Measurement and Scaling of Hydrodynamic Interactions in the Presence of Draining Channels
Measurement and Scaling of Hydrodynamic Interactions in the Presence of Draining Channels
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DOI：
10.1021/la303508x
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发表时间：
2012-10-16
期刊：
影响因子：
3.9
通讯作者：
Frechette, Joelle
中科院分区：
文献类型：
--
作者：
Gupta, Rohini;Frechette, Joelle
树蛙粘附和运动的核心是它们的结构化趾垫，它由一系列 10 μm 六角形上皮细胞组成，这些上皮细胞被 1 μm 宽、10 μm 深的互连通道隔开。已经提出，通道有利于排出脚趾垫和接触表面之间截留的多余流体，从而减少接近过程中的流体动力排斥。我们使用表面力装置对从结构化表面和光滑表面之间的间隙排出流体期间的正常流体动力相互作用进行直接力测量。结构化表面由圆柱柱的六边形阵列组成，代表互连通道的网络。测得的水动力排水力与雷诺理论对大间距光滑表面的预测一致。在低于某个临界分离 (h(c)) 时观察到与理论的偏差，其特征是流体动力排斥力的减少，该临界分离与驱动速度无关。我们采用尺度分析来建立结构特征（通道深度、宽度和柱直径）与偏差发生的临界间距之间的关系。我们发现我们的实验和尺度分析之间存在一致性，这使我们能够估计特征长度尺度，该尺度对应于从径向挤出标称接触区域的流体到通过互连通道网络被挤出的过渡。
Central to the adhesion and locomotion of tree frogs are their structured toe pads, which consist of an array of 10 mu m hexagonal epithelial cells separated by interconnected channels that are 1 mu m wide and 10 mu m deep. It has been proposed that the channels facilitate the drainage of excess fluid trapped between the toe pads and the contacting surface, and thus reduce the hydrodynamic repulsion during approach. We performed direct force measurement of the normal hydrodynamic interactions during the drainage of fluid from the gap between a structured and a smooth surface using surface force apparatus. The structured surface consisted of a hexagonal array of cylindrical posts to represent the network of interconnected channels. The measured hydrodynamic drainage forces agree with the predictions from Reynolds' theory for smooth surfaces at large separations. Deviations from theory, characterized by a reduction in the hydrodynamic repulsion, are observed below some critical separation (h(c)), which is independent of drive velocity. We employ a scaling analysis to establish the relationship between structural features (channel depth, width, and post diameter) and the critical separation for the onset of deviations. We find agreement between our experiments and the scaling analysis, which allows us to estimate a characteristic length scale that corresponds to the transition from the fluid being radially squeezed out of the nominal contact area to being squeezed out through the network of interconnected channels.