Instabilities in Drying Colloidal Films: Role of Surface Charge and Substrate Wettability

TL;DR

The paper investigates how particle surface charge and substrate wettability jointly influence the evaporation dynamics, final deposition patterns, and crack/delamination behavior in drying sessile droplets of silica nanoparticles. Using Ludox TM50 (negative) and CL30 (positive) on glass, polystyrene, and PTFE, the study reveals distinct crystallization and crack morphologies: TM50 tends to radial cracking with pronounced delamination on highly wettable substrates, while CL30 shows irregular, random cracks with attenuated delamination. Evaporation modes transition between CCR and CCA depending on substrate, concentration, and particle–substrate interactions, with crack spacing and length following concentration-dependent power laws (e.g., $\lambda = 0.1\phi^{0.63}$, $l = 0.23\phi^{0.40}$) on certain substrates. A mechanistic framework attributes these behaviors to capillary forces, interfacial interactions, and charge-mediated particle–substrate adhesion, offering practical insights for controlled patterning and coating integrity in drying colloidal films.

Abstract

The drying of colloidal suspensions leads to complex deposition patterns, accompanied by instabilities such as cracking and delamination. In this study, we experimentally investigate the coupled influence of particle surface charge and substrate wettability on the evaporation dynamics, final deposition morphology, and crack patterns of sessile droplets containing silica nanoparticles. We examine the dynamics of two types of colloids, namely the negatively charged colloidal silica nanoparticles (Ludox TM50) and the positively charged silica nanoparticle (Ludox CL30), at concentrations ranging from 0.1 to 5.0 weight percentages, deposited on glass, polystyrene, and polytetrafluoroethylene (PTFE) substrates with distinct wettability. Side and top-view imaging techniques are employed to capture the evaporation process and analyze the resulting cracks. Our results reveal that the nature of the particle charge and substrate wettability significantly affect the evaporation mode, with transitions observed between constant contact radius (CCR), constant contact angle (CCA), and mixed modes. TM50-laden droplets consistently exhibit radial cracks, whereas CL30 droplets display more randomly oriented and irregular cracks. At higher particle concentrations, TM50 suspensions form thicker deposits that undergo delamination, particularly on highly wettable substrates like glass. Quantitative analysis reveals that crack spacing and length follow power-law relationships with particle concentration. Additionally, the delamination behavior is strongly influenced by both the particle concentration and the type of substrate. We propose a mechanistic framework to explain the role of particle-substrate interactions in governing the observed cracking and delamination behaviors.

Figures (14)

• Figure 1: Schematic of the experimental setup (customized goniometer for side-view imaging and digital microscope for top-view imaging) used to study the evaporation kinetics of nanoparticle-laden droplets. The top views recorded using digital microscopy and the side views of the droplet captured with the CMOS camera at different stages of evaporation are shown in the left and right insets of the figure, respectively.
• Figure 2: Final deposition patterns of Ludox TM50 at different concentrations ($\phi$) on glass, polystyrene, and PTFE substrates.
• Figure 3: Final deposition patterns of Ludox CL30 at different concentrations ($\phi$) on glass, polystyrene, and PTFE substrates.
• Figure 4: Temporal evolution (side view) of (a) Ludox TM50 and (b) Ludox CL30 sessile droplets at different concentrations $\phi$ (0.1 wt.%, 1.0 wt.%, and 3.0 wt.%) on a glass substrate.
• Figure 5: Temporal evolution (side view) of (a) Ludox TM50 and (b) Ludox CL30 sessile droplets at different concentrations $\phi$ (0.1 wt.%, 1.0 wt.%, and 3.0 wt.%) on a polystyrene substrate.