Flat Drops, Elastic Sheets, and Microcapsules by Interfacial Assembly of a Bacterial Biofilm Protein, BslA.

Protein adsorption and assembly at interfaces provide a potentially versatile route to create useful constructs for fluid compartmentalization. In this context, we consider the interfacial assembly of a bacterial biofilm protein, BslA, at air-water and oil-water interfaces. Densely packed, high modulus monolayers form at air-water interfaces, leading to the formation of flattened sessile water drops. BslA forms elastic sheets at oil-water interfaces, leading to the production of stable monodisperse oil-in-water microcapsules. By contrast, water-in-oil microcapsules are unstable but display arrested rather than full coalescence on contact. The disparity in stability likely originates from a low areal density of BslA hydrophobic caps on the exterior surface of water-in-oil microcapsules, relative to the inverse case. In direct analogy with small molecule surfactants, the lack of stability of individual water-in-oil microcapsules is consistent with the large value of the hydrophilic-lipophilic balance (HLB number) calculated based on the BslA crystal structure. The occurrence of arrested coalescence indicates that the surface activity of BslA is similar to that of colloidal particles that produce Pickering emulsions, with the stability of partially coalesced structures ensured by interfacial jamming. Micropipette aspiration and flow in tapered capillaries experiments reveal intriguing reversible and nonreversible modes of mechanical deformation, respectively. The mechanical robustness of the microcapsules and the ability to engineer their shape and to design highly specific binding responses through protein engineering suggest that these microcapsules may be useful for biomedical applications.

Mechanical stability of particle-stabilized droplets under micropipette aspiration.

We investigate the mechanical behavior of particle-stabilized droplets using micropipette aspiration. We observe that droplets stabilized with amphiphilic dumbbell-shaped particles exhibit a two-stage response to increasing suction pressure. Droplets first drip, then wrinkle and buckle like an elastic shell. While particles have a dramatic impact on the mechanism of failure, the mechanical strength of the droplets is only modestly increased. On the other hand, droplets coated with the molecular surfactant sodium dodecyl sulfate are even weaker than bare droplets. In all cases, the magnitude of the critical pressure for the onset of instabilities is set by the fluid surface tension.

Adsorption of sub-micron amphiphilic dumbbells to fluid interfaces.

We investigate the adsorption of submicrometer bulk-synthesized polymer dumbbells to oil-water interfaces using freeze-fracture, shadow-casting (FreSCa) cryo-scanning electron microscopy. We find that the dumbbells are amphiphilic and adsorb to the interface with a preferred orientation. Most particles adsorb in a tilted configuration, with the polar and apolar lobes intersecting the interface and pointing toward the water and oil, respectively. Some particles adsorb with only one lobe attached to the interface. Moreover, we find that each lobe has a preferred angle of contact with the interface, identical in all observed configurations. A simple geometrical calculation using these contact angles accurately predicts the dominant configuration of particles at the interface. This calculation provides insight into how the shape and composition of dumbbells can be tuned to stand upright and pack efficiently on curved interfaces.

Plasmon mode modifies the elastic response of a nanoscale charge density wave system.

The elastic response of suspended NbSe(3) nanowires is studied across the charge density wave phase transition. The nanoscale dimensions of the resonator lead to a large resonant frequency (~10-100 MHz), bringing the excited phonon frequency in close proximity of the plasmon mode of the electronic condensate-a parameter window not accessible in bulk systems. The interaction between the phonon and plasmon modes strongly modifies the elastic properties at high frequencies. This is manifested in the nanomechanics of the system as a sharp peak in the temperature dependence of the elastic modulus (relative change of 12.8%) in the charge density wave phase.

A surface-anchored molecular four-level conductance switch based on single proton transfer.

The development of a variety of nanoscale applications requires the fabrication and control of atomic or molecular switches that can be reversibly operated by light, a short-range force, electric current or other external stimuli. For such molecules to be used as electronic components, they should be directly coupled to a metallic support and the switching unit should be easily connected to other molecular species without suppressing switching performance. Here, we show that a free-base tetraphenyl-porphyrin molecule, which is anchored to a silver surface, can function as a molecular conductance switch. The saddle-shaped molecule has two hydrogen atoms in its inner cavity that can be flipped between two states with different local conductance levels using the electron current through the tip of a scanning tunnelling microscope. Moreover, by deliberately removing one of the hydrogens, a four-level conductance switch can be created. The resulting device, which could be controllably integrated into the surrounding nanoscale environment, relies on the transfer of a single proton and therefore contains the smallest possible atomistic switching unit.