Studies and applications of microscopic forces in optically trapped Brownian particles embedded in complex fluids

Kundu, Avijit (2022) Studies and applications of microscopic forces in optically trapped Brownian particles embedded in complex fluids. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Avijit Kundu (15RS035)) 15RS035.pdf - Submitted Version Restricted to Repository staff only Download (15MB)

Abstract

Optical tweezers (OT), since its discovery, performs a crucial role in the measurement and calibration of forces at mesoscopic length scales. As a scientific tool, OT has proven its ability in a broad regime in science including physics, biology, and chemistry - advancing research in these areas over the past 50 years. The potential capability of manipulating light itself, and thus - employing mesoscopic particles as short length scale probes - facilitates their use in fields as diverse as optics, soft matter, statistical mechanics, stochastic thermodynamics, biophysics, etc. Moreover, measuring the Brownian motion of the probe particles (passive method), as well as coupling external forces to them (active method) using OT, has improved measurement techniques in both fundamental and applied research. In this thesis we have mainly focused on exploring the active method for studying the interaction forces between particles and the fluid medium in which the particles are embedded. In our first work, we have shown the advantages our technique offers over the active method and quantified the Van der Waals force between an optically trapped polystyrene particle and a stationary silica particle, which are in very close proximity. We have found a good match with existing literature values for a similar system. Now, colloidal particles do not only interact with other similar particles in the medium, but also with the constituting particles of the medium itself. This interaction - manifested in the form of Brownian motion - reveals the characteristic properties (viscosity) of the medium. The measurement of the viscosity from the motion of microscopic collodial particles embedded in the concerned medium is known as microrheology. In our thesis, we have studied the properties of complex fluids using Oscillating Optical Tweezers (OOT). Now, a complex fluid exhibits a frequencydependent complex viscosity, and complex shear modulus (storage and loss moduli, also called viscoelastic properties). Our earlier studies have shown an efficient way of measuring the microrheological properties of a complex fluid. In our case, we trap a colloidal micro-particle in the fluid and spatially modulate the trap with a square-wave pulse which is nothing but the combination of odd sinusoidal harmonics of the fundamental frequency of the signal given. Then, we decompose the phase response of the particle in the Fourier domain, and using this measure the viscoelastic parameters of the fluid. We have applied this tool to differentiate storage moduli between a wildtype Lamin protein and its mutant one. Lamins are the architectural protein of the cell nucleus, which gives the nucleus mechanical stability. We have observed an increase in storage values of the mutant protein compared to the wild-type one as the excitation frequency increases. This implies a denser mesh size of the mutant protein network, which then has been confirmed by super-resolution microscopy. The higher values of storage moduli also lead us to infer the mechanical rigidity of the mutant protein - a mutation which is known to cause laminopathies. Following this, we have extended this idea for facilitating broadband microrheology, by increasing the range of frequency where the viscoelastictity can be measured with high signal to noise. To achieve this, we replace the square wave modulation with a sequence of sinusoidal waves, each chosen with an amplitude complementing the response of our optically trapped colloidal probe at the particular frequency of the sinusoid. We apply the multiple sinusoidal frequencies to the optical trap in a single shot, and extract the fluid response at individual frequencies of excitation by analysing the phase response of the probe. We apply this tool to various kinds of complex fluids and determine their microrheological properties. Finally, we study the interaction between two colloidal particles trapped in a complex fluid, and placed in very close proximity. The ensuing hydrodynamic interactions lead to exchange of energy between the two particles when one is excited externally, with the motion of the other also displaying a clear motional resonance at a particular frequency of the driving force. Thus, in a nutshell, we have studied both particle-particle, and particle-fluid interactions employing oscillating optical tweezers, and observed the characteristic properties of various systems where the tweezers have been deployed.

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