Takahiro Tsukahara
Tokyo University of Science, Japan
Title: Photo-induced flows relevant to laser-based droplet manipulations
Biography
Biography: Takahiro Tsukahara
Abstract
Non-invasive and non-contact manipulation of micro-scale droplet in air/liquid pool, which is relevant to Medical or Biological Engineering applications and chemical processes using lab-on-chip technology, has increasingly attracted attention in the fi eld of microfl uidics. Several techniques were proposed by utilizing a local variation of interfacial tension, because the eff ects of interfacial phenomena are dominant in microfl uidics relative to the inertia and buoyancy forces. In particular, laser-induced optical force may be used as a non-invasive and precise tool for droplet-based controls, but its force magnitude is limited on the order of a pico newton. Compared to the optical force, an optically-induced thermal Marangoni convection may provide a larger resultant force that provides the nano-newton order force. Th erefore, the photothermal Marangoni convection can be a powerful technique of on-demand bubble/ droplet handling in a micro-channel liquid. If allowed to change physical properties of surfactant solution liquid (e.g., azobenzene) in response to light, the cis-trans photoisomerization can be alternative non-invasive fl uid manipulation without adding heat. The cis-trans photoisomerization is a property that the cis and trans isomers are reversibly changed by light of a specifi c wavelength such as ultraviolet light. As the isomers of diff erent molecular structures are switched by light irradiation, physical properties such as the contact angle and interfacial tension are varied. We have performed direct numerical simulation of multi-phase fl ows of droplets that are accompanied by either photo-induced thermal Marangoni convection or cis-trans photoisomerization, in order to study quantitatively the force and mechanisms relevant to the laser-based droplet manipulation.