Stable Hydrothermal Waves at Steady State Evaporating Droplet Surface

Area of Science:

• Fluid Dynamics
• Thermodynamics
• Surface Science

Background:

• Hydrothermal waves (HTWs) are typically observed during the transient drying of sessile droplets.
• The influence of droplet shrinkage on HTW dynamics has been a confounding factor in previous studies.
• Understanding HTWs under steady-state evaporation is crucial for controlling interfacial phenomena.

Purpose of the Study:

• To investigate the existence and characteristics of stable hydrothermal waves (HTWs) on a sessile ethanol droplet under steady-state evaporation.
• To decouple the effect of droplet shrinkage from intrinsic HTW dynamics.
• To analyze the influence of substrate temperature on HTW formation, propagation, and temperature heterogeneity.

Main Methods:

• Experimental observation of HTWs on ethanol droplets under controlled steady-state evaporation.
• Analysis using dimensionless numbers to evaluate the role of thermocapillary instabilities.
• Quantitative analysis of HTW number and temperature patterns in relation to substrate temperature and droplet geometry.

Main Results:

• Stable HTWs were observed and maintained under steady-state evaporation conditions, distinct from transient drying behavior.
• The number of stable HTWs exhibited a linear relationship with a dimensionless factor involving temperature difference and droplet aspect ratio.
• Increasing substrate temperature enhanced thermocapillary instabilities, intensified HTW temperature heterogeneity, and altered propagation patterns from directional 'source-to-sink' to complex multi-source/sink behavior.

Conclusions:

• Stable HTWs can be sustained on sessile droplets during steady evaporation, independent of shrinkage effects.
• Substrate temperature is a critical parameter controlling HTW stability, number, and propagation dynamics.
• The findings provide new insights into interfacial fluid dynamics and heat/mass transfer under sustained evaporation.