Electrophoretic Mobility Measurement by Differential-Phase Optical Coherence Tomography

We report the use of differential-phase optical coherence tomography (DP-OCT) for measurement of electrophoretic mobility in low-conductivity solvents. Weakly charged particles are common in low-permittivity solvents, particularly in practical applications that contain water as a result of ambient humidity. Use of DP-OCT with transparent electrodes enables close electrode spacing (0.18 mm) and thus high electric fields despite low applied electric potential, to avoid electrohydrodynamic instability and electrochemical interference. Further advantages include small sample volume requirement (20 μL), the ability to analyze highly turbid colloids, and avoidance of electro-osmosis. This phase-sensitive method is demonstrated on weakly charged TiO₂ particles dispersed in toluene with Aerosol-OT surfactant at a relatively high water content (50 mM), with small mobility of 2.8−3.0 × 10^-10 m²/V s (ζ potential 11−13 mV). Mobility is independent of applied field strength (28−56 kV/m). Measurement reproducibility is comparable to that by phase analysis light scattering (PALS) for dispersions in low-permittivity media. Capabilities of DP-OCT, including high sensitivity, high spatial resolution, and small detection volume, offer potential for significant expansion of the field of charged colloids in low-permittivity media.