Drying crack patterns of sessile drops with tuned contact line

Abstract

Drying cracks formed in the deposits of films, paints, and coatings make the deposits failure and limit the corresponding applications. Extensive experimental attempts and theoretical understanding have been made to discover the underlying mechanisms and avoid cracks in the cases of uniform films. However, crack formation from drying drops is greatly complicated due to the strong coupling of the mass, internal flow and geometry constriction by the presence of the contact line, resulting in the heterogeneity of the deposit thickness. Interestingly, the complex physicochemical influences from colloidal particle, solution, surroundings can be geometrically reflected by the motion of the contact line, while the effect on crack formation is still lack of investigations. Hence, in this paper, we propose a model system to examine the dynamical motion of the contact line and correlate the corresponding mechanism of crack formation. The wettability of supporting substrates are successfully tuned in order to control the drop shapes with different contact angles, i.e. from hydrophilic to hydrophobic. Three drying regimes are recognized as pinned, stick-slip and receding by tuning the mobility of the contact line. The correlation between the crack spacing and the local thickness of deposits near the contact line is found as approximately linear, which is qualitatively discussed in the analysis of the scaling law. Finally, suppression of the cracks deposits are practically realized by freely moving contact line on the slippery substrates in receding cases with a wide range of suspension concentration.