  The boundary element method is applied to investigate the optical forces when
whispering gallery modes (WGMs) are excited by a total internally reflected
wave. Such evanescent wave is particularly effective in exciting the high-$Q$
WGM, while the low angular or high radial order modes are suppressed
relatively. This results in a large contrast between the forces on and off
resonance, and thus allows for high size-selectivity. We fully incorporate the
prism-particle interaction and found that the optical force behaves differently
at different separations. Optimal separation is found which corresponds to a
compromise between intensity and $Q$ factor.
