Wonjae Choi in Non Newtonian Fluids Lab

My Research Interest :

	Nanobubbles on hydrophobic surfaces As the hydrophobic surfaces has a tendency to repel water molecules, there is a very thin ( ~2nm ) less-dense water layer close to hydrophobic surfaces. With a proper condition, this less-dense water layer generates spontaneous air bubbles which are hundreds of nm wide and tens of nm thick. This phenomenon can explain many odd observations related to hydrophobic surfaces and particles, such as drag reduction and long-range attractive force.
	Enhancement of Hydrophobicity Hydrophobicity in nature is a combination of chemistry and surface texture. With a decent hydrophobicity and severe roughness, water cannot penetrate into holes or between pillars and make a composite interface with the surface – a fraction is water/air interface, other fraction is water/solid interface. This composite interface effectively reduces the whole interfacial energy and increases apparent hydrophobicity of the surface. In my 1^st research, I’ve used vertical pillar array for this rough texture, and got 154^o water contact angle.
Super-oleophobic surface Previous vertical texture has succeeded to enhance hydrophobicity, and it acts as an ‘hydrophobicity amplifier’. The limit of the vertical texture is that it can’t work if it is made of a hydrophilic material. As most of commercial surface material is oleophilic, I can’t make a strongly oleophobic surface with the previous approach. To overcome this limit, I have made a very unique nano structure which is under non-disclosure agreement yet. This structure can resist any penetration from any liquid as long as it is not perfectly wetting ( 0^o contact angle ) As a result, I got 160^o contact angle even with octane. This is the only superoleophobic surface as far as I know. Better part of this surface is, the contact angle is less sensitive to the liquid and the hysteresis will always remain at low value