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Drag Reduction and Drop Impact Dynamics on Superhydrophobic and Liquid-Infused Surfaces

Jeong-Hyun Kim, University of Massachusetts Amherst

Friday, April 28, 2017
1:30 p.m.
Hopeman 224

Reducing drag in fluid flow has been one of the most widely studied topics in fluid dynamics due to significant impact on improving operational efficiencies and cutting cost in applications from aerospace, automotive and naval industries. Over the past two decades, superhydrophobic surfaces have been in the spotlight due to their ability to reduce frictional drag on the wall surface in both laminar and turbulent flows. Despite the extensive previous research on superhydrophobic surfaces, there are still a number of remaining questions.

In this talk, we will discuss how shapes of an air-water interface on the superhydrophobic surfaces affect the drag reduction. A novel microfluidic device was designed to incorporate superhydrophobic pillars. The shape of the air-water interface varied with change to the static pressure in the channel. Slip along interface trapped within the superhydrophobic surface was found to result in significant drag reduction. However, the variation in flow geometry due to changes in the bubble shape dominated the effects of the slip. Reducing the bubble size amplified drag reduction, while increasing the bubble size reduced drag reduction and even led to drag enhancement.

Liquid-infused superhydrophobic surfaces have recently been developed and shown great promise to overcome many of the inherent limitations of the conventional air-infused superhydrophobic surfaces such as failure of the air-water interface under high pressure and limited oleophobicity. In this talk, we will also look into how water droplets advance and retract from the liquid-infused superhydrophobic surfaces by changing the viscosity ratio between the water droplet and infused oil. The increase in the viscosity ratio was found to increase a maximum diameter and a spreading/retraction rates of droplets.

Taken together, the two experimental research presented in this talk have allowed us to better understand and optimize the design of air-infused and liquid-infused superhydrophobic surfaces for drag reduction, droplet spreading and liquid mobility. With this new-found knowledge, a sense of new innovative ideas and applications has been or soon will be realized.

Bio:

Dr. Jeong-Hyun Kim is a postdoctoral research associate in the Department of Mechanical and Industrial Engineering at the University of Massachusetts Amherst. His current research topic is extensional rheology measurements of inkjet fluids using a microfluidic rheometry and a dripping capillary breakup extensional rheometer (D-CaBER). His research interests lie in interfacial flows, microfluidics, rheology, and conventional fluid dynamics including aerodynamics and turbulence. He studied wake flow of blunt body (i.e. external rear view mirrors of an automobile) and slender body (i.e. fixed and rotary wing) by measuring statistical and spectral characteristics of the flow at Yeungnam University in South Korea where he earned his Bachelor and Master degrees. Then, he joined non-Newtonian fluid dynamics laboratory at the University of Massachusetts Amherst and studied dynamic wetting and drag reduction on superhydrophobic surfaces. He received his Ph.D. at the University of Massachusetts Amherst in 2016.