Research stay at UC3M: DEPARTMENT OF MATHEMATICS
Project: Over the last ten years it has been realised that fluids adsorbed on such structured surfaces, crucial for an understanding micro-fluidics, exhibit a host of novel interfacial phase transitions which reveal the dramatic and suprising influence that substrate geometry has on wetting. Despite these advances the nature of fluid adsorption at structures surfaces and in confinement is still in its infancy. Consider, for example, a large volume of liquid in a tall vertical capillary-slit or pore which is capped at its bottom. We initally hold the capillary vertically and the ask what happens when the capillary is turned to the horizontal? Experience tells us the liquid will drain from the open end if the capillary is wide, as when water escapes from a tipped glass, but remains trapped if it is narrow, such as liquid in a drinking straw. It is surprising that this basic aspect of capillarity, in particular the emptying phase transition that occurs as the slit is widened, has not been investigated in depth. We wish to further study this phenomena and other examples of fluid adsorption at structured walls.In particular:
A) The study of emptying transitions in pores of different cross section including, circular, ellipitical and triangular. Our preliminary findings are highly intriguing. Depending on the specific shape, emptying transitions can occur even in the absence of a gravitational field i.e. for microscopically small channels. In this case the of the order of the phase transition and the nature of fluctuation effects is unknown but will be entirely different to that of the mesoscopic capillary slit. So, the question is, how do microscopic pores empty depending on their shape?
B) Similarly one may also generalise capillary emptying by considering the micropatterning of the capillary slit/pore walls by for example decorating the surfaces with hyrodophobic and hydrophillic regions? How does this influence the emptying? Can the transition be made first order - in which case the controlled jumping of the meniscus between two different states may be considered a micro fluidic switch. Our theoretical studies of these first two projects will be done in tandem with experiments at the University of Oxford, U.K.
C) The order of the wedge filling transition in acute wedges. Recently it was predicted that continuous wedge filling transitions become first-order as the wedge is made more acute, due to the self interaction of the meniscus - or equivalently the interaction between the wetting films on either side of the wedge . Numerical confirmation of this has recently appeared but the tricritical angle was lower than expected. This we shall investigate using our Non-local Interfacial Hamiltonian and explicitly incorporate the wave vector dependent surface tension arising in different density functional descriptions.
Stay Period: FEB 13 - AUG 13