Electrorotation of colloidal particles and cells depends on surface charge

Area of Science:

• Physics
• Colloid Science
• Electrochemistry

Background:

• Surface conductivity is vital for particle electrorotation, yet its direct link to the ionic double layer lacks experimental proof.
• Understanding surface charge effects is key to controlling particle behavior in electric fields.

Purpose of the Study:

• To experimentally demonstrate the direct relationship between surface charges and electrorotation.
• To investigate the role of surface charges across a wide range of electric field frequencies.
• To develop a model explaining surface charge influence on particle rotation.

Main Methods:

• Developed a novel three-electrode apparatus for precise electrorotation experiments (25 Hz–80 MHz).
• Characterized latex particle surface properties using conductometric titration and free-flow electrophoresis.
• Analyzed single-particle rotation at varying frequencies and pH to determine zeta-potential and surface charge effects.

Main Results:

• Confirmed that surface charges significantly determine particle rotation rates at low frequencies and near the Maxwell-Wagner frequency (MWF).
• Observed inversion of rotation direction at the MWF peak when surface charges were negligible.
• Developed an analytical model showing a linear dependence of MWF rotation velocity on surface charge.

Conclusions:

• Surface charges play a critical, quantifiable role in electrorotation, impacting both rotation speed and direction.
• The developed analytical model accurately describes the double layer and dissociation, validating the linear relationship between surface charge and MWF rotation.
• This research provides direct experimental evidence linking surface conductivity, ionic double layers, and particle electrorotation behavior.