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Assembly of polymer colloids at fluid interfaces through external fields and nanoscale surface topography
Author ORCID Identifier
Open Access Dissertation
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Peter J Beltramo
The superposition of dipolar repulsion and capillary attraction energies between colloidal particles pinned at fluid interfaces dictates their microstructural organization and therefore the macroscopic interfacial material properties of particle-stabilized emulsions and 2D monolayer materials. While isotropic, spherical, particles have been extensively utilized, expanding the possible applications and material property tunability via anisotropic particles has been a challenge due to the propensity of particles to form disordered aggregates at the interface. My thesis presents the synthesis of anisotropic polymer ellipsoids and the development and use of experimental tools to study their interfacial behavior to reveal how dipolar and capillary interactions can be manipulated through external fields and particle topography in order to ultimately create novel 2D monolayer interfacial assemblies.
We first demonstrated a technique to apply an elongation gradient to manufacture multiple batches of lab-scale quantities of colloidal ellipsoids in a single step. A common route to creating anisotropic colloids is the mechanical stretching of spherical polymer colloids into ellipsoids above the glass transition temperature of the polymer. While this general method has been well studied in the production of a single aspect ratio colloidal ellipsoid per batch, we extended the technique to cover the production of multiple monodisperse samples of colloidal ellipsoids. Because these colloidal ellipsoids each underwent the same synthetic procedure, we were able to modulate particle aspect ratio as an independent variable with no sample-to-sample batch variation. Further, we were able to model the elongation of the particles based on the film characteristics to predict their final aspect ratio.
Next, we used these particles to investigate the assembly of ellipsoids under external AC electric fields, both in bulk and at air-water interfaces. We investigated ellipsoidal particle alignment in bulk aqueous solution in order to narrow down the phase space of applied AC electric field strength and frequency, as well as ionic strength to conditions favorable to particle reorientation. We then used these results to inform studies on these same particles at an air-water interface. We developed a first of its kind Mirau interferometer that can measure the relative height profile of the fluid surrounding an interfacially pinned colloid with nanometer precision concurrent with external field application. Under static conditions, increasing the particle aspect ratio decreases the interfacial three-phase contact angle, but increases the relative interfacial deformation and therefore capillary attraction. Applied electric fields change the location of the particle relative to the fluid interface as well as how the fluid interface approaches the three-phase contact line with the particle surface. As the electric field strength increases, the contact angle increases for anisotropic particles. By controlling the contact angle with external fields, the interparticle capillary forces, and thus the final two-dimensional particle assembly, may be controlled in the future.
We also investigated the use of magnetic fields to interfacially assemble spherical polystyrene/iron oxide hybrid colloids. Different combinations of ionic strength and DC magnetic field strengths are applied to monolayers of particles, revealing an intricate state space transitioning between disordered clusters and hexagonal latices. Interfacial assemblies were characterized using 2D finite Fourier transforms to measure the amount of order under different solution-field conditions. The height of the fluid interface surrounding the pinned colloids during magnetic field application was measured using Mirau interferometry, indicating that alteration of capillary interactions via changes to interfacial pinning are occurring simultaneously with induced dipolar forces.
While our previous studies sought to modulate the interparticle potential via induced dipolar and changing capillary forces, we next discovered a way to control interfacial capillary forces through particle engineering. We accomplished this through the introduction of nanoscopic physical heterogeneity to the surface of polymer microellipsoids that alters the interparticle interactions when they are pinned at an aqueous-air interface. Leveraging the experimental tools we developed, we used a combination of Mirau interferometry and video microscopy to show that porous micron-sized ellipsoids at an aqueous-air interface behave in fundamentally different ways than their smooth counterparts. Particles with a nanoscale porous network show no quadrupolar deformation of the fluid interface, a trademark of smooth, homogeneous, colloidal ellipsoids. This causes the capillary interaction energy to be reduced by over an order of magnitude, a result that is confirmed by monitoring the dynamics of pairs of particles as they approach. Our measurements provide direct evidence of a shorter-range attraction with seemingly no orientational specificity between porous ellipsoids, in contrast to homogeneous, smooth ellipsoids. Taken together, these results indicate that incorporating nanoscale surface topography into anisotropic particles is an effective avenue to minimize capillary-driven aggregation and control interparticle interactions. As a result, such particles are promising candidates as building blocks for interfacial assemblies of anisotropic particles with long-range orientational and translational order.
In sum, this thesis presented a combination of novel anisotropic particle synthesis and experimental analysis of interfacial monolayer behavior to inform how the two-dimensional assembly of polymer colloids can be controlled through manipulating dipolar and capillary forces.
Trevenen, Samuel, "Assembly of polymer colloids at fluid interfaces through external fields and nanoscale surface topography" (2023). Doctoral Dissertations. 2784.
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