Soft Matter, Fluidics, and Interfaces Group, Mesa+ Institute for Nanotechnology, Department of Science and Technology, University of Twente, 7500 AE Enschede, The Netherlands.
Proc Natl Acad Sci U S A. 2013 May 21;110(21):8422-6. doi: 10.1073/pnas.1304403110. Epub 2013 May 6.
Tailoring the hydrodynamic boundary condition is essential for both applied and fundamental aspects of drag reduction. Hydrodynamic friction on superhydrophobic substrates providing gas-liquid interfaces can potentially be optimized by controlling the interface geometry. Therefore, establishing stable and optimal interfaces is crucial but rather challenging. Here we present unique superhydrophobic microfluidic devices that allow the presence of stable and controllable microbubbles at the boundary of microchannels. We experimentally and numerically examine the effect of microbubble geometry on the slippage at high resolution. The effective slip length is obtained for a wide range of protrusion angles, θ, of the microbubbles into the flow, using a microparticle image velocimetry technique. Our numerical results reveal a maximum effective slip length, corresponding to a 23% drag reduction at an optimal θ ≈ 10°. In agreement with the simulation results, our measurements correspond to up to 21% drag reduction when θ is in the range of -2° to 12°. The experimental and numerical results reveal a decrease in slip length with increasing protrusion angles when >/~ 10°. Such microfluidic devices with tunable slippage are essential for the amplified interfacial transport of fluids and particles.
定制流体动力边界条件对于减少阻力的应用和基础方面都至关重要。提供气液界面的超疏水基底上的流体动力摩擦可以通过控制界面几何形状来潜在地优化。因此，建立稳定和最佳的界面至关重要，但却颇具挑战性。在这里，我们展示了独特的超疏水微流控装置，允许在微通道边界处存在稳定和可控的微气泡。我们通过微粒子图像测速技术实验和数值研究了微气泡几何形状对高速滑移的影响。我们获得了大范围微气泡突出角θ的有效滑移长度，使用微粒子图像测速技术。我们的数值结果显示，在最佳θ≈10°时，有效滑移长度最大，对应的阻力减少了 23%。与模拟结果一致，当θ在-2°至 12°范围内时，测量结果对应着高达 21%的阻力减少。当θ大于或等于 10°时，实验和数值结果显示滑移长度随突出角的增加而减小。这种具有可调滑移的微流控装置对于增强流体和粒子的界面输运至关重要。
