Light induced manipulation of isotropic and anisotropic liquids absorbed on photosensitive surfaces

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The general goal of our proposal is to study the behavior of droplets, consisting of isotropic or anisotropic (liquid crystalline) liquids on photo-switchable polymer substrates, the wetting properties of which can be changed locally, dynamically and reversibly in response to suitable optical stimuli. In the long run, we shall create an adaptable platform on which elementary operations on adsorbed liquid droplets such as merging, splitting, mixing or transport can be established. The photosensitive substrate consists of azobenzene containing polymers. Only gentle irradiation is needed to trigger significant changes in topography and surface energy. It is well known that when azobenzene containing polymer films are irradiated with optical interference patterns the film topography changes drastically to form surface relief gratings: in the simplest case, the topography mimics the intensity distribution and deforms into a wave-like, sinusoidal manner, the amplitude of which may be comparable to the total film thickness. This process takes places in the glassy state without softening. We have previously shown that these gratings can be made to appear or disappear or even “set in motion”, that is, be made to move like ripples across water by exploiting repetitive polarization changes in the two interfering beams. With the current project, we shall introduce two new aspects to the study of functionalized surfaces with switchable wetting properties. First, adsorbed liquid droplets will be manipulated by finite, heterogeneous patterns of varying wettability; these changes are completely reversible rendering the system reconfigurable. Second, the dynamics by which changes in wettability occur will be considered explicitly. Inherent to our systems there are two time scales: surface energies are expected to change almost instantaneously, but the evolving changes in texture and topography will take significantly longer. These latter aspects may significantly influence how a convenient irradiation protocol may be defined to achieve swift operations on a droplet, additionally impacted by other conditions, such as nature of azobenzene molecule, polymer and the extent of polymer deformation.