A study of microbubble based diverse phenomena in Optical Tweezers

Ghosh, Subhrokoli (2020) A study of microbubble based diverse phenomena in Optical Tweezers. PhD thesis, Indian Institute of Science Education and Research Kolkata.

PDF (PhD thesis with supplementary materials of Subhrokoli Ghosh (12IP009)) 12IP009/12IP009.pdf - Submitted Version Restricted to Repository staff only Download (9MB)

Abstract

Microbubbles are evolving as an essential tool of micromanipulation in various soft matter applications ranging from viscous through viscoelastic fluids to living systems. However, trapping microbubbles using conventional optical means is not possible due to their refractive index mismatch with the surrounding fluid, which therefore makes the task rather challenging. We had earlier developed a novel technique of generating and manipulating microbubbles using thermo-optical forces on an absorbing material (or substrate) in standard optical tweezers. In this thesis, we expand the initial work considerably, open new domains of research with microbubbles in confined mesoscopic systems, and also construct diverse applications of such laser-induced microbubbles. We demonstrate that the laser-induced microbubbles thus generated are an excellent tool of micromanipulation in mesoscopic length scales through the generation of convection flows in the ambient fluid, which also causes directed self-assembly of a wide range of microparticles. We explore three different aspects of this system, starting from the investigation of microscopic energetics of the system, through the characterization of the flow induced by microbubbles in the ambient fluid, to its applications. We start with the microscopic characterization of the flow induced by the bubble, relying upon the stochastic thermodynamics picture of an optically trapped probe particle in the flow. Based on the thermodynamic inference scheme, we explore a new experimental strategy to infer the energy dissipation in the colloidal particle system, which exists in a nonequilibrium steady state. We carry out this exercise in the presence and absence of the flow by quantifying the rate of entropy production along with the corresponding thermodynamic force field. We also verify the fluctuation theorem for our system using the stochastic sliding parabola (SSP) model and demonstrate that entropy production can be obtained with arbitrary precision from the short-time limit of the thermodynamic uncertainty relation. Following this, we move from the confined probe particle system to free probe particles in the flow to study the effects of the flow over a larger length scale. Using a single microbubble and a pair of microbubbles in different configurations, we produce various kinds of interesting flow profiles, which are supported by our analytical calculations of the flowfield based on the Stokes and heat equations. Finally, utilizing this flow, we develop a novel technique of directed self-assembly based micropatterning of various mesoscopic particles for numerous applications. In particular, we focus on the microscale patterning of conductive polymers on transparent glass surfaces for the fabrication of organic microcircuits. We carry out the simultaneous heterophase synthesis, doping and patterning of well-known organic semiconducting materials - such as pyrrole and aniline - and achieve an order of magnitude higher conductivity in comparison to that obtained by the native polymers. We also extend this micropatterning technique towards developing chemical and biological chips for sensing applications.

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