Mizuguchi Takuho, Inasawa Susumu
Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan.
Soft Matter. 2019 May 15;15(19):4019-4025. doi: 10.1039/c9sm00280d.
A gravity effect was demonstrated for 10 nm particles drying in colloidal suspensions. The particles were well-dispersed and did not sediment. However, when a suspension was dried on a tilted directional cell, a clear downward flow of particles was observed around the packing front, which was the boundary between the packed particles layer and the suspension. Three particle sizes (10-110 nm) were examined, with the most pronounced effect being on the 10 nm particles. The primary origin of the downflow was attributed to condensation of particles near the packing front and the subsequent increase in the overall density of the condensed layer. Because of the flow, the packing front was not parallel to the drying interface and tilted cracks formed in the packed layer. A mathematical model was proposed that considered conservation of the suspended particles in the condensed layer. Three competing factors of particle transport (advection, particle consumption by packing, and particle transport by the downward flow) were used to explain the experimental results. Overall, the results suggested that simple substrate tilting would be useful to evaluate whether suspended particles are easily packed or not during drying.
在胶体悬浮液中干燥的10纳米颗粒表现出重力效应。颗粒分散良好，不会沉降。然而，当悬浮液在倾斜的定向池中干燥时，在堆积前沿（即堆积颗粒层与悬浮液之间的边界）周围观察到颗粒明显向下流动。研究了三种粒径（10 - 110纳米），其中对10纳米颗粒的影响最为显著。向下流动的主要原因是颗粒在堆积前沿附近凝聚以及凝聚层整体密度随后增加。由于这种流动，堆积前沿与干燥界面不平行，并且在堆积层中形成了倾斜的裂缝。提出了一个数学模型，该模型考虑了凝聚层中悬浮颗粒的守恒。颗粒传输的三个竞争因素（平流、堆积导致的颗粒消耗以及向下流动导致的颗粒传输）被用来解释实验结果。总体而言，结果表明简单的底物倾斜有助于评估干燥过程中悬浮颗粒是否易于堆积。
