A three dimensional finite element method is used to model the forces acting on red blood cells trapped on an optical waveguide surface. The parameters are chosen to correspond to strip waveguides made of tantalum pentoxide (Ta2O5). A wavelength of 1070 nm is used and the cells are taken to be spherical. Gradient and scattering forces experienced by the cells are studied and found to be highly dependent on the refractive index of the cells. Gradient forces are found to be one order of magnitude larger than scattering forces. Only the lower part of the cells is in contact with the evanescent field of the waveguide. For low refractive indices, we find that the lower 0.5-1 μm of the cells is sufficient to determine the optical forces. For the cell sizes considered, all forces increase with the size.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.