Dynamics in Charged Colloidal Suspensions near the freezing point
Creators: name: ALRUWAILI, Amani
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 510405
subject: Soft condensed matter

description: The fundamental physics of fluids, and the transition from the equilibrium fluid state to the equilibrium crystal state, are still not well understood. These problems have been investigated in detail for many years using the simplest model systems: Hard-Sphere (HS) like particles suspended in a fluid. HS colloidal suspensions exhibit fluid, fluid + crystal coexistence, and crystal phase boundaries at well-defined particle volume fractions. Above the freezing point, the inherent polydispersity and slow Brownian motion of such particles lead to long-lived metastable fluid states prior to crystallization. This enables the measurement of dynamics in these metastable states, and previous work has shown that there are qualitative changes in the dynamical properties of such systems when moving from the equilibrium fluid state to the metastable state, which have opened up intriguing questions about the fundamental nature of the phase transition.<br><br>
However, the HS potential is not directly relevant to atomic/molecular systems, and so it is unclear if these results are unique to hard spheres, or if they are more universal. Interaction potentials which more closely mimic atomic and molecular systems are therefore of strong interest.<br><br>
This thesis focusses on the experimental investigation of charged colloidal suspensions to understand their structural and dynamic behavior in the equilibrium fluid state, and to compare with the results from hard-sphere colloidal suspensions.<br><br>
For this study three different charged colloidal suspensions (System A - charged PMMA particles dispersed in water with long range interactions; System B - silica particles in water:DMSO with interactions of moderate range; and System C - silica particles dispersed in DMSO solvent with shorter range interactions) are chosen which have different charge characteristics, and therefore a different range of interactions. Static and dynamic light scattering experiment are conducted on these suspensions, concentrating on the equilibrium fluid range, (and some samples for Systems A and C being in the metastable state) and the results compared with hard spheres. Static structure factor (S(q)) and intermediate scattering functions (ISFs) are measured for all three systems as a function of concentration at different q-vectors. An effective mean-particle distance () (equivalent to HS interaction distance) is determined by considering the particle number density as a function of concentration. S(q) and ISF are rescaled with q to compare the structure and dynamics of charged systems with HS systems. Different higher order functions such as ln(ISF), width function (W(q,τ)) and scaled current correlation function C*(q,τ) are used to rationalize the dynamic behaviour.<br><br>
It is found that the S(q) peak shift to higher qR (where R is actual particle radius) with increasing concentration in all the three systems, and that plots of S(q) vs. q collapse on to single master curve with different peak heights at different concentrations. This trend is in agreement with HS system by assuming an effective particle size. /R increases with increasing concentration linearly for three systems, with different slopes related to their interaction strength: system A (long- range interactions) showed the steep slope, whereas System B and System C with shorter-range interactions showed small slopes.<br><br>
The short-time diffusion constant (Ds) is obtained from three different analyses (ISF, ln(ISF) and W(q,τ)) at different q and at various concentrations. Normalized inverse short time diffusion coefficient (D0/Ds) as a function of q showed a peak at the principal peak of S(q) in all cases. This confirms that there exists very good correlation between the structural rearrangements and dynamics of the colloidal particles, as observed in HS system. The peak value of D0/Ds(qm) plotted as a function of concentration is found to increase linearly in System A and System C (in both the equilibrium and metastable states), but is nearly independent in System B.<br><br>
Width function (W(q,τ)) analysis showed trends similar to HS, but with noticeable differences - in particular the linear region in the short-time intervals exhibits a continuous gentle curvature rather than a line of slope 1 over several decades, as is the case for HS systems. This is attributed to the presence of long-range interactions which are always present at any distance, meaning that at the concentrations observed true diffusive behaviour (as observed in HS) in only observed over a short delay-time range. The logarithmic derivative of W(q, τ) is observed to decrease as a function qR, and drops to minimum at the qR value corresponding to the S(q) peak. This minimum indicates the crossover to the anomalous diffusive regime. The minima are well defined in system A and System B, but not observable in System C, as the concentrations measured in the fluid state are well below the crystallization concentration.<br><br>
Finality, the scaled current correlation function (C*(q,τ)) showed trends very different from HS colloids. C*(q,τ) showed two distinct linear regimes in short-time and long-time intervals. The slope in the long-time interval exhibited a peak at the S(q) peak, highlighting the correlation between structure and dynamics.<br><br>
This thesis provides the first systematic study of the dynamics of charged colloidal suspensions with different ranged interactions, and highlights similarities and differences with the well-studied HS system. These results make a significant contribution to an improved understanding of dynamics in colloidal suspensions near the freezing point.