Nonstationary electro-osmotic flow in closed cylindrical capillaries. Theory and experiment

Both from the experimental and theoretical viewpoints it is of fundamental importance to know precisely which are the fluid flow characteristics in a (cylindrical, say) closed cell under the action of an externally applied electric field, parallel to the cell axis. This is so because in many cases the experimental determination of the electrophoretic mobility of dispersed particles is carried out in closed cells, whereby the motion of the particles in the laboratory reference system is the result of the superposition of their electrophoretic migration plus the liquid motion with respect to the cell. This makes it of utmost importance to analyze the above-mentioned fluid and particle movements. If, in particular, this evaluation is carried out in the presence of alternating fields of different frequencies, information about the dynamics and time scales of the processes involved can be obtained for different frequencies of the applied field. In the present contribution, we discuss experimental results based on the determination of the velocity of polystyrene latex particles in a closed, cylindrical electrophoresis cell, and compare them to our previous theoretical analysis of the problem. It is concluded that the theory explains with great accuracy the observed particle velocities. In addition to the use of the particles as probes for the fluid velocity distribution, this work intends to give additional clues on the frequencies and positions for which electrophoretic mobility measurements in closed cells can be more reliable.