ABSTRACT The overall goal of this work is to determine the characteristics of the motion of micro-sized and nano-sized par- ticles in a fluid, which has direct application to the study of the motion of intracellular contents during stem cell differentiation proposed in the parent grant. A clear assessment of the fluid properties is needed because it has a great impact on the accuracy of the computer simulation of stem cell differentiation. An optical tweezer apparatus will be used to observe the motion of micron-sized and nano-sized beads as they approach the tweezer's focal point to determine whether their motion is overdamped, underdamped or even critically damped. The fluid vis- cosity will also be varied to replicate that of cytoplasm and nucleoplasm. The idea is to characterize the transition between overdamped and underdamped motion and the conditions under which this transition occurs, if it occurs. This assessment is important because the proposed model of the cell includes objects of a size above and below that where the transition should occur. The result of this study, along with those proposed in the parent grant, should facilitate the development of an accurate high-speed model of stem cell differentiation that can assist in filling gaps in the current understanding of cellular mechanobiology, and facilitate future studies of other cellular mechanobiological processes such as wound healing. This study should also result in a combined experimental and theoretical optical tweezer tool for exploring the mechanobiology of cells.