Vertical oscillatory motion of a single colloidal particle adjacent to an electrode in an ac electric field

The vertical motion of a levitated colloidal particle immersed in aqueous solution, located near an electrode, and subjected to an alternating electric field normal to the electrode was recorded using total internal reflection microscopy. The particle's height was sinusoidal in time during the portion of the cycle above its average height; during the remainder of the cycle, however, hydrodynamic hindrance arising from the particle's proximity to the electrode did not allow it to descend below the average height with the same amplitude as above. The particle's height averaged over a full cycle exhibited a minimum in its frequency and voltage dependences in potassium hydroxide but increased monotonically with frequency in sodium bicarbonate. The phase between the response of the particle and the applied electric field varied significantly with frequency. Both the profoundly different response in potassium hydroxide and sodium bicarbonate and the significant variation in phase angle with frequency are not expected on the basis of colloidal and electrophoretic forces alone; another force is suggested. The electrohydrodynamic model of Sides (2002) is qualitatively consistent with these observations.