Maintaining Underwater Cassie State for Sustained Drag Reduction in Channel Flow
Water droplets tend to bead up on rough or textured hydrophobic surfaces by trapping air on the crevices underneath resulting in “Cassie” state of wetting. When a textured hydrophobic surface is immersed in water, the resulting underwater “Cassie” state can lead to significant drag reduction. The entrapped air pockets act as shear free regions and the composite interface consisting of alternate no slip and no shear regions thus formed can deliver substantial drag reduction during flow. The magnitude of drag reduction depends not only on the fractional coverage of air on the surface, but also on the size of the air pockets, with larger sized air pockets facilitating larger drag reduction. It is a common observance that Lotus leaf when kept immersed in water for a few minutes loses its water repellency due to the loss of entrapped air on the surface. Underwater Cassie state on textured hydrophobic surfaces is also not sustainable because of the depletion of air pockets caused by the diffusion of trapped air into water. This causes the drag reduction to diminish with time. Rate of diffusion of air across the water–air interface depends on the concentration gradient of air across the interface. Under flow conditions, removal of entrapped air is further enhanced by convection, leading to more rapid shrinkage of the air pockets. In order to sustain the Cassie state, it is thus necessary to continuously supply air to these air pockets. In this work, we explore the possibility of supplying air to the cavities on the textured surface inside a microchannel by controlling the solubility of air in water close to the surface. The solubility is varied by i) Controlling the absolute pressure inside the channel and ii) Localized heating of the surface To trap uniform air pockets, a textured surface containing a regular array of blind holes is used. The textured surface is generated by photo etching of brass and is rendered hydrophobic through a self-assembled monolayer. The sustainability of the underwater Cassie state of wetting on the surface is studied at various flow conditions. The air trapped on the textured surface is visualized using total internal reflection based technique, with the pressure drop (or drag) being simultaneously measured. Water which is initially saturated with air at atmospheric conditions, when subjected to sub-atmospheric pressures within the channel becomes supersaturated causing the air bubbles to grow in size. Further growth causes the bubbles to merge and eventually detach from the surface. The growth and subsequent merging of the air bubbles leads to a substantial increase in the pressure drop because as the air pockets grow in size, they project into the flow and start obstructing the flow. On the other hand, a pressure above the atmospheric pressure within the channel makes the water undersaturated with air, leading to gradual shrinkage and eventual disappearance of air bubbles. In this case, the air bubbles do cause reduction in the pressure drop with the minimum pressure drop (or maximum drag reduction) occurring when the bubbles are flush with the surface. The rate of growth or decay of air bubbles is found to be significantly dependent on the absolute pressure in the channel. Hence by carefully controlling the absolute pressure, the Cassie state of wetting can be sustained for extended periods of time. A drag reduction of up to 15% was achieved and sustained for a period of over 5 hours. Temperature of water also influences the solubility of air in water with higher temperatures resulting in reduced solubility. Thus locally heating the textured hydrophobic surface causes the air bubbles to grow, with the rate of growth being dependent on the heat input. The effect of trapped air bubbles on thermal transport is also determined by measuring the heat transfer rate through the surface in the presence and absence of trapped air bubbles. Even though the trapped air bubbles do cause a reduction in the heat transfer coefficient by about 10%, a large pressure drop reduction of up to 15% obtained during the experiments helps in circumventing this disadvantage. Hence for the same pressure drop across the channel, the textured hydrophobic surface helps to augment the heat transfer rate. The experiments show that, by varying the solubility of air in water either by controlling the pressure or by local heating, underwater Cassie state of wetting can be sustained on textured hydrophobic surfaces, thus delivering up to 15% drag reduction in both cases for extended periods of time. The results obtained hold important implications towards achieving sustained drag reduction in microfluidic applications.