Viscous Drag of Deformed Vesicles in Optical Trap: Experiments and Simulations

Traditionally, Stokes' Law is applied in the calculation of the maximum drag force induced by optical trap on a rigid sphere under hydrodynamics flow. Experimental results show that the spherical phospholipid vesicle, traveling in liquid medium, is significantly deformed by hydrodynamics force, and, thereby, the drag force may deviate from that of a rigid sphere. Thus, it is critical to elucidate whether a discrepancy exists in the calculated drag force when the trapped vesicle is deformed under flow. In this study, optical tweezers is applied to interrogate the shape deformation of a moving unilamellar vesicle, and a commercial computational fluid dynamics (CFD) software package is used to calculate the drag force and wall stress distribution on the vesicle surface. The results show that shear stress distributions are reduced for a deformed vesicle in the liquid medium. The drag force deviation approaches 22.2% even for a slightly deformed vesicle. Overall, this study provides new insights into the mechanics of suspending cellular entities under flow and interpretation of drag force of a deformable liposome.