Control of the colloidal deposition of cellulose nanocrystal films

Cellulose nanocrystals (CNCs) are a bio-based organic material that has unique properties, including renewability, biodegradability, sustainability, environmental friendliness, a high stiffness, an adaptable surface chemistry, chirality, a high surface area, and an anisotropic morphology. These properties make CNCs a promising candidate for applications in nanocomposites, films, paper products, membranes, cement, photonic devices, inks, and tissue engineering scaffolds. Cellulose nanocrystals are commonly obtained via sulfuric acid hydrolysis of cellulosic source yielding rod-like shaped and negatively charged nanocrystals which form stable aqueous colloidal dispersions. 

One of the unique interests of CNCs originates from their ability to self-assemble in bulk suspension. Rod-like shaped CNCs exhibit a phase transition from an isotropic phase to a chiral nematic phase with increasing concentration of the suspension. The chiral nematic structure can be preserved upon solvent evaporation to obtain films that display structural color. This property can be utilized to manufacture inks, coatings, sensors, or optical devices. Structured CNC films can be produced via evaporation-induced self-assembly of colloidal drops. However, particle deposition onto a substrate is affected by the flow dynamics inside sessile droplets, and usually yields a ring-shaped deposition pattern commonly referred to as the coffee-ring effect. Therefore, a good understanding of film formation is required to improve the thickness uniformity of the films and to control the optical properties of the obtained films. 

This thesis provides new insights into the colloidal deposition and its control for cellulose nanocrystal film formation. The structural coloration in CNC films obtained via evaporation of drop casted CNC suspensions onto solid substrates was studied. First, the role of particle concentration in the drying droplets on the deposition of CNCs was investigated. Determination of the concentration in the drying colloidal drop is crucial since CNCs exhibit different phases in the suspension at different concentration regimes. These are respectively from low to high concentration an isotropic phase, a biphasic transition regime, and a chiral nematic phase. 

Furthermore, the helical pitch of the chiral nematically assembled phase changes as a function of the concentration. Therefore, an analytical method was formulated to determine the concentration inside the colloidal drop during drying. 

In addition, the fluid flow inside the drying colloidal drops was characterized via analysis of Marangoni flow, particle diffusion, and the viscosity of the suspensions. After understanding the role of the concentration and fluid flow on the deposition pattern of CNC films, this study also aimed to control the colloidal deposition of CNCs, and to investigate the effect of the deposition pattern on the optical properties of the obtained films. 

To modulate the deposition pattern, a methodology was developed to generate uniform films by harnessing solutal Marangoni flow during evaporation. For this, colloidal drops of CNCs were exposed to an air/ethanol atmosphere with varying vapor composition. The coffee-ring effect was suppressed by increasing the Marangoni flow relative to capillary flow inside the droplet. The colloidal stability of the CNC suspensions in the ethanol-water mixtures was also characterized by rheological and zeta potential measurements to optimize the optical properties of the obtained films through the desired structure formation. Consequently, iridescent films with a uniform thickness were produced. 

Next, an alternative way to control the colloidal deposition was designed through restriction of the capillary flow inside a drying droplet by inducing gelation. The effect of gelation on the deposition pattern and on the self-assembly of CNCs was investigated for the dried films. CNC films were thus obtained by drop casting CNC suspensions containing NaCl and CaCl2salts. The system was studied using rheological measurements and depolarized dynamic light scattering. In addition, analysis of the suspension’s surface tension, viscosity, and yield stress were used to gain a deeper insight into the deposition process. Lastly, the understanding of the gelation behavior in the drying droplet was used to exert control over the deposit where the coffee-ring deposit could be converted to a dome-shaped deposit. 

Finally, the optical properties of the CNC films dried onto superhydrophobic substrates were also investigated. In this study, a method was proposed to investigate the contribution of the thermal Marangoni flow in the drying droplets.

To conclude, this research demonstrates key solutions to avoid problems related to non-uniform film formation in case of colloidal deposition of CNCs in drying droplets. The work presented here provides one of the first investigations into how strategies including exploiting solutal Marangoni flow and gelation-induced drying can be used to control the deposition of CNCs into films.