Functional Surfaces

Wettability-Controlled Functional (e.g., superhydrophobic) Surfaces, which can be realized when the roughened surfaces are chemically treated to have a hydrophobicity, is known to trap the air pockets between the surface topologies when immersed underwater, thereby creating an effective slip on the surface. This phenomena has a wide range of potential applications and we are trying to apply SHPo surfaces for turbulent flow control.

1. Regeneration of the plastron on a SHPo surface through air injection

Experimental setting that can quantify the amount of plastron through total internal reflection

Regeneration regime of random SHPo surface

Air flow rate required for maintaining plastron for sufficient drag reduction

Bubble movement on plane(no coating) and SHPo surface

Raw Aluminum

SHPo Surface

2. Separation control for the flow around a bluff body

Water tunnel experiment setup for the velocity measurement in the flow around a SHPo cylinder

A water droplet on a superhydrophobic surfaces(left), roughened Teflon(middle) and untreated Teflon(right)

Mean flow and normalized Urms contour of acrylic cylinder and superhydrophobic cylinder at ReD=7000

Instantaneous spanwise vorticity contour of acrylic cylinder(left) and superrhydrophobic cylinder(right) at ReD=7000

SEM image of spray coated superhydrophobic surfaces(left) and picture of the superhycrophobic circular cylinder(right)

3. Theoretical modeling of the air-water interface dynamics

Effects of the micro-grate geometry on the critical hydrostatic pressure

Modeling of the plastron on superhydrophobic surfaces (fluctuating pressure in a harmonic function)

Collapse transition on surface geometry and side walls in dynamic state

Effects of surface geometry(left and sidewalls) property(right) on plastron

4. Turbulent flow control around a hydrofoil with SHPo surface

PIV experimental setup for the velocity measurement of the wake around a hydrofoil

Pictures of hydrofoil with smooth surface(a,b) and that with SHPo surface(c,d) in the water tunnel (AOA=5deg, Re=5000)

Instantaneous spanwise vorticity contour of smooth hydrofoil and SHPo hydrofoil at Re=10,000

Normalized mean streamwise velocity and reynolds stress contour of smooth hydrofoil and SHPo hycrofoil; whte soild line-wake bubble (AOA=5deg, Re=10,000 & AOA=10deg, Re=10,000)

5. Experimental measurement of gas-liquid interface using RICM(Reflection Interface Contrast Microscopy)

Setup for RICM and its schematic

Representative results for hole and structure