Probing Molecular Assembly of Small Organic Molecules during Meniscus-Guided Coating Using Experimental and Molecular Dynamics Approaches

Abstract

Evaporation-based thin-film coating processes are ubiquitous in fields such as organic electronics, dye coatings, and the pharmaceutical industry. Based on experimental observations of enhanced thin-film alignment during meniscus-guided coating (MGC), a molecular dynamics simulation was developed to describe the thin-film formation that occurs due to the interplay between evaporation and fluid dynamics. The process of solvent evaporation from solution is first examined, which leads to the formation of a solid thin film after evaporation is complete. Next, an MGC process was simulated by introducing a flow profile coupled with the evaporative system. We gain insights into how molecules respond and reorient in the presence of a flow profile. These results indicate that both the evaporation and the flow profile have an impact on orienting small molecules in a thin film during the MCG process. Combining experimental techniques with simulations helps enhance the understanding of the forces that are significant in directing molecular aggregation during coating processes.

Selective Morphological and Polymorphic Control of CL-20 Thin Films Using Meniscus-Guided Coating

“…For the 22 and 50 °C drop-cast samples, we observe a nanocrystallite morphology in circular bands due to the coffee ring effect . The grain size for these samples increases as a function of temperature, as observed in similar studies with glycine …”

“…In this region, a limited number of nucleation events occur and growth is dominant, producing large areas of aligned crystals, as seen in other solution shearing studies. 16,25 The alignment can be determined by observing the change in intensity (optical activity) as a function of the sample rotation under cross-polarized light. Rotated crosspolarized images are included in the right column of Figure 23 and ε polymorphs.…”

“…31 The grain size for these samples increases as a function of temperature, as observed in similar studies with glycine. 19 MGC in this study utilizes processing parameters (substrate temperature and coating blade speed) to finely tune the evaporative crystallization pathway, each having an impact on the fluid dynamics (meniscus shape and length) and evaporation rate. 32 These dependencies create a complex system, where discerning the effects of each processing parameter on thin-film crystallization and film properties becomes difficult.…”

“…18 Guthrie et al have shown the selective polymorph formation and high control over the crystal morphology for glycine and acetaminophen thin films using solution shearing. 16,19 We build upon these studies to further develop an understanding of the EM crystallization pathways, as well as fabricate thin-film EM crystals for studies on stability and polymorphic interconversion.…”

Simulation of Crystal Nuclei at the Liquid-Air Interface toward Morphology Control via Surface Tension

“…As a comparison, molecular dynamics (MD) simulation is good at computing thermodynamic observables of time-evolving processes while providing microscopic insights, making it an ideal tool for revealing the role of surface tension in crystallization. Although MD simulation has been extensively used to examine organic crystallization from the air or solutions, − few studies have focused on interfacial crystallization in systems with an explicit liquid-air interface. − These systems were designed to simulate the nonequilibrium process of solvent evaporation, during which thermodynamic analysis is challenging. Therefore, it is crucial to design an equilibrium system that represents the key steps of crystallization.…”