Near-critical spreading of droplets

Area of Science:

• Physics
• Physical Chemistry
• Soft Matter Physics

Background:

• Droplet spreading is crucial in various physical and chemical processes.
• Tanner's law describes viscous droplet spreading but requires refinement near critical points.
• Understanding precursor film dynamics is key to droplet behavior.

Purpose of the Study:

• To investigate droplet spreading in near-critical phase-separated liquid mixtures.
• To verify and extend Tanner's law in this specific regime.
• To analyze the behavior of the microscopic precursor film.

Main Methods:

• Experimental observations of droplet spreading.
• Application of lubrication theory.
• Finite-element numerical simulations.

Main Results:

• Tanner's law for viscous droplet spreading is robustly verified near the critical temperature.
• A microscopic cut-off length scale, interpreted as precursor film thickness, was determined.
• Droplet spreading demonstrates universality, collapsing onto a Tanner-like master curve before the fluctuation regime.
• A counter-intuitive, sharp thinning of the precursor film was observed approaching the critical point.

Conclusions:

• Viscous spreading in near-critical mixtures follows universal behavior described by a modified Tanner's law.
• The precursor film's thinning is linked to the vanishing spreading parameter at the critical point.
• This study provides insights into fluid dynamics at critical phenomena.