Vibration-Induced Wenzel to Cassie Transition on a Superhydrophobic Surface

Speaker: 
Jonathan Boreyko
Date of Seminar: 
Thu, 2008-10-30 12:00
Semester & Year: 
Fall 2008
Seminar Location: 
Hudson Hall Room 216
Seminar Contact(s): 
Elizabeth Irish eri@duke.edu or Justin Jaworski jwj@duke.edu
Special Instructions: 
Lunch and beverages will be served
A drop on a roughened surface can exhibit either the Cassie state, where the drop sits on the air-filled textures, or the Wenzel state, where the drop wets the cavities of the textures. The superhydrophobic Cassie state, exhibiting a small contact angle hysteresis and high drop mobility, is the desired state for water repellant surfaces. Although the Cassie state is the stable configuration for superhydrophobic surfaces, condensation often forms pinned in a metastable Wenzel state, greatly diminishing the surface’s superhydrophobicity. An energy barrier must be crossed to transition metastable Wenzel drops to the stable Cassie state, which has never previously been accomplished without resorting to liquid-vapor phase change. Using metastable Wenzel drops on a superhydrophobic lotus leaf, we show that mechanical vibration can be used to cross this energy barrier and obtain a stable Cassie state. Water drops were trapped in the metastable Wenzel state in two different ways: by exploiting the differential evaporation rate of water and ethanol; and by condensing the lotus leaf. In both cases a complete Wenzel to Cassie dewetting transition is achieved. This dewetting transition occurs when the drop’s kinetic energy overcomes the work of adhesion pinning the drop to the surface. Minimal external forcing is required when the vibration is in resonance with the drop. The vibration-induced Wenzel to Cassie transition has been shown to restore a surface’s superhydrophobicity ever after condensation and is applicable to a variety of engineering systems requiring sustained antidew superhydrophobicity.