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We fabricated thermoresponsive colloidal molecules of ca. 250 nm size. Electron- and scanning force microscopy reveal the dumbbell-shaped morphology. The temperature dependence of the size and aspect ratio (ca. 1.4 to 1.6) is analyzed by depolarized dynamic light scattering and found to be in good agreement with microscopic evidence.
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Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) particles of different sizes are synthesized by varying the concentration of sodium dodecyl sulphate (SDS) in a one-pot method. The sizes, size polydispersities and the thermoresponsivity of the PNIPAM particles are characterized by using dynamic light scattering and scanning electron microscopy. It is observed that the sizes of these particles decrease with increase in SDS concentration. Swelling ratios of PNIPAM particles measured from the thermoresponsive curves are observed to increase with decrease in particle size. This observation is understood by minimizing the Helmholtz free energy of the system with respect to the swelling ratio of the particles. Finally, the dynamics of these particles in jammed aqueous suspensions are investigated by performing rheological measurements.
This is an integrated experimental and theoretical study of the dynamics and rheology of self-crosslinked, slightly charged, temperature responsive soft Poly(N-isopropylacrylamide) (pNIPAM) microgels over a wide range of concentration and temperature spanning the sharp change in particle size and intermolecular interactions across the lower critical solution temperature (LCST). Dramatic, non-monotonic changes in viscoelasticity are observed with temperature, with distinctive concentration dependences in the dense fluid, glassy, and soft-jammed states. Motivated by our experimental observations, we formulate a minimalistic model for the size dependence of a single microgel particle and the change of interparticle interaction from purely repulsive to attractive upon heating. Using microscopic equilibrium and time-dependent statistical mechanical theories, theoretical predictions are quantitatively compared with experimental measurements of the shear modulus. Good agreement is found for the nonmonotonic temperature behavior that originates as a consequence of the competition between reduced microgel packing fraction and increasing interpar-ticle attractions. Testable predictions are made for nonlinear rheological properties such as the yield stress and strain. To the best of our knowledge, this is the first attempt to quantitatively understand in a unified manner the viscoelasticity of dense, temperature-responsive microgel suspensions spanning a wide range of temperatures and concentrations.
Single and double paramagnetic colloidal particles are placed above a magnetic square pattern and are driven with an external magnetic field precessing around a high symmetry direction of the pattern. The external magnetic field and that of the pattern confine the colloids into lanes parallel to a lattice vector of the pattern. The precession of the external field causes traveling minima of the magnetic potential along the direction of the lanes. At sufficiently high frequencies of modulation only the doublets respond to the external field and move in direction of the traveling minima along the lanes, while the single colloids cannot follow and remain static. We show how the doublets can induce a coordinated motion of the single colloids building colloidal trains made of a chain of several single colloids transported by doublets.
Topological insulators insulate in the bulk but exhibit robust conducting edge states protected by the topology of the bulk material. Here, we design a colloidal topological insulator and demonstrate experimentally the occurrence of edge states in a classical particle system. Magnetic colloidal particles travel along the edge of two distinct magnetic lattices. We drive the colloids with a uniform external magnetic field that performs a topologically non-trivial modulation loop. The loop induces closed orbits in the bulk of the magnetic lattices. At the edge, where both lattices merge, the colloids perform skipping orbits trajectories and hence edge-transport. We also observe paramagnetic and diamagnetic colloids moving in opposite directions along the edge between two inverted patterns; the analogue of a quantum spin Hall effect in topological insulators. We present a new, robust, and versatile way of transporting colloidal particles, enabling new pathways towards lab on a chip applications.
Colloidal crystals formed by size-asymmetric binary particles co-assemble into a wide variety of colloidal compounds with lattices akin to ionic crystals. Recently, a transition from a compound phase with a sublattice of small particles to a metal-like phase in which the small particles are delocalized has been predicted computationally and observed experimentally. In this colloidal metallic phase, the small particles roam the crystal maintaining the integrity of the lattice of large particles, as electrons do in metals. A similar transition also occurs in superionic crystals, termed sublattice melting. Here, we use energetic principles and a generalized molecular dynamics model of a binary system of functionalized nanoparticles to analyze the transition to sublattice delocalization in different co-assembled crystal phases as a function of T, number of grafted chains on the small particles, and number ratio between the small and large particles $n_s$:$n_l$. We find that $n_s$:$n_l$ is the primary determinant of crystal type due to energetic interactions and interstitial site filling, while the number of grafted chains per small particle determines the stability of these crystals. We observe first-order sublattice delocalization transitions as T increases, in which the host lattice transforms from low- to high-symmetry crystal structures, including A20 to BCT to BCC, Ad to BCT to BCC, and BCC to BCC/FCC to FCC transitions and lattices. Analogous sublattice transitions driven primarily by lattice vibrations have been seen in some atomic materials exhibiting an insulator-metal transition also referred to as metallization. We also find minima in the lattice vibrations and diffusion coefficient of small particles as a function of $n_s$:$n_l$, indicating enhanced stability of certain crystal structures for $n_s$:$n_l$ values that form compounds.
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