Multi-axis alignment of Rod-like cellulose nanocrystals in drying droplets

Hypothesis: Radial capillary flow in evaporating droplets carry suspended nanoparticles to its periphery where they are deposited and form a coffee-ring. Rod-like nanoparticles seeking to minimize their cap-illary energy will align with their long-axis parallel to the contact line. Particles exhibiting electrostatic repulsion, such as cellulose nanocrystals (CNCs), establish a competition between capillary flow-induced impingement against a growing coffee-ring and entropic minimization leading to enhanced par-ticle mobility. Therefore, balancing these effects by manipulating the local particle concentration in dry-ing droplets should result in deposition with a controlled orientation of CNCs. Experiments: The dynamic local order in aqueous suspensions of CNCs in evaporating sessile droplets was investigated through time-resolved polarized light microscopy. The spatial distribution of alignment in deposited CNCs was explored as a function of nanoparticle concentration, droplet volume, initial degree of anisotropy, and substrate hydrophobicity. Computational analysis of the rotational Peclet number dur -ing evaporation was also investigated to evaluate any effects of shear-induced alignment. Findings: Multiple modes of orientation were identified suggesting local control over CNC orientation and subsequent properties can be attained via droplet-based patterning methods. Specifically, high local par-ticle concentrations led to tangential alignment and lower local particle concentrations resulted in new evidence for radial alignment near the center of dried droplets. (c) 2021 Elsevier Inc. All rights reserved.

Automated summary

The study found that controlled orientation deposition of CNCs can be achieved by manipulating the local particle concentration in drying droplets. High particle concentrations lead to tangential alignment, while low concentrations result in radial alignment near the center of dried droplets.