RESUMEN
Simulations and experiments are reported for nonequilibrium steady-state assembly of small colloidal crystal clusters in rotating magnetic fields vs frequency and amplitude. High-dimensional trajectories of particle coordinates from image analysis of experiments and from Stokesian Dynamic computer simulations are fit to low-dimensional reaction coordinate based Fokker-Planck and Langevin equations. The coefficients of these equations are effective energy and diffusivity landscapes that capture configuration-dependent energy and friction for nonequilibrium steady-state dynamics. Two reaction coordinates that capture condensation and anisotropy of dipolar chains folding into crystals are sufficient to capture high-dimensional experimental and simulated dynamics in terms of first passage time distributions. Our findings illustrate how field-mediated nonequilibrium steady-state colloidal assembly dynamics can be modeled to interpret and design pathways toward target microstructures and morphologies.
RESUMEN
We report a novel approach to directly measure the interactions and deposition behavior of functional capsule delivery systems on glass substrates versus the concentration of an anionic surfactant sodium lauryl ether sulfate (SLES) and a cationic acrylamide-acrylamidopropyltrimonium copolymer (AAC). Analyses of three-dimensional optical microscopy trajectories were used to quantify lateral diffusive dynamics, deposition lifetimes, and potentials of mean force for different solution conditions. In the absence of additives, negatively charged capsule surfaces yield electrostatic repulsion with the negatively charged substrate, which inhibits deposition. With an increasing SLES concentration below the critical micelle concentration (CMC), capsule-substrate electrostatic repulsion is mediated by the charged surfactant solution that decreases the Debye length. Above the SLES CMC, depletion attraction causes enhanced deposition until eventually depletion repulsion inhibits deposition at concentrations â¼10 wt %. Addition of an ACC causes deposition via capsule-substrate bridging at all concentrations; the weakest deposition occurs at intermediate AAC concentrations from a competition of steric repulsion and attraction via a few extended bridges. The novel measurements and models of capsule interactions and deposition on substrates in this work provide a basis to fundamentally understand and rationally design complex rinse-off cleansing formulations with optimal characteristics.
RESUMEN
Colloidal matter exhibits unique collective behaviors beyond what occurs at single-nanoparticle and atomic scales. Treating colloidal particles as building blocks, researchers are exploiting new strategies to rationally organize colloidal particles into complex structures for new functions and devices. Despite tremendous progress in directed assembly and self-assembly, a truly versatile assembly technique without specific functionalization of the colloidal particles remains elusive. We develop a new strategy to assemble colloidal matter under a light-controlled temperature field, which can solve challenges in the existing assembly techniques. By adding an anionic surfactant (that is, cetyltrimethylammonium chloride), which serves as a surface charge source, a macro ion, and a micellar depletant, we generate a light-controlled thermoelectric field to manipulate colloidal atoms and a depletion attraction force to assemble the colloidal atoms into two-dimensional (2D) colloidal matter. The general applicability of this opto-thermophoretic assembly (OTA) strategy allows us to build colloidal matter of diverse colloidal sizes (from subwavelength scale to micrometer scale) and materials (polymeric, dielectric, and metallic colloids) with versatile configurations and tunable bonding strengths and lengths. We further demonstrate that the incorporation of the thermoelectric field into the optical radiation force can achieve 3D reconfiguration of the colloidal matter. The OTA strategy releases the rigorous design rules required in the existing assembly techniques and enriches the structural complexity in colloidal matter, which will open a new window of opportunities for basic research on matter organization, advanced material design, and applications.
RESUMEN
Non-equilibrium, steady-state effective pair potentials of micron-sized superparamagnetic particles in rotating magnetic fields are obtained vs. field frequency and amplitude. Trajectories of center-to-center distance between particle pairs from Brownian dynamic simulations, which were previously matched to experimental measurements, are analyzed to obtain local drift and diffusion coefficients. These coefficients are used to obtain effective interaction potentials from solving a one-dimensional Fokker-Planck equation. Biased sampling of the effective energy landscape was implemented by intermittent switching between the field of interest and a repulsive field. Our findings show how the shape and attractive well-depth of pair interactions can be tuned by changing field frequency and amplitude.
RESUMEN
Video microscopy (VM) experiments and Brownian dynamics (BD) simulations were used to measure and model superparamagnetic colloidal particles in rotating magnetic fields for interaction energies on the order of the thermal energy, kT. Results from experiments and simulations were compared for isolated particle rotation, particle rotation within doublets, doublet rotation, and separation within doublets vs field rotation frequency. Agreement between VM and BD results was obtained at all frequencies and amplitudes only by including exact two-body hydrodynamic interactions and relevant relaxation times of magnetic dipoles. Frequency-dependent particle forces and torques cause doublets to rotate at low frequencies via dipolar interactions and at high frequencies via hydrodynamic translation-rotation coupling. By matching measurements and simulations for a range of conditions, our findings unambiguously demonstrate the quantitative forms of dipolar and hydrodynamic interactions necessary to capture nonequilibrium, steady-state dynamics of Brownian colloids in magnetic fields.