Renewable Materials and Nanotechnology Research Group, Department of Chemical Engineering, KU Leuven, Campus Kulak Kortrijk , Etienne Sabbelaan 53, 8500 Kortrijk, Belgium.
Langmuir. 2017 Jan 10;33(1):228-234. doi: 10.1021/acs.langmuir.6b03724. Epub 2016 Dec 30.
Cellulose nanocrystals (CNCs), ribbonlike crystalline nanoparticles, are a biobased material that can be a great alternative to obtaining films with tunable optical properties. Iridescent and light-diffracting films can be readily obtained via the drying of a suspension of these cellulose nanocrystals. The characteristics of the particle deposition process together with the self-assembly in the precluding suspension has a direct effect on the optical properties of the obtained films. 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. We set out to measure and describe the drying kinetics under different conditions. We found that the Marangoni flow inside the droplet was too small to counteract the capillary flow that deposits CNCs at the edges, resulting in the coffee-ring effect, irrespective of the atmospheric humidity. By varying the amount of ethanol in the atmosphere, we were able to find a balance between (1) colloidal stability in the droplet, which is reduced by ethanol diffusion into the droplet, and (2) increasing Marangoni flow relative to capillary flow inside the droplet by changing the droplet surface tension. We could thus make iridescent films with a uniform thickness.
纤维素纳米晶体（CNC）是一种带状结晶纳米颗粒，是一种生物基材料，可以很好地替代具有可调光学性质的薄膜。通过干燥这些纤维素纳米晶体的悬浮液，可以很容易地获得彩虹色和光衍射薄膜。颗粒在预排除悬浮液中的沉积过程以及自组装过程直接影响到获得的薄膜的光学性质。颗粒在基底上的沉积受到液滴内的流动动力学的影响，通常会产生通常称为咖啡环效应的环形沉积模式。我们着手测量和描述不同条件下的干燥动力学。我们发现，液滴内的马拉高尼流太小，无法抵消将 CNC 沉积在边缘的毛细流动，导致咖啡环效应，而与大气湿度无关。通过改变大气中的乙醇量，我们能够找到平衡（1）液滴中的胶体稳定性，这是由于乙醇扩散到液滴中而降低的，以及（2）通过改变液滴表面张力，使液滴内的马拉高尼流相对于毛细流动增加。因此，我们可以制作具有均匀厚度的彩虹色薄膜。
