Session D40 - Poster Session I.
POSTER session, Monday afternoon, March 12
Exhibit Hall, Washington State Convention Center
Moving and steering fluids on a very small length scale, ranging from nanons to microns, is an important aspect of small devices, including MEMS, NEMS and (bio)chem-labs on chips. On large length scales fluids are typically moved by pumping or gravitational action. On small length scales, fluids can be transported by exploiting another driving force: spatial surface free energy gradients. A fluid droplet will experience such a gradient when the surface with which it is in contact exhibits a spatial variation in wettability ("hydrophilicity"). We have recently embarked on the modeling of drops on hetereogenous surfaces with a powerful lattice-gas modeling approach that combines both equilibrium and non-equilibrium techniques (i.e. Monte Carlo and density functional theory) that can include chemical reactions and be applied in both open and closed ensembles. We present modeling results on the unbalanced Young equation using droplets equilibriated on gradient surfaces in the presence of gravity and we will discuss the effects of contact angle hysteresis. We will also report on calculations involving moving droplets on gradient surfaces and, in addition, we will investigate the effects of having present a component that reacts with the surface to vary the wettability.