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Author

Wei HeFollow

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Physics

Year Degree Awarded

2018

Month Degree Awarded

September

First Advisor

Anthony D. Dinsmore

Subject Categories

Statistical, Nonlinear, and Soft Matter Physics

Abstract

Particle-laden interfaces have promising potentials in many fields because the particulate nature can endow the surface with physical properties that are not readily obtained from molecular-scale surfactants. In this dissertation, we first focus on measuring capillary forces on particles at fluid interfaces in order to assess the key parameters that yield effective stabilizing particles. In experiment, the force and the displacement of a millimeter-scale particle passing through a liquid interface were recorded. We find that the peak force needed to detach a particle from an interface crowded with other particles is consistently smaller than the force at a clean interface. By ruling out other possibilities, we attribute the force reduction to the perturbation of interface shape due to the constraints imposed by free particles. Then we study the mechanics of particulate assemblies by measuring the force response under a normal indentation. We find that there exist two linear regions with different slopes. The small-slope regime starts at the beginning and persists over a range of indentations much less than capillary length. Once the system entered the higher plateau region, it has the same stiffness as a pure liquid interface. Further, from top-view images, we showed that, as long as the indenter was larger than the size of a single particle, the azimuthal compression can be relaxed through the in-plane rearrangement of particles. These features are independent of the difference in fluid mass densities, the radius of the indenter and the species of particles. Although the presence of floating particles at an interface does not change its capillary nature under a wide range of poking depth, we show that the existence of the particle raft makes the original interface tougher in terms of both the maximum force it can sustain and the largest indentation an indenter can reach. These results provide an important step toward understanding the mechanics of particulate assemblies at interfaces.

Finally, we study the formation of organic 2D material in aqueous media for the purpose of potential applications in passivating objects in suspension. We optimized the conditions for the self-assembly of bola-amphiphilic molecules, and directly observed 2D sheets in optical microscope under dark-field illumination. We find that stacking is not preferred by sheets because of the likely electrostatic repulsion. Our method provides an effective way to better understand the properties of those sheets.

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