Scaling mechanical instabilities in drying micellar droplets

Abstract

Drying-induced mechanical instabilities in soft matter droplets occur primarily due to the interplay of evaporative compressive stresses, leading to a stretching effect of the liquid region at the receding wet...

Assessing Activation Quality through Evaporative Drying Patterns of Zr-MOF (UiO-66) Colloidal Droplets

“…The hydrodynamic stress (σ h ), and elastocapillary stress (σ ec ), are the consequence of outward capillary flow, that results in a coffee ring-like deposition at the droplet edge . The direction of elastocapillary stress is primarily inward, which pulls the packed bed inward to minimize its surface energy and that eventually causes cracking or wrinkling. ,,− By balancing the extra surface energy associated with the new surface formation and elastic energy release on cracking, Sprakel and co-workers expressed a relation between shrinkage stress and critical shrinkage stress (σ s,c ) . Combining the well-known Griffith’s equilibrium crack propagation criterion and Routh and Russel’s stress–strain criteria for a network of identical elastic spheres, they introduced the following expression for critical shrinkage stress , : σ normals , normalc = 0.3754 true( 2 γ h true) 2 / 3 true( italicEM φ rep 2 true) 1 / 3 , where h denotes the film thickness, M is the coordination number, φ rep is the particle volume fraction at random close packing, E is the particle modulus, and γ denotes liquid–air interfacial tension.…”

“…From droplet evaporation dynamics, it is well-established that increasing the substrate temperature causes a temperature gradient within the droplet. The portion of the drop closer to the hot substrate becomes hotter, and the apex or top of the drop, far from the substrate contact, becomes the cooler zone. ,, Now, the temperature gradient creates a surface tension gradient; therefore, a flow from higher temperature (lower surface tension) to lower temperature (higher surface tension) is generated within the droplet, known as a thermal Marangoni flow (Figure a). This inward flow works opposite to the evaporation-driven outward capillary flow field.…”

“…We prepared cross-linked PDMS substrates by following the standard method reported by Whitesides and co-workers . The Young’s modulus of soft cross-linked PDMS and glass substrate was checked following our previous work . Glass substrates were cleaned with acetone, ethanol, and deionized water and dried well before the drop-casting and drying experiment.…”

“…39 The Young's modulus of soft crosslinked PDMS and glass substrate was checked following our previous work. 40 Glass substrates were cleaned with acetone, ethanol, and deionized water and dried well before the drop-casting and drying experiment. In both the cases of pore-inactivated UiO-66 MOF and pore-activated UiO-66 MOF, various concentrations within 0.01−0.1 w/v% were studied thoroughly.…”

“…All controlled experiments were conducted at 23 °C and with a relative humidity of 40%. To study droplet patterns at different drying temperatures, we set up the substrate temperatures (T S ) with the help of INSTEC Mk2000 equipped with an enclosed hot stage following the processes mentioned by Dewangan et al 40 To capture the in situ drying process, the whole setup was placed under an optical microscope (OLYMPUS-BX53M, attached to a digital CCD camera). A similar procedure was followed for hard glass and soft cross-linked PDMS substrates.…”