We use Brownian Dynamics (BD) simulations and continuum models to study the microstructures and mechanics in the colloidal film drying process. Colloidal suspensions are compressed between a planar moving interface and a stationary substrate. In the BD simulations, we develop a new Energy Minimization Potential-Free (EMPF) algorithm to enforce the hard-sphere potential in confined systems and to accurately measure the stress profile. The interface moves either at a constant velocity U_w or via a constant imposed normal stress Σ_e. Comparing the interface motions to the particle Brownian motion defines the Péclet numbers Pe_U = U_wa/d₀ and Pe_Σ = Σ_ea³/k_BT, respectively, where d₀ = k_BT/ζ with k_BT the thermal energy scale, ζ the single-particle resistance, and a the particle radius. With a constant interface velocity, thermodynamics drives the suspension behavior when Pe_U ≪ 1, and homogeneous crystallization appears when the gap spacing between the two boundaries pushes the volume fraction above the equilibrium phase boundary. In contrast, when Pe_U ≫ 1, local epitaxial crystal growth appears adjacent to the moving interface even for large gap sizes. Interestingly, the most amorphous film microstructures are found at moderate Pe_U. The film stress profile develops sharp transitions and becomes step-like with growing Péclet number. With a constant imposed stress, the interface stops moving as the suspension pressure increases and the microstructural and mechanical behaviors are similar to the constant velocity case. Comparison with the simulations shows that the model accurately captures the stress on the moving interface, and quantitatively resolves the local stress and volume fraction distributions for low to moderate Péclet numbers. This work demonstrates the critical role of interface motion on the film microstructures and stresses.

中文翻译：

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胶膜干燥过程中的微观结构和力学

我们使用布朗动力学（BD）模拟和连续模型来研究胶体膜干燥过程中的微观结构和力学。胶态悬浮液在平面移动界面和固定基底之间被压缩。在BD模拟中，我们开发了一种新的无能量最小化势能（EMPF）算法，以在受限系统中施加硬球势，并精确测量应力分布。以恒定的速度移动，接口任Ù_瓦特或经由一个恒定施加正应力Σ _ë。将界面运动与粒子布朗运动进行比较，可以定义Péclet数Pe _U = U _w a / d ₀和PE _Σ = Σ _Ë一个³ / ķ_乙Ť表示，其中d ₀ = ķ_乙Ť / ζ与ķ_乙Ť热能规模，ζ单粒子性和一个粒子半径。以恒定的速度接口，热力学驱动器时，PE中的悬浮液行为_ù «1和均质结晶出现时，两个边界之间的间隔的间隙推动平衡相边界上方的体积分数。相比之下，当Pe _U≫ 1，即使对于较大的间隙尺寸，在移动界面附近也会出现局部外延晶体生长。有趣的是，在中等的Pe _U处发现了最无定形的薄膜微结构。膜的应力分布会随着Péclet数的增加而出现急剧的过渡并呈阶梯状。在施加恒定应力的情况下，随着悬架压力的增加，界面将停止移动，并且微观结构和力学行为与恒定速度情况相似。与仿真的比较表明，该模型准确地捕获了运动界面上的应力，并定量解析了中低Péclet数的局部应力和体积分数分布。这项工作证明了界面运动对薄膜微结构和应力的关键作用。