Development of optothermal fluidic platforms for nanoparticle patterning and light modulation

Abstract

Fluid-fluid interfaces are emerging as a potential alternative to solid-based platforms, newlineowing to their dynamic, reconfigurable, and self-healing capabilities, with applications newlineranging from reconfigurable optical devices to colloidal assembly. An interface between newlinetwo fluids can be actuated utilizing external stimuli like the electric field, magnetic field, newlinetemperature, pressure, and light. Among them, light-induced actuation is advantageous newlineas they provide a non-contact and programmable mode of manipulating the interface. In newlinecomparison, all the other actuation techniques necessitate peripheral components/specific newlinefluids, limiting their applicability. This PhD thesis addressed two challenging problems newlinein the field of microfluidics, i) Light-directed patterning of nanoparticles/binary colloids newlineand ii) Reconfigurable light modulation by realizing optical control over the temperature newlinedistribution of the fluidic systems. newlineIn the first phase of the thesis, an efficient optothermal strategy to realize the ondemand assembly of Gold nanoparticles (Au NPs) and Au-polymer hybrid structures newline(binary colloids) over solid substrates is presented. The method relies on newlinethermoplasmonically controlled liquid flow inside an evaporating sessile drop, which newlinecounteracts the naturally occurring evaporation-driven convective flow. Excitation at the newlineplasmonic wavelength (532 nm) generates the required temperature gradient (~ 18 °C newlinebetween the irradiated region and the three-phase contact line of the pinned droplet), newlineresulting in the particle assembly at the irradiation zone in response to the thermocapillary newlineflow created inside the droplet. The existence of a strong thermocapillary flow that newlinecounteracts the naturally occurring evaporative convection flows was confirmed from newlineParticle Streak Velocimetry experiments and analysis. Further, to understand the strength newlineand magnitude of the Marangoni flow, the Marangoni number (Ma) and temporal newlineevolution of the Marangoni velocity (and#119907;and#119898;), during the evaporation process, was estimated.