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Droplets Wicking in Thin Materials Exhibit Universal Drying Dynamics.
Garam Lee1, Samira Shiri1,2, James C Bird1
1Boston University, Department of Mechanical Engineering, Boston, Massachusetts 02215, USA.
Physical Review Letters
|October 25, 2025
Summary
Liquid drops on porous surfaces spread and then shrink due to evaporation. This wicking behavior, crucial for cooling textiles and forensic analysis, is predictable with a single parameter, showing a universal expansion-then-contraction dynamic.
Area of Science:
- Fluid dynamics
- Materials science
- Surface science
Background:
- Wicking, the radial spread of liquid on porous surfaces, is utilized in applications like cooling textiles.
- Understanding wicking dynamics is crucial for forensic stain analysis, where liquid spread complicates interpretation.
- The interplay between liquid spreading and evaporation in porous media is not fully understood.
Purpose of the Study:
- To investigate and model the wicking dynamics of a liquid drop on a thin porous surface, focusing on the influence of evaporation.
- To determine how evaporation affects the spreading and receding behavior of the liquid front.
- To identify predictable parameters governing the wicking process.
Main Methods:
- Experimental measurements of liquid drop spreading and evaporation on porous substrates.
- Development of a mathematical model to describe the coupled wicking and evaporation phenomena.
- Analysis of the influence of substrate and liquid properties on wicking dynamics.
Main Results:
- Evaporation can cause the liquid front to reverse direction, leading to a shrinking wetted patch after initial spreading.
- A single dimensionless parameter, dependent on substrate and liquid properties, predicts both maximum wetted diameter and total evaporation time.
- A universal dynamic was observed: drops expand for approximately 25% of their lifetime and shrink for the remaining 75%.
Conclusions:
- The wicking dynamics of drops on porous surfaces are governed by a predictable interplay between spreading and evaporation.
- The maximum stain size and evaporation time are not linearly dependent on the initial droplet volume, contrary to simple assumptions.
- The findings offer insights into optimizing applications like cooling textiles and improving forensic analysis techniques.
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