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
