Directed assembly of complex shaped particles at fluid interfaces

Abstract

Particles situated at fluid interfaces occur in nature, with the particles ranging from pollen to insects which walk on water. Particles at interfaces are exploited in classical applications like Pickering emulsions, in which particles stabilize emulsions, and froth flotation, in which ore particle adsorption to fluid interfaces is used as a means of separating and recovering metal ores. They also occur in emerging applications in which nanomaterials are organized at interfaces.

In this thesis, the assembly of particles into ordered structures via capillary interactions is studied. Early work in this field focused primarily on spherical particles that distort fluid interfaces and create excess area. The particles assembled by capillary interactions which occur because the excess area created by the particles decreases as the particles approach each other. In this thesis, particles with shape anisotropy are studied. Such particles create undulations with excess area that can be locally elevated at certain locations around the particle. The local elevation of excess area makes these sites locations for preferred assembly. Hence, particles can orient and aggregate in preferred orientations. Such self assembly is often termed directed assembly. Three key issues in directed assembly are means of controlling the object orientation, alignment, and the sites for preferred assembly, including means of promoting registry of features on particles. Each of these issues is addressed in detail in for the example of a right circular cylinder using analysis, experiment and numerics. A series of other shapes are then studied to illustrate the generality of the concepts developed.

Anisotropic particles can have non-unique orientations at fluid interfaces which satisfy mechanical equilibrium. For example, a cylinder placed on a fluid interface can assume an end-on or side-on orientation, or it can immerse itself in the surrounding bulk phases. Any of these orientations can satisfy a mechanical force balance when the particle is small enough that gravitational effects are negligible. The stable orientation is determined by the surface energies of the fluid-solid, fluid-vapor and vapor-solid surfaces. A comparison of the energy of each state allows phase diagrams to be defined in terms of the scaled aspect ratio [special characters omitted] and the contact angle &thetas;*, where L_cyl and R_cyl denote the cylinder length and radius, respectively. Line tension can also influence the orientations by changing the equilibrium contact angle &thetas;* and by increasing the energetic cost of the contact line. Phase diagrams accounting for positive line tensions Σ are also constructed. These phase diagrams can be divided into two classes. In the first, over some range of x and Σ cylinders can be driven from side-on to end-on orientations with increasing˜ Σ. This transition terminates at a triple point where the side-on, end-on and immersed energies are the same. In the second class, there is no triple point and, for a range of Σ values, cylinders of all aspect ratios x prefer an end-on orientation. In all cases, for high enough Σ, line tension drives a wetting transition similar to that already noted in the literature for spherical particles. The zero line tension predictions are compared favorably to experiment on the nanoscale and on the microscale. In the nanoscale experiments, functionalized gold nanowires made by template synthesis are spread at aqueous-gas interfaces, immobilized using a gel-fixation technique, and observed by SEM. The small aspect ratio particles (disks) were in an end-on configuration, while the longer nanowires were in a side-on orientation, in agreement with the theory. On the microscale, cylinders made using photolithographic techniques orient in agreement with prediction.

Anisotropic particles have preferred alignment on interfaces with unequal principle radii of curvature. Thus, the underlying shape of a fluid interface can be used to influence the assemblies that form. Depending on the shape of the interface, particles have different preferred alignments. This concept is developed by considering micron-scale cylindrical particles at air-water interfaces with unequal principle radii of curvature and observing the rotation and alignment by optical microscopy. (Abstract shortened by UMI.)