Analysis of the Field-Assisted Permanent Assembly of Oppositely Charged Particles

We characterize experimentally and analyze analytically a novel electric-field-assisted process for the assembly of permanent chains of oppositely charged microparticles in an aqueous environment. Long chains of oppositely charged particles are rapidly formed when an external electric field is applied and break up into permanent linear fragments upon switching off the field. The resulting secondary chains are stabilized by attractive electrostatic and van der Waals interactions between the particles. We find that the length of the permanent chains is strongly dependent on the relative size (microsphere diameter <i>D</i>) of small and large particles and can be tuned by varying the particle size ratio <i>s</i> = <i>D</i>_sm/<i>D</i>_lg and particle number ratio <i>r</i> = <i>N</i>_sm/<i>N</i>_lg. Three latex microsphere systems of different particle size ratio, <i>s</i> = 0.9, 0.45, and 0.225, were characterized at different particle number ratios <i>r</i> by determining experimentally the length distribution of the permanent chains. The results are compared with statistical models based on a one-step or two-step process of forming the primary chains. We find that the one-step model is applicable to the system of similarly sized particles (<i>s</i> = 0.9) and the two-step chaining model is applicable to the system of dissimilarly sized particles (<i>s</i> = 0.225), where the large particles form chains first and the small ones serve as binders, which are later drawn in the junctions. Long permanent chains are formed only from particles of dissimilar size for which our model predicts a linear increase in the mean chain length with increasing <i>r</i>. On the basis of these results, we formulate a set of assembly rules for permanent colloidal chain formation by oppositely charged particles. The results make possible the precise large-scale formation of particle chains of any length, which can serve as components in new gels, biomaterials, and fluids with controlled rheology

Fabrication of Photoreactive Biocomposite Coatings via Electric Field-Assisted Assembly of Cyanobacteria

We report how dielectrophoresis (DEP) can be used as a tool for the fabrication of biocomposite coatings of photoreactive cyanobacteria (<i>Synechococcus</i> PCC7002) on flexible polyester sheets (PEs). The PE substrates were precoated by a layer-by-layer assembled film of charged polyelectrolytes. In excellent agreement between experimental data and numerical simulations, the directed assembly process driven by external electric field results in the formation of 1D chains and 2D sheets by the cells. The preassembled cyanobacteria chains and arrays became deposited on the substrate and remained in place after the electric field was turned off due to the electrostatic attraction between the negatively charged cell surfaces and the positively charged polyelectrolyte-coated PE. The DEP-assisted packing of cyanobacteria is close to the maximal surface coverage of ∼70% estimated from convectively assembled monolayers. Confocal laser scanning microscopy and spectrophotometry confirm that the photosynthetic pigment integrity of the <i>Synechococcus</i> cells is preserved after DEP immobilization. The significant decrease of the light scattering and the enhanced transmittance of these field-assembled cyanobacteria coatings demonstrate reduced self-shading compared to suspension cultures. Thus, we achieved the assembly of structured cyanobacteria coatings that optimize cell surface coverage and preserve cell viability after immobilization. This is a step toward the development of flexible multilayered cell-based photoabsorbing biomaterials that can serve as components of “biomimetic leaves” for utilizing solar energy to recycle CO₂ into fuels or chemicals

Assembling Wormlike Micelles in Tubular Nanopores by Tuning Surfactant–Wall Interactions

Threadlike molecular assemblies are excluded from narrow pores unless attractive interactions with the confining pore walls compensate for the loss of configurational entropy. Here we show that wormlike surfactant micelles can be assembled in the 8 nm tubular nanopores of SBA-15 silica by adjusting the surfactant–pore-wall interactions. The modulation of the interactions was achieved by coadsorption of a surface modifier that also provides control over the partitioning of wormlike aggregates between the bulk solution and the pore space. We anticipate that the concept of tuning the interactions with the pore wall will be applicable to a wide variety of self-assembling molecules and pores

Synergistic Role of Temperature and Salinity in Aggregation of Nonionic Surfactant-Coated Silica Nanoparticles

The adsorption of nonionic surfactants onto hydrophilic nanoparticles (NPs) is anticipated to increase their stability in aqueous medium. While nonionic surfactants show salinity- and temperature-dependent bulk phase behavior in water, the effects of these two solvent parameters on surfactant adsorption and self-assembly onto NPs are poorly understood. In this study, we combine adsorption isotherms, dispersion transmittance, and small-angle neutron scattering (SANS) to investigate the effects of salinity and temperature on the adsorption of pentaethylene glycol monododecyl ether (C12E5) surfactant on silica NPs. We find an increase in the amount of surfactant adsorbed onto the NPs with increasing temperature and salinity. Based on SANS measurements and corresponding analysis using computational reverse-engineering analysis of scattering experiments (CREASE), we show that the increase in salinity and temperature results in the aggregation of silica NPs. We further demonstrate the non-monotonic changes in viscosity for the C12E5–silica NP mixture with increasing temperature and salinity and correlate the observations to the aggregated state of NPs. The study provides a fundamental understanding of the configuration and phase transition of the surfactant-coated NPs and presents a strategy to manipulate the viscosity of such dispersion using temperature as a stimulus

Capillary Bridging as a Tool for Assembling Discrete Clusters of Patchy Particles

Janus and patchy particles are emerging as models for studying complex directed assembly patterns and as precursors of new structured materials and composites. Here we show how lipid-induced capillary bridging could serve as a new and nonconventional method of assembling patchy particles into ordered structures. Iron oxide surface patches on latex microspheres were selectively wetted with liquid lipid, driving the particle assembly into two- and three-dimensional clusters via interparticle capillary bridge formation. The liquid phase of the bridges allows local reorganization of the particles within the clusters and assists in forming <i>true</i> equilibrium configurations. The temperature-driven fluid-to-gel and gel-to-fluid phase transitions of the fatty acids within the bridge act as a thermal switch for cluster assembly and disassembly. By complementing the experiments with Monte Carlo simulations, we show that the equilibrium cluster morphology is determined by the patch characteristics, namely, their size, number, and shape. This study demonstrates the ability of capillary bridging as a versatile tool to assemble thermoresponsive clusters and aggregates. This method of binding particles is simple, robust, and generic and can be extended further to assemble particles with nonspherical shapes and complex surface chemistries enabling the formation of sophisticated colloidal molecules

Capillary Bridging as a Tool for Assembling Discrete Clusters of Patchy Particles

Janus and patchy particles are emerging as models for studying complex directed assembly patterns and as precursors of new structured materials and composites. Here we show how lipid-induced capillary bridging could serve as a new and nonconventional method of assembling patchy particles into ordered structures. Iron oxide surface patches on latex microspheres were selectively wetted with liquid lipid, driving the particle assembly into two- and three-dimensional clusters via interparticle capillary bridge formation. The liquid phase of the bridges allows local reorganization of the particles within the clusters and assists in forming <i>true</i> equilibrium configurations. The temperature-driven fluid-to-gel and gel-to-fluid phase transitions of the fatty acids within the bridge act as a thermal switch for cluster assembly and disassembly. By complementing the experiments with Monte Carlo simulations, we show that the equilibrium cluster morphology is determined by the patch characteristics, namely, their size, number, and shape. This study demonstrates the ability of capillary bridging as a versatile tool to assemble thermoresponsive clusters and aggregates. This method of binding particles is simple, robust, and generic and can be extended further to assemble particles with nonspherical shapes and complex surface chemistries enabling the formation of sophisticated colloidal molecules

Capillary Bridging as a Tool for Assembling Discrete Clusters of Patchy Particles

Janus and patchy particles are emerging as models for studying complex directed assembly patterns and as precursors of new structured materials and composites. Here we show how lipid-induced capillary bridging could serve as a new and nonconventional method of assembling patchy particles into ordered structures. Iron oxide surface patches on latex microspheres were selectively wetted with liquid lipid, driving the particle assembly into two- and three-dimensional clusters via interparticle capillary bridge formation. The liquid phase of the bridges allows local reorganization of the particles within the clusters and assists in forming <i>true</i> equilibrium configurations. The temperature-driven fluid-to-gel and gel-to-fluid phase transitions of the fatty acids within the bridge act as a thermal switch for cluster assembly and disassembly. By complementing the experiments with Monte Carlo simulations, we show that the equilibrium cluster morphology is determined by the patch characteristics, namely, their size, number, and shape. This study demonstrates the ability of capillary bridging as a versatile tool to assemble thermoresponsive clusters and aggregates. This method of binding particles is simple, robust, and generic and can be extended further to assemble particles with nonspherical shapes and complex surface chemistries enabling the formation of sophisticated colloidal molecules