需要了解固体表面和流动流体之间复杂的相互作用。开发的流体-表面配对可以实现微米级而不是纳米级的滑移长度。如果滑移长度在通道间隙宽度的数量级上，则可以显著提高体积流量，从而显著降低阻力。最近的研究集中在量化滑移长度的大小及其与润湿性和表面粗糙度等参数的依赖关系。我们的主要目标之一是评估滑移对层流和槽道流动中减阻的影响。*我们将研究如何利用超疏水表面(SHSS)来降低层流中的阻力。微尺度表面粗糙度的疏水性防止液体进入表面粗糙度峰值之间的空间，从而产生由柱子支撑的气液界面。因此，在SHSS上的流动中，与固体柱接触的流体不会发生滑移，但微细或纳米特征之间支撑的气液界面基本上是无剪切的。我们打算采用两相(气-液)模型来模拟这种SHSS上的流动。*投入了大量的精力来使用SHSS来设计大滑移以减少阻力。这些表面通过支持液体在其上滑动的无剪切气液界面来减少其接触角滞后，从而增强了液滴的流动性。在层流中，SHSS的使用代表了能够减少大于分子规模的设备中阻力的首批技术之一。这些表面的发展可能会深刻地影响各种重要的现有技术，从微流控设备到海洋船只。我们研究了流体应用范围广泛的SHSS上的流动。

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.