Effect of Gelation on the Colloidal Deposition of Cellulose Nanocrystal Films

One of the most important aspects in controlling colloidal deposition is manipulating the homogeneity of the deposit by avoiding the coffee-ring effect caused by capillary flow inside the droplet during drying. After our previous work where we achieved homogeneous deposition of cellulose nanocrystals (CNCs) from a colloidal suspension by reinforcing Marangoni flow over the internal capillary flow (Gençer et al. Langmuir 2017, 33 (1), 228-234), we now set out to reduce the importance of capillary flow inside a drying droplet by inducing gelation. In this paper, we discuss the effect of gelation on the deposition pattern and on the self-assembly of CNCs during droplet drying. CNC films were obtained by drop casting CNC suspensions containing NaCl and CaCl₂ salts. A mixed methodology using rheological and depolarized dynamic light scattering was applied to understand the colloidal behavior of the CNCs. In addition, analysis of the mixture's surface tension, viscosity, and yield stress of the suspensions were used to gain deeper insights into the deposition process. Finally, the understanding of the gelation behavior in the drying droplet was used to exert control over the deposit where the coffee-ring deposit can be converted to a dome-shaped deposit.