Investigating non-equilibrium effects in the fluctuation-dissipation relation using optical tweezers

Jain, Umang (2018) Investigating non-equilibrium effects in the fluctuation-dissipation relation using optical tweezers. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

According to the fluctuation dissipation theorem the linear response of a system in thermal equilibrium is expressed in terms of fluctuation properties of the system under effect of an external perturbation. If a Brownian particle is suspended in a fluid, random impacts because of collisions of fluid molecules act as a random driving force and the same impacts also give rise to frictional force to a forced motion. The origin of both these forces is the same i.e. a collision from fluid molecules opposing the motion of particle in response to an external driving force. This report gives an insight to a project we performed to check the modifications needed in fluctuation dissipation relation, to be applied directly in a nonconservative system. Theoretically we have claimed its validity in a conservative system and invalidity in a non conservative system and then we have experimentally verified it. We use optical-tweezers to trap micro particles in fluid and measure its Brownian motion. In the case of conservative system we trap a colloidal particle and measure its response by manipulating it from external force. Since the energy of this system remains constant, we can safely say that system is in equilibrium and fluctuation dissipation theorem holds true. For the second part we trap a the particle alongside a bubble produced because of heating effects of a laser. In this system, the trapped particle experiences a continuous force in a particular direction because of the flow created by bubble (marangoni convection) and hence this system becomes non-conservative. We demonstrate the invalidity of the fluctuation dissipation relation in this case and provide a brief overview of a modified relation to find the velocity of medium molecules.

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