Electrophoretic Mobility Measurement by Differential-Phase Optical Coherence Tomography

Abstract

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.

Electrophoretic mobility of concentrated carbon black dispersions in a low-permittivity solvent by optical coherence tomography

“…The Doppler shift of light backscattered from moving particles was measured using DP-OCT velocimetry as described in a previous report [25]. The electrophoresis chamber was a cylindrical void (10 mm diameter and 180 lm thick) in the electrophoresis chamber body, which was cut from a microscope cover slip.…”

“…The electrodes were ITO-coated glass (Delta Technologies, CG-61IN-S115, 25 mm by 75 mm, 1.1 mm thick, 60-100 nm ITO layer). A 1 Hz frequency of the applied symmetric square wave electric field (10 V amplitude) was high enough to avoid significant accumulation of particles at the electrodes but low enough to provide a substantial duration (0.5 s) of electrophoresis between field reversals [25]. The applied electric field was low enough that stripping of counterions from the particle did not occur to increase the electrophoretic mobility [4].…”

“…The applied electric field was low enough that stripping of counterions from the particle did not occur to increase the electrophoretic mobility [4]. For TiO 2 particles in AOT-toluene, a linear increase in electrophoretic mobility was seen with electric field strength (28-56 kV/m), resulting in a constant calculated zeta potential [11,25]. The electrode separation d was measured as 180 lm using OCT, giving the constant electric field strength (56 kV/m) used in this study.…”

“…However, given the large electrode spacing (1-4 mm) [25], the small mobilities in nonpolar media require large potentials [26][27][28], which can produce undesired chemical reactions [24] and electrohydrodynamic instabilities [29]. Much smaller electric potentials may be used in differential-phase optical coherence tomography (DP-OCT) given the very narrow electrode spacing of 0.18 mm [25,30]. Compared to techniques such as LDE and PALS, more turbid, concentrated dispersions can be analyzed with DP-OCT since only a small coherence detection volume is sampled.…”

“…Compared to techniques such as LDE and PALS, more turbid, concentrated dispersions can be analyzed with DP-OCT since only a small coherence detection volume is sampled. Previously, it was shown that concentrated dispersions (up to 0.12 wt.%) of highly scattering TiO 2 particles dispersed in AOT-toluene could be measured straightforwardly using DP-OCT [25]. Mobilities as low as approx.…”