Contact-line pinning controls how quickly colloidal particles equilibrate with liquid interfaces.

Area of Science:

• Colloid and Interface Science
• Soft Matter Physics
• Materials Science

Background:

• Spherical colloidal particles exhibit slow relaxation to equilibrium after adsorbing to liquid-liquid interfaces.
• This slow dynamics is attributed to transient pinning and depinning of the contact line on particle surfaces.
• The precise nature of these pinning sites has remained largely uncharacterized.

Purpose of the Study:

• To investigate the nature of pinning sites on colloidal particles at liquid interfaces.
• To quantify the timescales and energetics of contact line pinning.
• To understand the implications for colloidal particle behavior in interfacial applications.

Main Methods:

• Digital holographic microscopy was employed to track various colloidal spheres (inorganic, organic, stabilized, aqueous, non-aqueous) breaching liquid interfaces.
• Comparison of experimental relaxation data with theoretical models of pinning dynamics.
• Inference of defect area and energy from observed logarithmic time relaxation.

Main Results:

• Nearly all examined colloidal particles displayed logarithmic time relaxation over extended timescales, exceeding predictions from viscous dissipation alone.
• Inferred area per defect is approximately a few square nanometers for all colloids studied.
• Energy per defect varies significantly, from a few kT for non-aqueous/inorganic spheres to tens of kT for aqueous polymer particles.

Conclusions:

• Topographical features, such as surface roughness (silica) and grafted polymer "hairs" (polymer particles), are identified as likely pinning sites.
• The slow relaxation dynamics due to contact line pinning must be considered in experiments and applications like Pickering emulsions.
• Aqueous polymer particles exhibit strong contact line pinning, making this effect particularly significant for them.