Under the proper conditions it is possible to press together two drops of the same liquid without experiencing coalescence or to press a liquid droplet against a surface normally wetted by the liquid without wetting occurring.  By permanent noncoalescence and nonwetting we distinguish cases in which the phenomena may be observed for unlimited time from transient examples such as two drops of liquid bouncing off one another or a liquid droplet bouncing off a solid wall.  To achieve permanent noncoalescence or nonwetting, a mechanism is needed for establishing a lubricating film of surrounding fluid (usually air) and sustaining this film as the liquid/liquid or solid/liquid surfaces are moved toward each other.

 

One technique for achieving these lubricating films is the use of thermocapillarity, the variation of a liquid’s surface tension with temperature. We have demonstrated that nonwetting may be achieved through non-contact, optical heating and that this may be further employed to levitate droplets above solid surfaces and to translate them along these surfaces.  The implications of this technique for lab-on-a-chip applications will be discussed, along with items of more fundamental scientific interest.

 

G. Paul Neitzel has been a Professor in The George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology since 1990; he presently also serves as Associate Chair for Graduate Studies.  Prior to arriving at Georgia Tech, he served for eleven years on the faculty of the Department of Mechanical and Aerospace Engineering at Arizona State University and worked ten years at the U.S. Army Ballistic Research Laboratory, during which time he received his Ph.D. in fluid mechanics from The Johns Hopkins University.  He has conducted research on the hydrodynamic stability of unsteady swirling flows and flows associated with materials processing, vortex breakdown, suppression of coalescence/wetting and bioreactor fluid dynamics.  He is a Fellow of the American Physical Society and the American Society of Mechanical Engineers, an Associate Fellow of the American Institute of Aeronautics and Astronautics and the recipient of a National Science Foundation Presidential Young Investigator Award and an Alexander von Humboldt Fellowship.  He has served as a visiting professor at Universität Karlsruhe (Germany), Imperial College of Science and Technology (London) and the Université d’Aix-Marseille II and a visiting scientist at Forschungszentrum Karlsruhe (Germany).