Hypothesis:

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.

Experiments:

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.

Findings:

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.

中文翻译：

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通过界面辅助蒸发组装具有细胞状棋盘格结构的胶体单层

假设：

蒸发含有表面活性胶体的无固定液滴是自组装二维纳米结构的有前途的途径。标准协议是首先在水蒸气界面自组装表面活性纳米级颗粒，然后将其转移到固体表面上。利用这种自下而上的自组装技术，已经制成了形态很少的胶体单层膜。然而，在受控湿度条件下的蒸发动力学可能会极大地改变液体-蒸汽界面处以及固体表面上自组装胶体单层的微观结构，这在现有研究中尚未得到充分解决。

实验：

为此，我们提出了一种实验性研究，研究了无柄液滴中的软聚（N-异丙基丙烯酰胺）（pNIPAM）微凝胶颗粒的蒸发驱动自组装。表面活性的微凝胶颗粒自发地形成水蒸气界面，有利于抑制咖啡环效应和形成单层污渍。详细研究了在受控湿度条件下蒸发动力学对吸附到溶剂-空气界面的胶体微观结构以及转移到固体表面上的胶体单层形态的影响。

结果：

对于在饱和湿度条件下蒸发的无固定液滴，观察到在水-蒸汽界面处形成无颗粒和富含颗粒的区域。我们表明，蒸发引起的界面面积的收缩和吸附到界面上的微凝胶颗粒的面密度的增加导致载有颗粒的界面发生重组。沿着水-蒸汽界面的类似于去湿辅助图案的微凝胶颗粒的重排在溶剂完全蒸发后转移到固体基质上。沉积物中的微凝胶颗粒组装成结晶顺序增强的区域。提出了通过标准蒸发和慢速蒸发途径获得的Voronoi熵在单层沉积模式上的演变。