On the Phase Behaviour of Soft Matter: Understanding Complex Interactions via Quantitative Imaging

Abstract

The effect of microscopic colloid interactions on the resultant macroscopic phase behaviour is a frequently studied topic in soft matter resarch, and lies at the heart of this thesis. Key structural and dynamic properties of colloidal model systems across liquid-solid transitions are tracked using optical imaging techniques.
The first studied system comprises of thermosensitive microgels. These are soft, crosslinked polymer networks of colloidal size, which have been used as model systems to investigate various phase transitions. They display a rich phase behaviour due to their soft potential and internal core-corona structure. Especially, their thermosensitivity allows us to use temperature as an external control to tune particle size, volume fraction and effective interaction potential in situ. However, a thorough understanding of the effective interactions between microgels is lacking, and constitutes a key research question in this thesis.
We therefore quantitatively compare experimental and numerical pair correlation functions (g(r)s) across the phase diagram, obtained from confocal microscopy and simulations. We find that neutral, swollen microgel interactions are temperature-dependent, but also hinge on whether the core or corona of the microgel is explored.
This approach is repeated for ionic microgels with varying crosslinker density, where the introduction of acrylic acid complicates the resultant swelling behaviour. For this reason, we start by decoupling the core and corona swelling response to various charge regimes via light scattering experiments, and found that dangling polymer strands can extend up to several 100 nm outside of the network. Dangling ends had a pronounced effect on the interactions and phase behaviour of ionic microgels, but their contribution is missing within the current theoretical framework.
Finally, liquid-solid transitions in concentrated protein solutions are investigated. Two well studied globular proteins, lysozyme and γB-crystallin, were used as model systems with completely different interactions. No unambiguous experimental demonstration of the existence of an arrested glassy state had been published so far for either protein. A combination of two passive microrheology techniques now allowed us to confirm the formation of a glass phase at concentrations above a critical arrest concentration, and to obtain quantitative insight into the concentration dependence of the zero shear viscosity prior to arrest.

Abstract (Swedish)

A first lesson in physical chemistry invariably starts with an example of water existing in three states, or phases - gaseous (water vapour), liquid (water from the tap) and solid (ice cubes). Such phase transitions are easily observed, but their underlying cause is not so evident. Atoms and molecules are difficult to follow due to their minute size and rapid movements. So, in order to investigate phase transitions, so-called colloids are often employed. Colloids are tiny particles ranging between 1-1000nm in size: their size is large enough, and their dynamics are slow enough to be detectable with various instruments, yet at the same time, their motion and interactions still resemble molecules and atoms. Like magnets, interactions can either cause colloids to move away from each other (repulsive interactions) or draw closer to each other (attractive interactions). The microscopic interactions between colloids drive their macroscopic phase transitions, and so by investigating the phase behaviour of colloids with different interactions, we can learn more about what triggers phase transitions in atomistic and molecular matter. In recent years, the phases and phase transitions of increasingly complex colloids have been researched.

My thesis follows suit, and focuses on two vastly different systems, for which we try to predict phase behaviour based on the interactions between the colloids. The first system consists of so-called microgels, which are tiny, soft polymer networks. The microgel is comparable to a microscopic sponge saturated with water, which can be squeezed (relatively) dry. Changes in sample environment, for example an increased temperature, will lead to the squeezing out of water, which can also be done via the packing of many microgels in a small space. A swollen microgel softly repulses its neighbour, while a so-called collapsed microgel, i.e. a compressed sponge, experiences attractions. As a result, the macroscopic behaviour of a microgel sample will change significantly as the temperature is increased. The aim of my thesis is to find a model which correctly predicts the interactions between microgels as a function of temperature and packing fraction.

The second leg of my thesis revolves around the interactions and phase behaviour of two proteins, lysozyme and γB-crystallin, which occur naturally in the body. Proteins - consisting of many different amino acids, each with their own specific interactions - can be considered as very complex colloids. A recurring theme in physical chemistry is therefore the attempt to describe proteins with models based on simpler colloids. In the absence of salt, lysozyme is slightly attractive but mainly repulsive, and these mixed interactions leads to the formation of clusters. There is an ongoing debate whether these clusters will eventually jam and stop moving with increasing protein concentration, i.e. transitions from a liquid state into a solid one. γB-crystallin possesses attractive patches on its surface. Under physiological conditions, these patches dominate its phase behaviour and at high enough concentration, a network is formed. Any simple colloid with an overall attractiveness - as opposed to patches - would not form such a network. Predictive theories describing at what concentrations such patchy particles will arrest, and how the concentration affects the macroscopic viscosity, are not readily available. The second aim of my thesis is therefore to experimentally explore the liquid-solid transitions for these two proteins.

• author: Bergman, Maxime
• supervisor: Peter Schurtenberger; Emanuela Zaccarelli; Johan Bergenholtz
• opponent: Professor Hellweg, Thomas, Bielefeld University, Bielefeld, Germany
• organization: Physical Chemistry
• publishing date: 2019-02
• type: Thesis
• publication status: published
• subject: Physical Chemistry
• keywords: Microgels, Lysozyme, eye lens proteins, confocal microscopy, interaction potential, phase behaviour, simulations
• pages: 270 pages
• defense location: Sal B, Kemicentrum, Naturvetarvägen 14, Lund
• defense date: 2019-03-14 10:00:00
• ISBN: 978-91-7422-626-3
• ISBN: 978-91-7422-627-0
• language: English
• LU publication?: yes
• id: 89250e52-7f5a-4f60-a3ec-a2a190d789aa
• date added to LUP: 2019-02-15 11:58:27
• date last changed: 2020-04-15 11:53:10