[Paper Review] Zeta potential dependent Self-electrophoresis of Pt-coated Janus particles in hydrogen peroxide solutions

Cheng‐Tse Wu, Yen-Wei Lin+1 more|arXiv (Cornell University)|Feb 13, 2017

Micro and Nano Robotics3 citations

TL;DR

This study demonstrates that the self-propulsion direction of Pt-coated Janus particles in hydrogen peroxide solutions is governed by their zeta potential, with negative zeta potential causing motion toward the non-catalytic side and positive zeta potential reversing motion to the catalytic side. The findings support a self-electrophoresis mechanism driven by self-generated electric fields, experimentally confirmed through surface charge manipulation and direct motion tracking.

ABSTRACT

We provide experimental results to show that self-propulsion of Janus particles made by coating platinum on the hemisphere of dielectric particles in hydrogen peroxide solution is similar to selfelectrophoresis. By different surface treatments and measuring the motion of particles and their ζ-potentials, we find that the speed and direction of motion are determined by the ζ-potential in a given concentration of hydrogen peroxide solution. When sign of ζ-potential is changed from negative to positive, the direction of motion reverses from toward non-catalytic side to catalytic side. We also find that the angular distribution of Janus particle is more polarized with increasing of the concentration of hydrogen peroxide, which support the self-electrophoresis mechanism.

Motivation & Objective

• To determine the mechanism behind the bidirectional self-propulsion of Pt-coated Janus particles in hydrogen peroxide solutions.
• To investigate the role of surface charge, quantified by zeta potential, in controlling the direction and speed of particle motion.
• To experimentally verify whether self-electrophoresis, rather than bubble propulsion or self-diffusiophoresis, is the dominant mechanism.
• To establish a direct correlation between zeta potential and self-propulsion behavior through surface modification and surfactant adsorption.

Proposed method

• Preparation of 2 µm silica particles with asymmetric Pt coating via DC sputter deposition to form Janus particles.
• Measurement of individual particle zeta potential using a custom-built electrophoresis device with ITO-coated glass plates and CMOS imaging of diffraction ring shifts.
• Tracking particle motion in H2O2 solutions (2–30 wt%) using high-speed microscopy (20 fps) and particle tracking algorithms to determine velocity and direction.
• Surface modification using APTES to reverse zeta potential from negative to positive, and surfactant adsorption (DDAB, STAC, TWEEN 20, SDS) at varying CMC levels to tune surface charge.
• Application of external DC electric fields in a microfluidic channel to align particle dipoles and measure angular distribution as a function of H2O2 concentration.
• Correlation of particle motion direction and velocity with zeta potential and H2O2 concentration to infer the dominant propulsion mechanism.

Experimental results

Research questions

• RQ1 Does the direction of self-propulsion of Pt-coated Janus particles in H2O2 depend on their zeta potential?
• RQ2 Can surface charge reversal via chemical modification (e.g., APTES) reverse the direction of particle motion?
• RQ3 How does increasing H2O2 concentration affect the alignment and angular distribution of Janus particles, and what does this imply about the underlying mechanism?
• RQ4 Is self-electrophoresis the dominant mechanism for self-propulsion, as opposed to bubble propulsion or self-diffusiophoresis?
• RQ5 Can the motion of individual Janus particles be predicted based on their measured zeta potential and H2O2 concentration?

Key findings

• Janus particles with negative zeta potential move toward the non-catalytic (Pt-free) side, while those with positive zeta potential move toward the catalytic (Pt-coated) side.
• Reversal of zeta potential via APTES surface treatment successfully reverses the direction of self-propulsion, confirming zeta potential as the key determinant.
• The angular distribution of particles becomes more polarized with increasing H2O2 concentration, indicating stronger self-generated dipole moments consistent with self-electrophoresis.
• Particle velocity increases with H2O2 concentration, and motion is best described by a two-parameter fitting model within the rotational diffusion timescale (~20 s).
• The observed motion direction and alignment under external electric fields strongly support a self-electrophoresis mechanism driven by self-generated electric fields from asymmetric catalytic reactions on Pt.
• The results challenge previous assumptions that motion is always toward the non-catalytic side and suggest that surface charge engineering can be used to control microswimmer function.