Structure and mechanical properties of model colloids at liquid interfaces
Colloidal particles adsorbed to liquid interfaces appear in a broad range of industries such as foods, mining and oil recovery. Yet our understanding of such systems remains incomplete. This thesis aims to characterise a model sterically-stabilised colloid system. We will explore their structure, stress-strain relationship, and the theoretical models used to describe them. Since the high barrier to adsorption and high desorption energy of particles are inﬂuenced strongly by size, we focus on the eﬀects of varying particle size. This also allows us to begin connecting our micron-sized colloids to smaller surface active species such as proteins. We focus on the mechanical properties of the particle-laden interface, which are crucial to applications such as emulsion stabilisation, and complement this investigation with microscopic imaging. For simplicity, we focus on the less frequently investigated sterically-stabilised particles, as opposed to chargestabilised particles. The imaging reveals that for unsonicated samples, the characteristic structure changes with particle size: while large ( 1 µm radius) particles are relatively ordered in a two-dimensional crystal, as particle size decreases aggregates start to appear. This apparent diﬀerence persists in their stress response - which we measure using a Langmuir-Pockels trough - where the largest and most ordered particles can withstand the most stress before buckling the interface. However, once the number and size of aggregates are reduced by sonication, the variation in mechanical properties with particle size disappears and all sizes (from 1 µm radius down to 0.2 µm radius) show a comparable response, consistent with the behaviour of charge-stabilised particles. With particle size shown to be unimportant in our range, we focus on the smallest particles and use another technique - oscillating pendant drop tensiometry: ﬁrst, to further explore the interfacial rheology, and second to verify our Langmuir-Pockels trough measurements by another method. The second point is particularly important because literature reports of pendant drop and trough measurements seem to show a surprising inconsistency: drop measurements often only model the eﬀect of colloid adsorption while trough measurements often only include colloid interactions, and each model is consistent with their own data. We demonstrate a pendant drop experiment which can be modelled with interactions, and use this to develop the theoretical understanding of how colloidal particles aﬀect the interfacial rheology, thereby oﬀering an explanation for this apparent inconsistency. We also quantitatively characterise the scaling of our interactions with the surface density of particles, and ﬁnd that it is not inconsistent with interfacial electrostatic interactions, as with charge-stabilised particles. This result agrees with an independent report which more directly measured interparticle interactions. Directly comparing the results of our pendant drop and trough measurements, we ﬁnd consistency at low surface pressures and deviation at higher surface pressures. This is attributed to the limitations of the trough and our modelling at high surface pressures. In chapter 7 we present the ﬁrst observations in particle-laden interfaces of a new mechanism of particle expulsion we call collective particle detachment, which was predicted to occur for particle-laden interfaces in earlier works on elastic sheets at liquid interfaces. In this process, thousands of particles collectively detach from the interface after wrinkling, producing long tubular structures, much like those produced by the highly elastic BslA protein in similar conditions. This provides a clear and novel link between particle and protein behaviour. Finally, having investigated particles as a model system for proteins, we perform oscillating pendant drop measurements on a model particle-like protein - ferritin - to explore the applicability of our colloidal understanding to proteins. We ﬁnd that its dilational elastic modulus is linear with surface pressure, as it was for particles. By applying our particle-based model to ferritin, we ﬁnd that its interactions are short-ranged, consistent with previous studies. We conclude with a discussion of particle-protein similarities - such as their high desorption energy - and diﬀerences, such as the compressibility and unfolding potential of proteins. This thesis explores a model colloidal system and adds to the literature a new approach where we use the adaptability of colloidal particles (at the synthesis stage) to explore key variables for fundamental interfacial properties. In the process we develop an improved model for the eﬀect of adsorbed particles on the interfacial rheology, which allows us to measure interparticle interactions at the interface. We ﬁnd the novel phenomenon of collective particle detachment, and show that our sterically-stabilised particles behave as charge-stabilised particles at the interface. Future work might explore other crucial variables for interfacial properties such as the contact angle or anisotropy.