Colloidal particles at a liquid-liquid interface: interactions and rheology
Creators: name: Muntz, Iain Alexander Aylett
Publisher: name: The University of Edinburgh
Year: 2021

description: Colloidal particles at fluid interfaces are present in many industries and applications, including the food, pharmaceutical and cosmetics industries. Much work has focussed on the behaviour of charge stabilised colloidal particles at fluid interfaces, investigating both the interactions between particles and the flow behaviour of a particle laden interface. However, there is markedly less work on sterically stabilised particles at fluid interfaces, which can also be used to create systems with high interfacial area, such as Pickering emulsions. In this work I consider sterically stabilised poly(methyl methacrylate) (PMMA) particles adsorbed to a water-dodecane interface. I investigate the interaction between these particles and develop a novel method for characterising the rheology of a particle laden interface. I begin by investigating the long-range interaction between PMMA particles adsorbed to a liquid interface. A theory for the interaction between point charges at the interface between two dielectric media with finite screening lengths is developed, which I will argue is relevant further on. The result of this shows that there are three possible contributions to the interaction: a screened monopole term, a screened dipole term and a screened 1/r² term. As it turns out, the screened dipole term is experimentally inaccessible for my system. For PMMA stabilised by poly(lauryl methacrylate) (PLMA), blinking optical trap (BOT) measurements indicate that the particles are (close to) neutrally charged in oil, while qualitative evidence indicates they also acquire no charge in water. However, radial distribution functions (g(r), measured using fluorescence microscopy) when the PMMA-PLMA particles are adsorbed to an interface evince an unexpectedly long-range interaction. Interparticle potentials, U(r), are extracted from g(r) using two methods: Ornstein-Zernike (OZ) inversion at low surface fraction and reverse Monte Carlo (RMC) at a higher surface fraction. U(r) are also measured at the interface using a BOT. In each case, a single screened monopole potential can be used to describe the data, with no dipolar contribution evident. To corroborate these findings, a Bayesian model comparison was performed on the BOT data, showing that the single screened monopole potential was ∼ 40 times more likely to describe the data than a combination of screened monopole and unscreened dipole. I also show that a screened 1/r² provides a better fit to our data than an unscreened dipole. The comparison of the screened 1/r² and the single screened monopole shows that at low separations the single screened monopole provides a better fit while at high separations the screened 1/r² provides a better fit. I propose that this longrange interaction arises as the neutrally charged particles behave as neutral holes in the charged plane of the water-dodecane interface. g(r) at varying aqueous salt concentration and pH are consistent with this physical model, providing a method for varying the surface charge density of the fluid interface. In Chapter 4, I perform Monte Carlo simulations with a bimodal distribution of particles using the single screened monopole interaction discussed. I show that, while well-ordered structures have been observed experimentally for particles interacting with a dipolar potential, particles interacting via a screened monopolar potential with experimentally relevant parameters exhibit no such long-range order. I also show that the method for loading particles on to the interface affects the local structure of the particles. At low surface fraction, a sequential deposition of particles leads to greater local hexagonal ordering. However, at a higher surface fraction, a one-step deposition leads to more local hexagonal ordering. I attribute this effect to particles becoming stuck in areas of the same size particle at high surface fraction in a one step deposition, while in a sequential deposition the larger particles can first rearrange to have larger spacings before the smaller particles are introduced. The possible separation of large and small particles in the one step deposition would lead to greater local hexagonal arrangements but little long-range order. To probe the rheological response of the particle-laden interface I have developed a novel method for performing interfacial rheology which requires no probe attached directly to the interface, described in Chapter 5. I argue that this method is applicable to applications where, for example, an emulsion being sheared indirectly deforms the droplet interface via deformation of the continuous phase. In addition, the interface probed is purely a particle laden, liquid-liquid interface with no large probe immersed therein. My method uses simultaneous confocal microscopy to track the response of the interface, while shearing the upper oil phase using a parallel plate rheometer. Using this method I measure steady shear material properties such as the interfacial viscosity for fluid-like interfaces and the interfacial elastic modulus for solid-like interface. These measurements are consistent with recent studies on a similar system using a more direct probe, however using my indirect technique I can measure lower interfacial viscosities than have previously been reported using a double wall ring interfacial rheometer. As this technique uses simultaneous confocal imaging, it lends itself to structural analysis and I have correlated the rheological response of the interface to the structural behaviour under shear. I show that the structural properties of the interface have an effect on the shear behaviour, thereby the results from Chapter 4 become particularly relevant, and shearing the interface can have an irreversible effect on the interfacial structure. Finally, in Chapter 6, I use the indirect rheometry setup from Chapter 5 to measure stress propagation across the liquid-liquid interface. Using tracer particles in the lower water phase, I show, using a velocimetry technique, that the rheological properties of the interface play a key role in stress propagation across the interface. When the interface behaves as a fluid, there is little barrier for stresses to propagate to the lower phase. On the other hand, when the interface behaves as a solid, the response of the lower phase closely follows the response of the interface, i.e. the interface “shields” the lower phase from external stresses. This has profound implications for droplet-like systems in external shears, where the internal phase may need to be protected to maintain its functionality. Considering these results together, I have improved the understanding of particleladen interfaces by adding the behaviour of interfacially adsorbed (uncharged) sterically stabilised particles to the existing literature. This has been achieved from a theoretical, simulational, and experimental standpoint, demonstrating new physics in this field. Additionally, I have provided a novel method for probing these systems’ rheological properties in an industrially relevant manner, including considering stress profiles across a particle-laden interface which is important for many droplet-like systems in an external flow field. This novel method also allows measurements of remarkably low interfacial viscosities which can be seen for relatively weak rheological responses of, for instance, PMMA particles at water-oil interfaces.