Rotational diffusion of partially wetted colloids at fluid interfaces.

Area of Science:

• Colloid and surface science
• Soft matter physics
• Brownian motion

Background:

• Colloidal particle dynamics at fluid interfaces are crucial for self-assembly and microrheology.
• Previous studies observed slowed translational diffusion for spherical particles at the air-water interface.
• Similar slowing of rotational dynamics was hypothesized for partially wetted colloids.

Purpose of the Study:

• To experimentally investigate the rotational Brownian diffusions of Janus colloids at the air-water interface.
• To quantify the slowing down of rotational diffusion compared to theoretical predictions.
• To understand the role of wetting dynamics in particle rotational motion.

Main Methods:

• Utilized optical microscopy, including bright-field and fluorescent techniques.
• Fabricated Janus colloids to exploit their unique geometry for rotational measurements.
• Measured both in-plane and out-of-plane rotational diffusions of the particles.

Main Results:

• Observed a significant slowing down of out-of-plane rotational diffusion (Dr,⊥), linked to contact line motion and wetting/dewetting dynamics.
• Also detected a slowing down of in-plane rotational diffusion (Dr,||) about the interfacial normal.
• Contact line fluctuations due to partial wetting dynamics were identified as a source of rotational line friction.

Conclusions:

• Partial wetting dynamics and associated contact line fluctuations severely impede rotational Brownian motion of Janus colloids.
• The observed rotational slowing can be modeled by considering rotational line friction.
• Findings contribute to understanding particle behavior at fluid interfaces, relevant for self-assembly and microrheology.