Scaling mechanical instabilities in drying micellar droplets.

Area of Science:

• Physical Chemistry
• Materials Science
• Fluid Dynamics

Background:

• Drying of liquid droplets can induce mechanical instabilities like cracking and wrinkling due to surface tension gradients and material stresses.
• Micellar solutions exhibit complex behavior during drying, influenced by surfactant concentration and substrate interactions.
• Understanding these instabilities is crucial for controlling the morphology of dried deposits in various applications.

Purpose of the Study:

• To investigate the unique wrinkling patterns formed by evaporating sessile micellar aqueous droplets on different substrates.
• To explore the influence of surfactant concentration, substrate elastic modulus, and substrate temperature on wrinkle formation.
• To establish scaling relationships between deposit morphology and material properties.

Main Methods:

• Formation and observation of sessile micellar droplets (CTAB solution) on rigid (glass) and soft (PDMS) substrates at elevated temperatures.
• Systematic variation of surfactant concentration (C_CTAB) and substrate temperature (T_S).
• Analysis of resulting dried deposit morphologies, including wrinkle patterns, and correlation with substrate properties.

Main Results:

• Coffee-ring morphologies observed at low CTAB concentrations (≤ 0.0364 wt%) without wrinkling.
• Wrinkle formation initiated at higher CTAB concentrations (≥ 0.0364 wt%) and temperatures (≥ 85 °C), with radial patterns on glass and annular patterns on PDMS.
• Wrinkle characteristics (width, length) scale with initial surfactant concentration and contact angle, showing substrate-dependent interdependence with elastic modulus.

Conclusions:

• Evaporation of micellar droplets leads to distinct wrinkling patterns governed by a complex interplay of concentration, temperature, and substrate properties.
• The study confirms existing theories on mechanical instabilities in drying deposits and highlights substrate-dependent scaling relationships.
• Found an antagonistic interdependence between the elastic modulus of the dried deposit and the initial surfactant concentration, modulated by the substrate.