Dynamic deformation of a soft particle in dual-trap optical tweezers

A dual-trap optical tweezers is used for deforming the red blood cell (RBC) in suspension and studying its elasticity. The radiation force is applied directly to the cell without physical contact. The 3D radiation stress distribution was computed by ray tracing, the generalized Lorentz-Mie theory with the T-matrix and the FDTD via the Maxwell stress tensor. The 3D deformation of the cells was computed with the elastic membrane theory. The calculated deformation can fit to experimental data resulting in cell's elasticity coefficient. The static approach is valid only for small deformation (5- 10%). For a large deformation such as that of the RBC, we consider re-distribution of the radiation stress on the morphologically deformed cell. This stress re-distribution in turn induces subsequent deformation of the deformed cell and new stress re-distribution. The recursive process continues until a final equilibrium state is achieved. This iterative computation was implemented with the finite element method using the COMSOLTM multi-physics models. The deformation results can fit to the experimental data for cell's deformation up to 20%.