Superhydrophobicity, drag reduction and microfluidic flow

微薄膜和液体在微器件中的流动，具有微米或更小的厚度，强烈地受到固液界面处的流体动力学特性的影响。我们的研究的目的是澄清之间的连接的疏水性（缺乏润湿性）的固体表面和微流体流动，并阐明减阻的机制，导致滑移。三种不同的配置将在理论上进行检查，并从实验密切指导。鉴于一般边界层（BL）流动的根本重要性及其与薄膜的密切联系，我们将首先深入探讨一般边界层（BL）流动。相反，粘附液体流动，滑移液体的BL流是不相似的性质，因此更难以治疗。射流冲击水平板和水跃的流动将研究下一步，作为广泛的实验工作已经进行了光滑和波纹板。最后，微通道流动和微射流将检查附近的通道出口。 随着微流体器件的广泛应用，越来越需要了解固体表面和流动流体之间的复杂相互作用。流体表面配对的发展，可以实现微米而不是纳米量级的滑移长度。如果滑移长度是通道间隙宽度的数量级，则体积流率可以显著提高，从而导致阻力的显著减小。最近的研究集中在量化的滑移长度的大小和它的依赖参数，如润湿性和表面粗糙度。我们的主要目标之一是评估滑移对BL和槽道流中减阻的影响。 我们将研究如何超疏水表面（SHS）可以用来减少层流中的阻力。微尺度表面粗糙度的疏水性防止液体移动到表面粗糙度的峰之间的空间中，导致由柱支撑的气液界面。因此，在SHS上的流动中，与固体柱接触的流体不经历滑动，但是支撑在微米或纳米特征之间的气-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SHS的流动。 人们投入了大量的精力来使用SHS来设计大滑移以减少阻力。这些表面通过支撑液体在其上滑动的无剪切气-液界面来减少液滴的接触角滞后，从而增强液滴的流动性。在层流中，SHS的使用代表了能够在大于分子尺度的装置中减少阻力的首批技术之一。这些表面的发展可能会深刻影响各种重要的现有技术，从微流体设备到海洋船舶。我们研究了SHS在广泛的流体应用中的流动。

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.