Chungil Ha, Sung-Jin Kim, Hyuk Kyu Pak
Transportation and delivery of microscopic materials in very small and complex systems such as biological organisms are mainly done by physical diffusion. This phenomenon in a fluid system with a low Reynolds number can be explained using the Stokes-Einstein relation D = k B T/β, where D is the diffusion coefficient, T is the temperature of the system, and β is the viscous friction coefficient of the background fluid. For a spherical particle with radius a in a fluid of viscosity η, β = 6πηa. As far as we know, all the experimental tests of this relation before ours measured only D, η, a, and T due to the experimental difficulties in measuring β directly. In this research, we tested this relation from a different perspective. The diffusion coefficient D and the viscous friction coefficient β were experimentally measured in the same system by using a position tracking method and an oscillating optical tweezers technique, respectively. We found that our experimental results supported the Stokes-Einstein relation very well.
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