Colloidal particles at a liquid-liquid interface: interactions and rheology

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

Electrostatic potential between charged particles at an oil-water interface

Electrostatic interactions between point charges embedded into interfaces separating dielectric media are omnipresent in soft matter systems and often control their stability. Such interactions are typically complicated and do not resemble their bulk counterparts. For instance, the electrostatic potential of a point charge at an air-water interface falls off as $r^{-3}$, where $r$ is the distance from the charge, exhibiting a dipolar behaviour. This behaviour is often assumed to be generic, and is widely referred to when interpreting experimental results. Here we explicitly calculate the in-plane potential of a point charge at an interface between two electrolyte solutions with different dielectric permittivities and Debye screening lengths. We show that the asymptotic behaviour of this potential is neither a dipole, which characterises the potential at air-water interfaces, nor a screened monopole, which describes the bulk behaviour in a single electrolyte solution. By considering the same problem in arbitrary dimensions, we find that the physics behind this difference can be traced to the asymmetric propagation of the interaction in the two media. Our results are relevant, for instance, to understand the physics of charged colloidal particles trapped at oil-water interfaces.Comment: 11 pages, 7 figure

Contactless Interfacial Rheology: Probing Shear at Liquid-Liquid Interfaces without an Interfacial Geometry via Fluorescence Microscopy

Interfacial rheology is important for understanding properties such as Pickering emulsion or foam stability. Currently, the response is measured using a probe directly attached to the interface. This can both disturb the interface and is coupled to flow in the bulk phase, limiting its sensitivity. We have developed a contactless interfacial method to perform interfacial shear rheology on liquid/liquid interfaces with no tool attached directly to the interface. This is achieved by shearing one of the liquid phases and measuring the interfacial response via confocal microscopy. Using this method we have measured steady shear material parameters such as interfacial elastic moduli for interfaces with solid-like behaviour and interfacial viscosities for fluid-like interfaces. The accuracy of this method has been verified relative to a double-wall ring geometry. Moreover, using our contactless method we are able to measure lower interfacial viscosities than those that have previously been reported using a double-wall ring geometry. A further advantage is the simultaneous combination of macroscopic rheological analysis with microscopic structural analysis. Our analysis directly visualizes how the interfacial response is strongly correlated to the particle surface coverage and their interfacial assembly. Furthermore, we capture the evolution and irreversible changes in the particle assembly that correspond with the rheological response to steady shear.Comment: 14 pages, 11 figure

Interaction between Nearly Hard Colloidal Spheres at an Oil-Water Interface

We show that the interaction potential between sterically stabilized, nearly hard-sphere [poly(methylmethacrylate)-poly(lauryl methacrylate) (PMMA-PLMA)] colloids at a water-oil interface has a negligible unscreened-dipole contribution, suggesting that models previously developed for charged particles at liquid interfaces are not necessarily applicable to sterically stabilized particles. Interparticle potentials, $U(r)$, are extracted from radial distribution functions [$g(r)$, measured by fluorescence microscopy] via Ornstein-Zernike inversion and via a reverse Monte Carlo scheme. The results are then validated by particle tracking in a blinking optical trap. Using a Bayesian model comparison, we find that our PMMA-PLMA data is better described by a screened monopole only rather than a functional form having a screened monopole plus an unscreened dipole term. We postulate that the long range repulsion we observe arises mainly through interactions between neutral holes on a charged interface, i.e., the charge of the liquid interface cannot, in general, be ignored. In agreement with this interpretation, we find that the interaction can be tuned by varying salt concentration in the aqueous phase. Inspired by recent theoretical work on point charges at dielectric interfaces, which we explain is relevant here, we show that a screened $\frac{1}{r^2}$ term can also be used to fit our data. Finally, we present measurements for poly(methyl methacrylate)-poly(12-hydroxystearic acid) (PMMA-PHSA) particles at a water-oil interface. These suggest that, for PMMA-PHSA particles, there is an additional contribution to the interaction potential. This is in line with our optical-tweezer measurements for PMMA-PHSA colloids in bulk oil, which indicate that they are slightly charged.Comment: 8 pages, 8 figure

Surface Pressure of Liquid Interfaces Laden with Micron-Sized Particles

We consider the surface pressure of a colloid-laden liquid interface. As micron-sized particles of suitable wettability can be irreversibly bound to the liquid interface on experimental timescales, we use the canonical ensemble to derive an expression for the surface pressure of a colloid-laden interface. We use this expression to show that adsorption of particles with only hard-core interactions has a negligible effect on surface pressures from typical Langmuir-trough measurements. Moreover, we show that Langmuir-trough measurements cannot be used to extract typical interparticle potentials. Finally, we argue that the dependence of measured surface pressure on surface fraction can be explained by particle coordination number at low to intermediate particle surface fractions. At high surface fractions, where the particles are jammed and cannot easily rearrange, contact-line sliding and/or deformations of the liquid interface at the length scale of the particles play a pivotal role.Comment: 13 pages, 5 figure

Modelling metastatic colonization of cholangiocarcinoma organoids in decellularized lung and lymph nodes

Cholangiocarcinoma (CCA) is a type of liver cancer with an aggressive phenotype and dismal outcome in patients. The metastasis of CCA cancer cells to distant organs, commonly lung and lymph nodes, drastically reduces overall survival. However, mechanistic insight how CCA invades these metastatic sites is still lacking. This is partly because currently available models fail to mimic the complexity of tissue-specific environments for metastatic CCA. To create an in vitro model in which interactions between epithelial tumor cells and their surrounding extracellular matrix (ECM) can be studied in a metastatic setting, we combined patient-derived CCA organoids (CCAOs) (n=3) with decellularized human lung (n=3) and decellularized human lymph node (n=13). Decellularization resulted in removal of cells while preserving ECM structure and retaining important characteristics of the tissue origin. Proteomic analyses showed a tissue-specific ECM protein signature reflecting tissue functioning aspects. The macro and micro-scale mechanical properties, as determined by rheology and micro-indentation, revealed the local heterogeneity of the ECM. When growing CCAOs in decellularized lung and lymph nodes genes related to metastatic processes, including epithelial-to-mesenchymal transition and cancer stem cell plasticity, were significantly influenced by the ECM in an organ-specific manner. Furthermore, CCAOs exhibit significant differences in migration and proliferation dynamics dependent on the original patient tumor and donor of the target organ. In conclusion, CCA metastatic outgrowth is dictated both by the tumor itself as well as by the ECM of the target organ. Convergence of CCAOs with the ECM of its metastatic organs provide a new platform for mechanistic study of cancer metastasis

The role of cell-matrix interactions in connective tissue mechanics

Living tissue is able to withstand large stresses in everyday life, yet it also actively adapts to dynamic loads. This remarkable mechanical behaviour emerges from the interplay between living cells and their non-living extracellular environment. Here we review recent insights into the biophysical mechanisms involved in the reciprocal interplay between cells and the extracellular matrix and how this interplay determines tissue mechanics, with a focus on connective tissues. We first describe the roles of the main macromolecular components of the extracellular matrix in regards to tissue mechanics. We then proceed to highlight the main routes via which cells sense and respond to their biochemical and mechanical extracellular environment. Next we introduce the three main routes via which cells can modify their extracellular environment: exertion of contractile forces, secretion and deposition of matrix components, and matrix degradation. Finally we discuss how recent insights in the mechanobiology of cell-matrix interactions are furthering our understanding of the pathophysiology of connective tissue diseases and cancer, and facilitating the design of novel strategies for tissue engineering

Contactless Interfacial Rheology: Probing Shear at Liquid-Liquid Interfaces without an Interfacial Geometry via Fluorescence Microscopy

Interfacial rheology is important for understanding properties such as Pickering emulsion or foam stability. Currently, the response is measured using a probe directly attached to the interface. This can both disturb the interface and is coupled to flow in the bulk phase, limiting its sensitivity. We have developed a contactless interfacial method to perform interfacial shear rheology on liquid/liquid interfaces with no tool attached directly to the interface. This is achieved by shearing one of the liquid phases and measuring the interfacial response via confocal microscopy. Using this method we have measured steady shear material parameters such as interfacial elastic moduli for interfaces with solid-like behaviour and interfacial viscosities for fluid-like interfaces. The accuracy of this method has been verified relative to a double-wall ring geometry. Moreover, using our contactless method we are able to measure lower interfacial viscosities than those that have previously been reported using a double-wall ring geometry. A further advantage is the simultaneous combination of macroscopic rheological analysis with microscopic structural analysis. Our analysis directly visualizes how the interfacial response is strongly correlated to the particle surface coverage and their interfacial assembly. Furthermore, we capture the evolution and irreversible changes in the particle assembly that correspond with the rheological response to steady shear. A Jupyter Notebook for image analysis is available here: https://datashare.ed.ac.uk/handle/10283/4746

Computer-aided design of elastin-like polypeptides with controlled viscoelastic and structural properties

The biofabrication of structural proteins with controllable properties via amino acid sequence design is interesting for biomedicine and biotechnology, yet design rules that link amino acid sequence to material properties remain largely unknown. Molecular dynamics (MD) simulations can help in unveiling such rules, but the lack of a standardised framework to interpret the outcome of those simulation hinders their predictive value for the design of de novo structural proteins, To address this, we developed a model that unambiguously classifies a library of de novo elastin-like polypeptides (ELPs) with varying numbers and locations of hydrophobic/hydrophilic and physical/chemical-crosslinking blocks according to their thermoresponsiveness at physiological temperature. Our approach does not require long simulation times or advanced sampling methods. Instead, we apply (un)supervised data analysis methods to a dataset of molecular properties from relatively short MD simulations (150 ns). We also investigate the rheological properties and microstructure of ELP hydrogels, revealing handles to tune them: chain hydrophilicity/hydrophobicity or block distribution control the viscoelasticity and thermoresponsiveness, whereas ELP concentration defines the network permeability. Our findings provide an avenue to accelerate the design of de novo ELPs with bespoke material properties