Frequency dependence of the electrophoretic mobility for single colloids as measured using optical tweezers

Optical tweezers accomplished with fast (100 mu s) particle tracking are employed to determine the frequency (1-1000 Hz) dependence of the electrophoretic mobility mu of a single colloid in an external oscillating electric (ac) field. The colloid under study is held in a microfluidic channel filled with monovalent or divalent salt (KCl and CaCl2) solutions of varying concentrations (10-4 to 10-1 molar), and its response to an external ac field is measured. A pronounced steplike frequency dependence of the single colloid electrophoretic mobility (SCE-mu) is observed with a plateau on the low-frequency side (1-100 Hz) and a corner frequency, f(c), which shifts with increasing concentration of the salt solution. The results are explained by considering the characteristic time needed for polarizing the electric double layer surrounding the colloidal particle under study.