Role of Mixed Boundaries on Flow in Open Capillary Channels with Curved Air鈥揥ater Interfaces

详细信息    查看全文

• 作者：Wenjuan Zheng ; Lian-Ping Wang ; Dani Or ; Volha Lazouskaya ; Yan Jin
• 刊名：Langmuir
• 出版年：2012
• 出版时间：September 4, 2012
• 年：2012
• 卷：28
• 期：35
• 页码：12753-12761
• 全文大小：424K
• 年卷期：v.28,no.35(September 4, 2012)
• ISSN：1520-5827

文摘

Flow in unsaturated porous media or in engineered microfluidic systems is dominated by capillary and viscous forces. Consequently, flow regimes may differ markedly from conventional flows, reflecting strong interfacial influences on small bodies of flowing liquids. In this work, we visualized liquid transport patterns in open capillary channels with a range of opening sizes from 0.6 to 5.0 mm using laser scanning confocal microscopy combined with fluorescent latex particles (1.0 渭m) as tracers at a mean velocity of 0.50 mm s^鈥?. The observed velocity profiles indicate limited mobility at the air鈥搘ater interface. The application of the Stokes equation with mixed boundary conditions (i.e., no slip on the channel walls and partial slip or shear stress at the air鈥搘ater interface) clearly illustrates the increasing importance of interfacial shear stress with decreasing channel size. Interfacial shear stress emerges from the velocity gradient from the adjoining no-slip walls to the center where flow is trapped in a region in which capillary forces dominate. In addition, the increased contribution of capillary forces (relative to viscous forces) to flow on the microscale leads to increased interfacial curvature, which, together with interfacial shear stress, affects the velocity distribution and flow pattern (e.g., reverse flow in the contact line region). We found that partial slip, rather than the commonly used stress-free condition, provided a more accurate description of the boundary condition at the confined air鈥搘ater interface, reflecting the key role that surface/interface effects play in controlling flow behavior on the nanoscale and microscale.