Synthesis and characterization of colloids
I’ve been working on the synthesis and characterization of colloids of which the refractive index and density can be simultaneously matched in mixtures of polar and non-volatile solvents. Current experimental systems for studying the physics of colloidal suspensions have several drawbacks; the volatility and toxicity of the suspending fuids, constraints on interaction potential modification and irreproducible colloidal polymerization. The new system is designed to overcome these drawbacks. We can modify the surface of the colloids using surface-initiated polymerization methods, changing the interactions from hard sphere-like to a long-ranged electrostatic or steric repulsion. The tunability of the interactions, non-volatility of the solvents and ability for simultaneous matching of both refractive index and density opens up new possibilities for exploring the physics of model colloidal systems.
Particle size range achieve with different solvent ratios.
Confocal microscopy images and 3D rendering of a colloidal crystal formed when the colloids possess a highly repulsive potential; i.e. polar solvent with very low ionic strength (~0.3 micro Siemens).
Colloidal Gel Failure
Colloidal gels are networks of aggregated colloids. Because these bonds have a finite bond strength they break and reform over the lifetime of the network. If too many of these bonds break due to some stress or simply thermal rearrangement, the network many no longer be able to bear its own weight and collapse. The process by with this structural integrity is lost is complicated by the mesostructure of the fractal network – gels are comprised of individual bond between colloids that aggregate and form strands with a cross-sectional thickness comprised of many colloids (Fig.1).
If a gel is subjected to a large constant shear stress - a creep experiment in a rheometer, the network fails catastrophically after some delay time which scales to the exponential of the stress. However as the stress decreases, the thermal, finite nature of the individual bonds begins to play a role in how the structure degrades. At a low enough applied stresses individual bonds break but due to the strand being composed of multiple bonds in a cross-section, the strand does not instantaneously fail; individual bonds may also reform. It is only when all the bonds in a cross-section fail simultaneous does the strand no longer bear a stress. Therefore the time to fail after the application of a small constant stress becomes proportional to the exponential of the stress times the number of bonds in a cross-section (Fig.2) (See. J.Sprakel et.al. PRL 2011 for more information).
How does such a competition play out at the microscopic length scale? In order to observe the entire colloid gel in 3D, we use a confocal microscope with a shearing geometry that directly observes the gradient shear plane (Fig3), unlike conventional shear cells with observe the vorticity plane. This geometry allows us to observe fast deformation with limited 3D scanning – typically the rate limiting step in confocal experiments. We apply a constant shear to the system (Fig.4) and follow individual strand deformation and failure quantifying the local strand thickness using a convering radius transform applied to each stack (Fig.5) and the loss of connectivity by computing the local bond number for each colloid. Also we observing the entire gap – this allows us quantify any local non-affinity due to stand failure with respect to the entire structure.
The colloids used in these confocal shearing experiments are composed of a copolymer of poly-t-butyl methacrylate and polytrifluoroethyl methacrylate. The copolymer ratio is chosen such that they are refractive index and density matched to a polar non-volatile binary continuous phase. Colloids are attractive due to depletion of a non-absorbing polymer (polystyrenesulfonate) added to the continuous phase. See the section Synthesis and Characterization of Colloids for more information. The non-volatile nature of the continuous phase allows the shearing to take place on the confocal without the need for complicated solvent traps.