Mayarani M, Basavaraj Madivala G, Satapathy Dillip K
Soft Materials Laboratory, Department of Physics, IIT Madras, Chennai, India.
Polymer Engineering and Colloid Sciences Laboratory, Department of Chemical Engineering, IIT Madras, Chennai, India.
J Colloid Interface Sci. 2021 Feb 1;583:683-691. doi: 10.1016/j.jcis.2020.09.019. Epub 2020 Sep 16.
Evaporating sessile drops containing surface active colloids is a promising route to self-assemble two-dimensional nanostructures. The standard protocol is to first self-assemble surface active nanoscale particles at the water-vapour interface and subsequently transfer it on to a solid surface. Colloidal monolayers with very few morphologies have been fabricated, exploiting this bottom-up self-assembly technique. However, the evaporation kinetics under controlled humidity conditions may dramatically alter the microstructure of self-assembled colloidal monolayers at the liquid-vapor interface and that on the solid surfaces, an aspect that has not been fully addressed in the prior studies.
To this end, we present an experimental study of evaporation driven self-assembly of soft poly(N-isopropylacrylamide) (pNIPAM) microgel particles loaded in a sessile drop. The surface-active microgel particles spontaneously populate the water-vapour interface facilitating the suppression of the coffee-ring effect and the formation of monolayer stains. The role of evaporation kinetics under controlled humidity conditions on the colloid's microstructure adsorbed to the solvent-air interface and on the morphology of the colloidal monolayer transferred onto the solid surface are studied in detail.
The formation of particle-free and particle-rich regions at the water-vapor interface is observed for sessile drops evaporated under saturated humidity conditions. We show that the evaporation induced shrinkage of the interface area and the enhancement of the areal density of microgel particles adsorbed onto the interface leads to a restructuring of the particle-laden interface. The rearrangement of microgel particles along the water-vapor interface resembling the de-wetting assisted patterns is transferred to the solid substrate upon complete evaporation of the solvent. The microgel particles in the deposit assemble into domains with enhanced crystalline order. The evolution of Voronoi entropy across the monolayer deposit patterns obtained by the standard and slow evaporation routes are presented.
蒸发含有表面活性胶体的固着液滴是自组装二维纳米结构的一条很有前景的途径。标准流程是首先在水汽界面自组装表面活性纳米级颗粒，随后将其转移到固体表面。利用这种自下而上的自组装技术，制备出的胶体单层形态非常少。然而，在受控湿度条件下的蒸发动力学可能会显著改变液 - 气界面以及固体表面上自组装胶体单层的微观结构，而此前的研究尚未充分探讨这一方面。
为此，我们对固着液滴中负载的软质聚（N - 异丙基丙烯酰胺）（pNIPAM）微凝胶颗粒的蒸发驱动自组装进行了实验研究。表面活性微凝胶颗粒会自发地聚集在水汽界面，有助于抑制咖啡环效应并形成单层污渍。详细研究了在受控湿度条件下蒸发动力学对吸附在溶剂 - 空气界面的胶体微观结构以及转移到固体表面的胶体单层形态的作用。
对于在饱和湿度条件下蒸发的固着液滴，在水汽界面观察到了无颗粒区域和富颗粒区域的形成。我们表明，蒸发引起的界面面积收缩以及吸附在界面上的微凝胶颗粒面密度的增加导致了载有颗粒的界面发生重构。微凝胶颗粒沿水汽界面的重排类似于去湿辅助模式，在溶剂完全蒸发后会转移到固体基质上。沉积物中的微凝胶颗粒组装成具有增强结晶有序度的区域。展示了通过标准蒸发途径和缓慢蒸发途径获得的单层沉积物图案的Voronoi熵的演变。
