From coffee rings to coffee eyes

Area of Science:

• Fluid dynamics
• Materials science
• Surface science

Background:

• Evaporation of colloidal suspensions commonly results in a ring-shaped stain due to capillary forces and particle deposition.
• The coffee-stain effect is a well-known phenomenon in fluid dynamics and materials science.

Purpose of the Study:

• To investigate the effect of substrate heating on the morphology of stains formed by evaporating liquid suspensions.
• To understand the underlying physical mechanisms driving the observed changes in stain morphology.

Main Methods:

• Depositing liquid suspensions onto glass and plastic substrates.
• Heating the substrates to controlled temperatures during the evaporation process.
• Observing and analyzing the resulting stain morphology using microscopy and image analysis.

Main Results:

• Increasing substrate temperature alters stain morphology from a simple ring to an 'eye' shape, characterized by a central deposit and outer ring.
• Both the size and relative volume of the central stain component increase with substrate temperature.
• The phenomenon is attributed to enhanced Marangoni recirculation flow on heated substrates.

Conclusions:

• Substrate temperature is a critical parameter for controlling the morphology of evaporative stains.
• Enhanced Marangoni flow on hot substrates drives the transition to the 'eye' stain shape.
• This controlled morphology tuning has potential applications in self-assembly and precision ink-jet printing.