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Corners, cusps, and pearls in running drops.
T Podgorski1, J M Flesselles, L Limat
1Physique et Mécanique des Milieux Hétérogènes, UMR 7636 CNRS-ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France.
Physical Review Letters
|July 20, 2001
Summary
Small drops change shape as they slide, forming different patterns like smooth lines, corners, or cusped tails based on their speed (capillary number). Dynamic contact angle explains these shape bifurcations.
Area of Science:
- Fluid Dynamics
- Surface Science
- Microfluidics
Background:
- Understanding droplet behavior on surfaces is crucial for various applications.
- The shape of sliding droplets is influenced by surface properties and flow conditions.
Purpose of the Study:
- To investigate the shape bifurcations of small sliding drops.
- To correlate droplet shapes with the capillary number (Ca).
- To explore the role of dynamic contact angle in these phenomena.
Main Methods:
- Experimental observation of small drops sliding on a partially wetting substrate.
- Varying the capillary number (Ca) to induce different flow regimes.
- Analysis of droplet contact line dynamics and shape evolution.
Main Results:
- Droplet shapes bifurcate into distinct categories based on capillary number.
- At low Ca, drops have smooth contact lines.
- At intermediate Ca, drops develop trailing edge corners (0-60 degrees).
- At higher Ca, drops exhibit cusped tails, leading to secondary droplet ('pearl') emission.
- These observed bifurcations are explained by dynamic contact angle variations.
Conclusions:
- The dynamic contact angle is a key factor governing the shape bifurcations of sliding drops.
- The study provides a framework for predicting droplet behavior based on capillary number.
- Findings have implications for microfluidic devices and coating technologies.