Buckling of fluid interfaces laden with plate-like particles

Abstract

The remarkable properties of 2D nanomaterials make them promising candidates for the development of sustainable energy materials. However, the primary challenge in producing 3D materials from 2D nanosheets lies in the precise control of their microstructure. Previous studies have shown that buckling can be leveraged to control the microstructure of 3D materials assembled from nanosheets. Buckling is achieved by compressing fluid interfaces with adsorbed nanosheets. Therefore, understanding the buckling of fluid interfaces with adsorbed plate-like particles is crucial for producing functional 3D materials from 2D nanosheets. In this dissertation, we focus on two techniques used to control the microstructure of assembled nanosheets: the Langmuir-Blodgett assembly and spray drying.

In the Langmuir-Blodgett assembly, nanosheets adsorbed at planar fluid interfaces are compressed by barriers. The compression results in buckling of the fluid interface laden with a monolayer of nanosheets. To understand the buckling of a monolayer of nanosheets, we studied a simplified model system comprising millimetric Mylar sheets at a fluid-fluid interface. This model system allowed the precise measurement of both the buckling force and the buckling wavelength. The wavelength was found to be of the order of a few particle diameters. We developed a theoretical model based on energy minimization, which agrees well with the experimentally measured buckling force and wavelength. Building on insights from the model systems and accounting for van der Waals interactions between overlapping 2D nanosheets, we proposed a theoretical model to explain the buckling wavelengths observed in monolayers of nanosheets.

In spray drying, the evaporation of water drops containing particles results in the formation of buckled capsules. Previous studies on spherical colloids have shown that evaporation leads to accumulation of particles at the air-water interface. This accumulated particle layer (shell) buckles under further compression as evaporation proceeds. However, the following questions remain unanswered: (1) how particle adsorption at the interface affects evaporation rate, (2) what criterion governs onset of buckling, (3) how this criterion depends on particles adsorption at the interface, and (4) how the evaporation rate affects the final buckled morphology. To address these questions, we studied the evaporation of a single water drop containing graphene oxide nanosheets deposited on superhydrophobic substrates.

We found that particle adsorption at the interface had a negligible effect on the evaporation rate of drops. We explain this by adapting mathematical models from an analogous electrostatic problem. The model predicts that when the particles are uniformly distributed at the interface and are much smaller than the drop, the evaporation rate is identical to that of a pure water drop. In contrast, the onset of buckling strongly depends on particle adsorption at the interface. To explain this dependence, we modeled the shell as a particle bilayer. The bilayer buckles when the total interfacial tension becomes negative, which is qualitatively in agreement with the experiments. Finally, the buckling wavelength of the dried capsule decreased with increasing evaporation rate. For a fixed solid fraction, faster evaporation results in thinner shells. In thin shells, the low bending energy compared to the stretching energy favors high-curvature deformations, producing shorter buckling wavelengths.

In conclusion, this dissertation advances the fundamental understanding of the buckling of interfaces laden with plate-like particles. The results obtained provide practical ways to control the microstructure of industrially produced 3D materials made from 2D nanosheets.