Exploring the Dynamics and Interactions of Brownian Particles in Simple and Complex Fluids using Optical Tweezers

Paul, Shuvojit (2019) Exploring the Dynamics and Interactions of Brownian Particles in Simple and Complex Fluids using Optical Tweezers. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Shuvojit Paul (12IP006)) 12IP006.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

In this thesis we explore the dynamics and interactions of Brownian particles in simple and complex fluids by employing dual-beam optical tweezers. Other than careful experiments, we also carry out theoretical analyses and numerical simulations to comprehend and explain our experimental results. First, we develop several techniques to calibrate the optical tweezers system in time and frequency domains and validate them by experiments. Then, we conduct basic studies on our system of two optically trapped and hydrodynamically coupled Brownian particles in a viscous medium (water) in the context of the equilibrium fluctuation-dissipation relation to understand how a system comes back to equilibrium after we perturb it. We first verify experimentally whether the fluctuation-dissipation relation at equilibrium holds for such a system where the entities are coupled non-conservatively. In the process of this verification, we observe that, very interestingly, the mutual response function of these two particles possesses resonance characteristics, although the surrounding medium is highly over-damped. We also observe that the dynamics of an individual particle is non-Markovian, i.e., the system possesses memory. The memory ensures that this system can store energy in it, even though, it is in a purely viscous medium and inertial effects are negligible. We propose a method of quantifying the storage and loss of the energy in the system, providing corresponding analytical expressions. Then, we shift our focus to the study of viscoelastic fluids and examine the Brownian motion of single free and confined micro-particles in a viscoelastic Stokes-Oldroyd B fluid. We show from the Stokes-Oldroyd B model that the parameters of Jeffrey’s viscoelastic model are linked to the properties of the constituents of a viscoelastic fluid. Then we move on to applications of our findings. First, we develop a two-point active microrheology technique using the resonance characteristics of the mutual response function of a two-particle system and present an accurate viscosity measurement procedure. Later, using the theory developed during our studies in viscoelastic fluids, we experimentally demonstrate the procedure of extracting the intrinsic parameters of the constituents of a viscoelastic fluid, and design a fast phase-sensitive active microrheology technique for such fluids from the phase response of an optically trapped particle under external perturbations. The phase response is less affected by noise compared to the amplitude, and does not require detector sensitivity measurements which are mandatory for the amplitude measurements, so that the accompanying errors do not compromise the accuracy and precision of the calculated rheological parameters. The phase response-based microrheology technique we develop is thus more robust and also fast, thereby reducing the risk of any drifts in time in the measurement process.

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