The equilibrium phase behavior and the dynamics of colloidal assemblies composed of soft, spherical, colloidal particles with attractive pair potentials have been studied by digital video microscopy. The particles were synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAm), acrylic acid (AAc), and N,N'-methylene bis(acrylamide) (BIS), yielding highly water swollen hydrogel microparticles (microgels) with temperature- and pH-tunable swelling properties. It is observed that in a pH = 3.0 buffer with an ionic strength of 10 mM, assemblies of pNIPAm-AAc microgels crystallize due to a delicate balance between weak attractive and soft repulsive forces. The attractive interactions are further confirmed by measurements of the crystal melting temperatures. As the temperature of colloidal crystals is increased, the crystalline phase does not melt until the temperature is far above the lower critical solution temperature (LCST) of the microgels, in stark contrast to what is typically observed for phases formed due to purely repulsive interactions. The unusual thermal stability of pNIPAm-AAc colloidal crystals demonstrates an enthalpic origin of crystallization for these microgels.

We report direct measurements of the pairwise interparticle potential between poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-coAAc) colloidal microgels as a function of pH, as determined through Ornstein-Zernike analysis of the pair distribution function of quasi-2D dilute colloidal suspensions. The interaction potential ranges from purely repulsive at high pH due to electrosteric interactions to weakly attractive at low pH due to hydrogen bonding, which explains previous observations on the unique phase behavior of these particles in concentrated suspensions.Colloidal suspensions display various phases and microscopic structures, ranging from random disordered to self-assembled ordered structures. 1 The phase behavior is determined by the effective volume fraction of the suspension and interparticle forces, such as van der Waals attractions, Coulombic interactions, steric repulsions and hydrogen-bonding. Understanding the interplay between these intermolecular forces is essential to control colloidal phase behavior, for example to fabricate ordered colloidal structures for specific applications. The majority of studies to date have been devoted to hard-sphere systems, which have simple intermolecular forces dominated by entropic excluded volume effects 2 and to other colloidal systems with relatively well-understood additional interactions, such as Coulombic repulsion and depletion attractions. 3 Only fairly recently, soft and deformable particles, in particular star polymers and microgels, have attracted significant attention under the impulse of improved synthesis techniques. These soft colloidal systems are characterized by tunability of their softness and complex interparticle interactions, which result in rich phase diagrams and unique hydrodynamic behavior. 4,5 In particular, poly(N-isopropylacrylamide) (pNIPAm)-based microgels have been studied extensively because of their reversible temperature-responsiveness. 6,7 By incorporating ionizable functional groups, e.g. acrylic acid (AAc), as co-monomers, one can synthesize ionic microgels that respond not only to temperature, but also to changes in pH and ionic strength. 8 In spite of several studies on the phase behavior of colloidal microgels as a model system for soft spheres, both from our groups and from other researchers, 7,9-12 direct measurements of the underlying interparticle interactions have not been reported. However, previous experimental observations in our groups have indicated that these interactions are non-trivial, especially in pNIPAm-co-AAc microgels. While the phase behavior of pure pNIPAm microgels can be explained relatively well by defining an effective volume fraction of particles and using hard-sphere-like interactions, the incorporation of AAc adds significant complexity to the system. Hard sphere theory is not sufficient to explain the dynamics of these pH-responsive microgels, as illustrated by the following examples from our own research. First, pNIPAm microgel suspensions are liquid-like below the hard sphere freezing volu...

Quantitative microscopy measurements have been made on poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgel dispersions as a function of time, temperature, pH, and volume fraction. These studies reveal an extreme degree of complexity in the physical aging and phase behavior of the dispersions; this complexity arises from a convolution of the system energetics at the colloidal, polymer-chain, and molecular scales. Superficially, these dispersions display the classic colloidal phases observed for spherical particles (i.e., gas, fluid, crystal, and glass). However, unlike simple repulsive hard spheres, pNIPAm-AAc dispersions are observed to evolve from a diffusive, fluidlike state immediately after being introduced into rectangular capillary tubes, to very slow crystalline or glassy phases after days or weeks of aging. In addition to this structural evolution, the free volume accessible to the microgels in crystalline or glassy phases (i.e., the cage size) decreases with time, indicating that the physical aging process does not end following assembly, but instead continues to evolve as the dispersion slowly proceeds to an equilibrium state. The temperature dependence of pNIPAm-AAc microgel swelling and how it influences the colloidal assembly was evaluated during the aging process as well. These thermal melting experiments revealed an enhancement in the thermal stability (i.e., a decrease in the influence of temperature on the phase behavior) of the assemblies during the aging process that we associate with an evolution of attractive interparticle interactions during aging. These attractive interactions dictate the time scale for assembly (aging), the final phase adopted by the dispersion, the dynamics of the final state, and the ultimate thermal stability. The culmination of these studies is the pseudoequilibrium phase behavior of pNIPAm-AAc microgel dispersions, which we present as a function of pH and volume fraction following approximately 1 month of aging. This diagram reveals highly complex dispersion characteristics that appear to be intrinsically tied to the degree of AAc protonation. In general, we find that, at pH < pK(a), the final dispersions behave in a manner that can be associated with attractive interparticle interactions, whereas at pH > pK(a), repulsive interactions appear to be dominant. These results are discussed in the context of the slow evolution of microgel swelling and attractive interaction potentials arising from reorganization and association of polymer chains via multiple weak hydrogen-bonding interactions.

We report an in situ method for three-dimensionally resolved temperature measurement in microsystems. The temperature of the surrounding fluid is correlated from Brownian diffusion of suspended nanoparticles. We use video-microscopy in combination with image analysis software to selectively track nanoparticles in the focal plane. This method is superior with regards to reproducibility and reduced systematic errors since measuring Brownian diffusivity does not rely on fluorescence intensity or lifetime of fluorophores. The efficacy of the method is demonstrated by measuring spatial temperature profiles in various microfluidic devices that generate temperature gradients and by comparing these results with numerical simulations. We show that the method is accurate and can be used to extract spatial temperature variations in three dimensions. Compared to conventional methods that require expensive multiphoton optical sectioning setups, this technique is simple and inexpensive. In addition, we demonstrate the capability of this method as an in situ tool for simultaneously observing live cells under the microscope and monitoring the local temperature of the cell medium without biochemical interference, which is crucial for quantitative studies of cells in microfluidic devices.

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