Drying mechanism of monodisperse colloidal film: Evolution of normal stress and its correlation with microstructure

Abstract

We investigate the drying process of monodisperse colloidal film over a wide range of Péclet numbers (Pe) by using the Brownian dynamics simulation. We analyze the detailed process in three aspects; accumulation front, normal stress, and microstructure. The evolution of particle distribution is quantified by tracking the accumulation front. The accumulated particles contribute to the continuous increase of the normal stress at the interface. At the substrate, the normal stress first stays constant and then increases as the accumulation front touches the substrate. We quantitatively analyze the stress development by a scaled normal stress difference between the two boundaries. At all tested Pe, the stress difference increases to the maximum, followed by a decrease during drying. Interestingly, a mismatch is observed between the stress difference maximum and the initial stress increase at the substrate. The microstructural analysis reveals that this mismatch is related to the microstructural development at the substrate.

Stratification Mechanism in the Bidisperse Colloidal Film Drying Process: Evolution and Decomposition of Normal Stress Correlated with Microstructure

“…34,35 We have verified our BD simulations with the drying model of Wang and Brady 36 in our previous work. 37 In ref 37, we compared the volume fraction profile of the monodisperse colloidal film and confirmed that our simulation is in good agreement with the model prediction for the drying conditions covered in this study.…”

“…In addition, the normalized average particle velocity observed in the monodisperse film is also shown in the same figure. 37 The normalized average velocity according to Pe L (Figure 4b) shares a similarity with the scaled normal stress difference (Figure 4a). In all Pe L 's, the normalized average velocity increases (−⟨v z ⟩/E ̇increases) from the beginning for both the large and small particles.…”

“…After the accumulation front touches the substrate (H/H 0 ∼ 0.3), a rapid normal stress increase at the substrate leads to a decrease in the stress difference. 37 In the bidisperse film, compared to the monodisperse film, the scaled normal stress difference is higher at the same film thickness before it decreases. Also, the difference reaches the maximum at higher film thicknesses.…”

“…In addition, we plot the results of Jeong et al in the figure, who observed the evolution of the scaled normal stress difference in the drying process of monodisperse colloidal films. 37 In the monodisperse colloidal film system, the particle radius a = a L , initial volume fraction ϕ 0 = 0.1 (the number of particles N = 2000), and other conditions are the same as in the bidisperse colloidal film system. In Figure 4a, the scaled normal stress difference increases from the beginning for all Pe L 's.…”

“…This potential well describes the nearly hard-sphere interaction, and its steepness is dependent on the exponents. 30,39,40 We have chosen the exponents where the normal stress evaluated using the WCA potential is comparable to the stress using the potential-free algorithm, 37 the overlap resolution method to impose the hard-sphere potential. 36 The potential between the particles i and j is as follows…”