Hollow Rims from Water Drop Evaporation on Salt Substrates.

Area of Science:

• Physical Chemistry
• Materials Science
• Fluid Dynamics

Background:

• Evaporation of sessile drops is a complex process involving fluid flow and solute transport.
• The coffee-stain effect describes the deposition of solutes at the contact line of evaporating drops.
• Understanding shell formation in evaporating salt solutions is crucial for various applications.

Purpose of the Study:

• To investigate the formation and morphology of salt shells from evaporating pure water drops on salt surfaces.
• To elucidate the underlying physical mechanisms driving shell formation.
• To develop a model explaining the observed shell structures.

Main Methods:

• Experimental observation of evaporating pure water drops on salt substrates.
• Analysis of shell morphology, including shape and structure.
• Development and application of a theoretical model based on advection-diffusion and evaporation principles.

Main Results:

• Thin salt shells form at the periphery of evaporating pure water drops on salt.
• Observed shell morphologies include rings of inclined walls and hollow toroidal rims.
• The molecular coffee-stain effect, driven by increased evaporation at the pinned contact line, was identified as the primary mechanism.
• Salt supersaturation near the triple line leads to crystallization at the liquid-air interface.

Conclusions:

• The formation of salt shells is a direct consequence of the molecular coffee-stain effect in evaporating salt solutions.
• The observed shell shapes are dictated by the interplay of salt advection, supersaturation, and crystallization at the liquid-air interface.
• A simple model supports the proposed mechanism of shell growth, providing insights into the physics of drop evaporation and salt crystallization.