Dynamic Self-Assembly in Ensembles of Camphor Boats

Millimeter-sized gel particles loaded with camphor and floating at the interface between water and air generate convective flows around them. These flows give rise to repulsive interparticle interactions, and mediate dynamic self-assembly of nonequilibrium particle formations. When the numbers of particles, N, are small, particle motions are uncorrelated. When, however, N exceeds a threshold value, particles organize into ordered lattices. The nature of hydrodynamic forces underlying these effects and the dynamics of the self-assembling system are modeled numerically using Navier−Stokes equations as well as analytically using scaling arguments

Nonconductive Noncharging Composites: Tunable and Stretchable Materials for Adaptive Prevention of Charging by Contact Electrification

Static charge generated by contact electrification can cause a wide range of undesirable consequences in our lives and in industry (e.g., adhesion of particles on surfaces, damage to electronics, and explosions). It has, however, been challenging to develop methods to prevent charging due to the vast types of materials that charge easily by contact electrification and the frequent changes in process and environmental conditions. The most common method is to use conductive materials for dissipating charge away; however, it is ineffective for many circumstances. Here, we propose a general and effective materials framework that involves a two-level consideration for preparing noncharging materials: (1) the variation of the proportion of a two-material composite and (2) the extent of stretching the composite material. This materials strategy is achieved by infusing particles within a stretchable bulk material. Importantly, the preparation of the noncharging surface for (1) is based on a novel fundamental mechanism that involves combining an appropriate amount of a material (e.g., the particles) that tends to charge positively with another material (e.g., the bulk material) that tends to charge negatively. This mechanism does not rely on conductivity; both the contacting materials naturally prevent the generation of static charge even when only nonconductive materials are involved. When the composite material is stretchable, the change in proportion of the surface coverage of the particles allows the charging response to be changed. Therefore, the variation in composition and stretching provide a wide two-dimensional parameter space for achieving noncharging response for the vast range of contacting materials that are used in industry and our lives. In addition, stretchability allows the composite material to flexibly adapt to changes in process and environmental conditions. This stretchable composite material was also demonstrated to be capable of preventing the adhesion of particles and separating particles of different materials

Swarming in Shallow Waters

A swarm is a collection of separate objects that move autonomously in the same direction in a concerted fashion. This type of behavior is observed in ensembles of various organisms but has proven inherently difficult to realize in artificial chemical systems, where the components have to self-assemble dynamically and, at the same time, propel themselves. This paper describes a class of systems in which millimeter-sized components interact hydrodynamically and organize into dissipative structures that swarm in thin fluid layers. Depending on the geometry of the particles, various types of swarms can be engineered, including ensembles that rotate, follow a “leader”, or are pushed in front of a larger particle

Blocking of Disulfide Adsorption by Coadsorbing ω-Functionalized Alkane Thiols Revealed by Wet Stamping and Fluorescence Microscopy

When alkane thiols and disulfides coadsorb onto gold, they do not necessarily create a mixed monolayer. In particular, when thiols are terminated in groups capable of hydrogen bonding, they can altogether eliminate adsorption of disulfides. Such elimination can be observed directly by using fluorescently labeled disulfides and monitoring their adsorption (or lack of) by fluorescence microscopy. These experiments suggest a mechanism in which adsorption of thiols is facilitated by hydrogen bonding

Correlating Material Transfer and Charge Transfer in Contact Electrification

Static charge on surfaces of materials is generated when two solid surfaces come into contact and are then separated. It is important to understand the phenomenon because the influence of static charge on surfaces is widely felt in our daily lives and can have a wide range of applications or undesirable effects in industry. Although the phenomenon has been observed since antiquity, the fundamental mechanism that underlies the generation of charge on insulating surfaces is still not known. After many decades of research, different mechanisms have been proposed, including electron and ion transfer. One other possibility has been discussed to a lesser extent: material transfer (i.e., the transfer of quantities of charged materials). This study seeks to investigate the significance of material transfer by correlating the amount of charge transferred and the amount of material transferred from one surface to another after contact. The investigation involved varying the degree of softness of a polymer (polydimethylsiloxane; PDMS), contact-charging it against another reference material, and analyzing the surfaces of the materials after contact. Results showed that when more material transferred, more charge was generated. An explanation for these results is that the surface of PDMS experienced heterolytic cleavage of bonds, which resulted in the generation of charge. When more cleavage of bonds occurred, more charge was generated, and more materials were transferred. Hence, material transfer seems to have an important contribution for the generation of charge by contact

Contact De-electrification of Electrostatically Charged Polymers

The contact electrification of insulating organic polymers is still incompletely understood, in part because multiple fundamental mechanisms may contribute to the movement of charge. This study describes a mechanism previously unreported in the context of contact electrification: that is, “contact de-electrification”, a process in which polymers charged to the same polarity discharge on contact. Both positively charged polymeric beads, e.g., polyamide 6/6 (Nylon) and polyoxymethylene (Delrin), and negatively charged polymeric beads, e.g., polytetrafluoroethylene (Teflon) and polyamide-imide (Torlon), discharge when the like-charged beads are brought into contact. The beads (both with charges of ∼±20 μC/m², or ∼100 charges/μm²) discharge on contact regardless of whether they are made of the same material, or of different materials. Discharge is rapid: discharge of flat slabs of like-charged Nylon and Teflon pieces is completed on a single contact (∼3 s). The charge lost from the polymers during contact de-electrification transfers onto molecules of gas in the atmosphere. When like-charged polymers are brought into contact, the increase in electric field at the point of contact exceeds the dielectric breakdown strength of the atmosphere and ionizes molecules of the gas; this ionization thus leads to discharge of the polymers. The detection (using a Faraday cup) of charges transferred to the cup by the ionized gas is compatible with the mechanism. Contact de-electrification occurs for different polymers and in atmospheres with different values of dielectric breakdown strength (helium, argon, oxygen, carbon dioxide, nitrogen, and sulfur hexafluoride): the mechanism thus appears to be general

Magnetic Levitation as a Platform for Competitive Protein–Ligand Binding Assays

This paper describes a method based on magnetic levitation (MagLev) that is capable of indirectly measuring the binding of unlabeled ligands to unlabeled protein. We demonstrate this method by measuring the affinity of unlabeled bovine carbonic anhydrase (BCA) for a variety of ligands (most of which are benzene sulfonamide derivatives). This method utilizes porous gel beads that are functionalized with a common aryl sulfonamide ligand. The beads are incubated with BCA and allowed to reach an equilibrium state in which the majority of the immobilized ligands are bound to BCA. Since the beads are less dense than the protein, protein binding to the bead increases the overall density of the bead. This change in density can be monitored using MagLev. Transferring the beads to a solution containing no protein creates a situation where net protein efflux from the bead is thermodynamically favorable. The rate at which protein leaves the bead for the solution can be calculated from the rate at which the levitation height of the bead changes. If another small molecule ligand of BCA is dissolved in the solution, the rate of protein efflux is accelerated significantly. This paper develops a reaction-diffusion (RD) model to explain both this observation, and the physical-organic chemistry that underlies it. Using this model, we calculate the dissociation constants of several unlabeled ligands from BCA, using plots of levitation height versus time. Notably, although this method requires no electricity, and only a single piece of inexpensive equipment, it can measure accurately the binding of unlabeled proteins to small molecules over a wide range of dissociation constants (<i>K</i>_d values within the range from ∼10 nM to 100 μM are measured easily). Assays performed using this method generally can be completed within a relatively short time period (20 min–2 h). A deficiency of this system is that it is not, in its present form, applicable to proteins with molecular weight greater than approximately 65 kDa

Photoswitchable Catalysis Mediated by Dynamic Aggregation of Nanoparticles

Catalytic activity of gold nanoparticles in a hydrosilylation reaction is controlled by irradiation with UV or visible light. When exposed to UV, the particles aggregate and the catalysis is effectively switched “off”. When the particles are exposed to visible light, the particles redisperse and catalysis can proceed

Rapid Deposition of Hydrophobic Nanoparticle Monolayers onto Hydrophilic Surfaces from Liquid−Liquid Interfaces

Dense, hydrophobic coatings comprising hydrophilic nanoparticles are deposited rapidly from water/toluene emulsions. The process of deposition is driven by a subtle interplay between interfacial phenomena, electrostatic interparticle repulsions, and hydrogen bonding between the NPs and the substrate(s). The packing fractions and the plasmonic properties of the coatings can be controlled by the pH of the aqueous phase. Once formed, the coatings can be further functionalized without a loss of mechanical integrity

Rapid Deposition of Hydrophobic Nanoparticle Monolayers onto Hydrophilic Surfaces from Liquid−Liquid Interfaces

Dense, hydrophobic coatings comprising hydrophilic nanoparticles are deposited rapidly from water/toluene emulsions. The process of deposition is driven by a subtle interplay between interfacial phenomena, electrostatic interparticle repulsions, and hydrogen bonding between the NPs and the substrate(s). The packing fractions and the plasmonic properties of the coatings can be controlled by the pH of the aqueous phase. Once formed, the coatings can be further functionalized without a loss of mechanical integrity