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pH-Tunable electrokinetic movement of droplets
Mansoureh Rashidi1, Anne M Benneker1
1Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, AB, T2N 1N4, Canada. anne.benneker@ucalgary.ca.
Soft Matter
|April 11, 2023
Summary
Controlling droplet motion in electric fields is key for microfluidics. This study shows pH changes can reverse oil-in-water emulsion droplet direction by altering amphoteric surfactant charge, enabling new separation strategies.
Area of Science:
- Physical Chemistry
- Materials Science
- Chemical Engineering
Background:
- Droplet manipulation in electric fields is crucial for microfluidics and separation techniques.
- Amphoteric surfactants, with both positive and negative head groups, offer tunable surface charge properties.
- Understanding electrokinetic (EK) phenomena is vital for controlling charged particle and droplet behavior.
Purpose of the Study:
- To investigate the electrokinetic motion of oil-in-water emulsions stabilized by amphoteric surfactants.
- To demonstrate pH-controlled manipulation of droplet motion in an electric field.
- To analyze the influence of pH, surfactant concentration, and droplet size on EK velocity.
Main Methods:
- Utilized a microfluidic system to study droplet behavior.
- Varied pH, surfactant concentration, and droplet size to observe effects on EK velocity.
- Measured EK velocity as a combination of electrophoresis (EP) and electro-osmotic flow (EOF).
Main Results:
- Reversed droplet motion direction by changing pH from acidic to alkaline.
- Observed minimal change in EK velocity at acidic/neutral pH due to competing EP and EOF effects.
- Found surfactant concentration significantly impacts EK velocity near the isoelectric point (pH 7).
- Demonstrated increased EK velocity with larger droplet size due to higher zeta-potential and wall effects.
Conclusions:
- pH-dependent control of amphoteric surfactant charge enables tunable droplet motion.
- Results validate theoretical predictions for droplet electrophoresis mobility.
- Findings support the development of on-chip droplet separation strategies using pH gradients.
- Applicable to biological systems and microfluidic devices with natural pH variations.
