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.

The time-dependent shape of an evaporating spherical water drop containing graphene oxide (GO) nanosheets is measured for varying solid concentration, humidity level, and pH. The drop is sitting on a superhydrophobic surface, depinned from it. Three different stages of evaporation are identified: isotropic retraction of the drop interface, buckling of the shell of particles accumulated at the fluid interface, and shrinking of the buckled shell at constant shell shape. Marked differences between acidic and basic drops are reported. It is argued that this feature is caused by the pH-dependent interfacial adsorption of the GO particles. For intermediate values of GO concentration, dried capsules with remarkably repeatable folding patterns could be obtained, whose mode numbers are compatible with those predicted by an inertialess, linear elastic shell model. When redispersed in water, the dried capsules from acidic drops retain their shape better than capsules from basic drops.

Particles trapped at a fluid-fluid interface by capillary forces can form a monolayer that jams and buckles when subject to uniaxial compression. Here we investigate experimentally the buckling mechanics of monolayers of millimeter-sized rigid plates trapped at a planar fluid-fluid interface subject to uniaxial compression in a Langmuir trough. We quantified the buckling wavelength and the associated force on the trough barriers as a function of the degree of compression. To explain the observed buckling wavelength and forces in the two-dimensional (2D) monolayer, we consider a simplified system composed of a linear chain of platelike particles. The chain system enables us to build a theoretical model which is then compared to the 2D monolayer data. Both the experiments and analytical model show that the wavelength of buckling of a monolayer of platelike particles is of the order of the particle size, a different scaling from the one usually reported for monolayers of spheres. A simple model of buckling surface pressure is also proposed, and an analysis of the effect of the bending rigidity resulting from a small overlap between nanosheet particles is presented. These results can be applied to the modeling of the interfacial rheology and buckling dynamics of interfacial layers of 2D nanomaterials.

We experimentally and numerically study the dynamics of a liquid jet issued from a rotating orifice, whose breakup is regulated by a vibrating piezo element. The helical trajectory of the spiralling jet yields fictitious forces varying along the jet whose longitudinal projections stretch and thin the jet, affecting the growth of perturbations. We show that by quantifying these fictitious forces, one can estimate the jet intact length and size distribution of drops formed at jet breakup. The presence of the locally varying fictitious forces may render high-frequency perturbations, that would otherwise be stable in the abscence of stretching, unstable, as observed similarly in the case of straight jets stretching under gravity. The perturbation amplitude then dictates how strong the perturbation is coupled to the jet compared with random noise that is inherently present in any experimental set-up. In the present study we exploit the slenderness of the jet to separate the calculation of the base flow and the growth of perturbations. The fictitious forces calculated from the base flow trajectory are then used in a nonlinear slender-jet model, which treats the spiralling jet as a quasi-straight jet with locally varying body forces. We show both experimentally and numerically that jet breakup characteristics (e.g. intact length and drop size distribution) can be controlled by finite-amplitude perturbations created by mechanically induced pressure modulations. Finally, we revisit the integrated net gain approach developed for straight jets under gravity and we provide simple analogous relations for spiralling jets.

The paper presents experimental data and results from a prediction tool for the performance of a desiccant loop cooling system. The experiments are performed under a variety of high humidity and hot ambient conditions and the system performance is described. One of the experimental conditions is typical of many Indian cities and the systems appropriate for those cities are established. A simulation program that can predict the performance of the desiccant loop is developed. The simulation results show that this system can work as effectively as vapor compression air-conditioning for certain ambient conditions whereas it can function as a pre-cooler to a vapor compression system under more severe conditions, resulting in a reduced power consumption. The results presented in the paper give a guideline to practicing engineers as to when a desiccant loop cooling system would be useful. A simple payback analysis and a lifecycle cost analysis shows that a desiccant cooling system with a waste heat recovery recuperator is an economically viable investment.