Sui Jize
Center of Soft Matter Physics and its Applications, Beihang University, Beijing 100191, China and School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China.
Phys Rev E. 2019 Jun;99(6-1):062606. doi: 10.1103/PhysRevE.99.062606.
Directional drying of colloidal suspensions, experimentally observed to exhibit mechanical instabilities, is a nonequilibrium procedure that is susceptible to geometric confinement and the properties of colloidal particles. Here, we develop an advection-diffusion model to characterize the transport dynamics for unidirectional drying of a suspension consisting of charged particles in a confined Hele-Shaw cell. We consider the electrostatic interactions by means of the Poisson-Boltzmann cell approach with the viscous flow confined to the cell. By solving the nonequilibrium transport equations, we clarify how the multiple parameters, such as drying rate, confinement ratio, and the monovalent slat concentration, affect the transport dynamics of charged colloidal particles. We find that the drying front recedes into the cell with linear behavior, while the liquid-solid transition front recedes with power law behaviors. The faster evaporation rate creates a rapid formation of the drying front and produces a thinner transition layer. We show that confinement is equivalent to raising the effective concentration in the cell, and, accordingly, the drying front appears earlier and grows more rapidly. Under geometric confinement, a longer fully dried film is created while the total drying time is shortened. Moreover, we have theoretically illustrated that low salt loadings cause a large collective diffusivity of charged colloidal particles, which results in a colloidal network by aggregation. Thus, the drying behavior alters dramatically as salt loadings decrease, since the resulting compacted clusters of charged particles eventually convert the suspension into a gel-like material instead of a simple fluid. Our model is consistent with the current experiments and provides a simple insight for applications in directional solidification and microfluidics.
实验观察到，胶体悬浮液的定向干燥会表现出机械不稳定性，它是一种非平衡过程，容易受到几何限制和胶体颗粒性质的影响。在此，我们开发了一种平流扩散模型，以表征在受限的Hele-Shaw池中由带电颗粒组成的悬浮液单向干燥的传输动力学。我们通过泊松-玻尔兹曼单元法考虑静电相互作用，粘性流被限制在单元内。通过求解非平衡传输方程，我们阐明了干燥速率、限制比和单价盐浓度等多个参数如何影响带电胶体颗粒的传输动力学。我们发现干燥前沿以线性行为向池内后退，而液-固转变前沿以幂律行为后退。更快的蒸发速率会使干燥前沿迅速形成，并产生更薄的过渡层。我们表明，限制等效于提高池内的有效浓度，因此，干燥前沿出现得更早且生长得更快。在几何限制下，会形成更长的完全干燥薄膜，同时缩短总干燥时间。此外，我们从理论上说明了低盐负载会导致带电胶体颗粒的集体扩散率增大，这会通过聚集形成胶体网络。因此，随着盐负载量的减少，干燥行为会发生显著变化，因为由此产生的带电颗粒紧密团簇最终会将悬浮液转变为凝胶状材料，而不是简单的流体。我们的模型与当前实验结果一致，并为定向凝固和微流体应用提供了一个简单的见解。
