Photon Control of Liquid Motion on Reversibly Photoresponsive Surfaces

Abstract

The movement of a liquid droplet on a flat surface functionalized with a photochromic azobenzene may be driven by the irradiation of spatially distinct areas of the drop with different UV and visible light fluxes to create a gradient in the surface tension. In order to better understand and control this phenomenon, we have measured the wetting characteristics of these surfaces for a variety of liquids after UV and visible light irradiation. The results are used to approximate the components of the azobenzene surface energy under UV and visible light using the van Oss-Chaudhury-Good equation. These components, in combination with liquid parameters, allow one to estimate the strength of the surface interaction as given by the advancing contact angle for various liquids. The azobenzene monolayers were formed on smooth air-oxidized Si surfaces through 3-aminopropylmethyldiethoxysilane linkages. The experimental advancing and receding contact angles were determined following azobenzene photoisomerization under visible and ultraviolet (UV) light. Reversible light-induced advancing contact-angle changes ranging from 8 to 16 degrees were observed. A large reversible change in contact angle by photoswitching of 12.4 degrees was achieved for water. The millimeter-scale transport of 5 microL droplets of certain liquids was achieved by creating a spatial gradient in visible/UV light across the droplets. A criterion for light-induced motion of droplets is shown to be consistent with the response of a variety of liquids. The type of light-driven fluid movement observed could have applications in microfluidic devices.

cis–trans Isomerisation of Substituted Aromatic Imines: A Comparative Experimental and Theoretical Study

“…Recently, these photo-sensitive systems were mounted on solid surfaces either by chemisorption, in an upright position, [2][3][4][5] or physisorption, in which case scanning tunnelling microscopy (STM) [6,7] or optically [8] induced cis-trans isomerisations of molecules lying flat on the surface have already been observed. These results show the capabilities of these systems to act as nanoscale molecular motors.…”

Light Responsive Polymer Membranes: A Review

“…This means that light-induced contact angles should be greater than contact angle hysteresis (Δ θ = θ adv − θ rec ) in the trans state [69]. In this respect, it is instructive to describe the equations that express the forces for the light-driven liquid motion.…”

Photoresponsive thin films containing an azobenzene derivative intercalated with a layered double hydroxide

“…The reversible change in the sessile water contact angle is attributed to the trans-cis isomerization of AO5 triggered by UV and visible light. It is known that there is a significant difference in dipole moments (μ) between the trans and cis azobenzene isomers [35,36], and the film containing the cis-isomer (high polarity) is considerably more hydrophilic, whereas the trans-isomer (low polarity) makes the surface more hydrophobic. The reversible photoinduced change in the sessile water contact angle was found to be ∼11°, much larger than other reported values.…”