Georgios Polizos

Air-stable droplet interface bilayers on oil-infused surfaces

Significance By suspending a lipid bilayer in an aperture or between water droplets, single-molecule transport through membrane channels can be electrically detected. To date, all suspended bilayers have been confined to fluid reservoirs. This study demonstrates that droplet interface bilayers can be created in an ambient environment using noncoalescing water droplets on an oil-infused surface. Air-stable droplet interface bilayers are easy to manipulate and electrically characterize, and could potentially allow for the biosensing of airborne molecules. Droplet interface bilayers are versatile model membranes useful for synthetic biology and biosensing; however, to date they have always been confined to fluid reservoirs. Here, we demonstrate that when two or more water droplets collide on an oil-infused substrate, they exhibit noncoalescence due to the formation of a thin oil film that gets squeezed between the droplets from the bottom up. We show that when phospholipids are included in the water droplets, a stable droplet interface bilayer forms between the noncoalescing water droplets. As with traditional oil-submerged droplet interface bilayers, we were able to characterize ion channel transport by incorporating peptides into each droplet. Our findings reveal that droplet interface bilayers can function in ambient environments, which could potentially enable biosensing of airborne matter.

Properties of a nanodielectric cryogenic resin

Physical properties of a nanodielectric composed of in situ synthesized titanium dioxide (TiO2) nanoparticles (≤5 nm in diameter) and a cryogenic resin are reported. The dielectric losses were reduced by a factor of 2 in the nanocomposite, indicating that the presence of small TiO2 nanoparticles restricted the mobility of the polymer chains. Dielectric breakdown data of the nanodielectric was distributed over a narrower range than that of the unfilled resin. The nanodielectric had 1.56 times higher 1% breakdown probability than the resin, yielding 0.64 times thinner insulation thickness for the same voltage level, which is beneficial in high voltage engineering.

Nonfunctionalized Polydimethyl Siloxane Superhydrophobic Surfaces Based on Hydrophobic−Hydrophilic Interactions

Superhydrophobic surfaces based on polydimethyl siloxane (PDMS) were fabricated using a 50:50 PDMS-poly(ethylene glycol) (PEG) blend. PDMS was mixed with PEG, and incomplete phase separation yielded a hierarchic structure. The phase-separated mixture was annealed at a temperature close to the crystallization temperature of the PEG. The PEG crystals were formed isothermally at the PDMS/PEG interface, leading to an engineered surface with PDMS spherulites. The resulting roughness of the surface was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The PDMS spherulites, a few micrometers in diameter observed from SEM images, were found to have an undulated (rippled) surface with nanometer-sized features. The combination of micrometer- and nanometer-sized surface features created a fractal surface and increased the water contact angle (WCA) of PDMS more than 60°, resulting in a superhydrophobic PDMS surface with WCA of >160°. The active surface layer for the superhydrophobicity was approximately 100 μm thick, illustrating that the material had bulk superhydrophobicity compared to conventional fluorocarbon or fluorinated coated rough surfaces. Theoretical analysis of the fractal surface indicates that the constructed surface has a fractal dimension of 2.5, which corresponds to the Apollonian sphere packing.

Strong and Electrically Conductive Graphene-Based Composite Fibers and Laminates

Graphene is an ideal candidate for lightweight, high-strength composite materials given its superior mechanical properties (specific strength of 130 GPa and stiffness of 1 TPa). To date, easily scalable graphene-like materials in a form of separated flakes (exfoliated graphene, graphene oxide, and reduced graphene oxide) have been investigated as candidates for large-scale applications such as material reinforcement. These graphene-like materials do not fully exhibit all the capabilities of graphene in composite materials. In the current study, we show that macro (2 inch × 2 inch) graphene laminates and fibers can be produced using large continuous sheets of single-layer graphene grown by chemical vapor deposition. The resulting composite structures have potential to outperform the current state-of-the-art composite materials in both mechanical properties and electrical conductivities (>8 S/cm with only 0.13% volumetric graphene loading and 5 × 10(3) S/cm for pure graphene fibers) with estimated graphene contributions of >10 GPa in strength and 1 TPa in stiffness.

Breakdown in liquid nitrogen in the presence of thermally generated bubbles for different electrode geometries

Liquid nitrogen is used as the cryogen and dielectric for many high temperature superconducting high voltage applications. When a quench in the superconductor occurs, bubbles are generated which can affect the dielectric properties of the liquid nitrogen. An experiment has been set up to generate bubbles in liquid nitrogen. Bubbles were generated using a kapton heater. Three different electrode geometries were applied: plane-plane, sphere-sphere, and sphere-plane. Breakdown measurements were made in both open bath liquid nitrogen and in a pressurized dewar at pressures up to 200 kPa absolute. The voltage applied was 60 Hz AC. For sphere-plane it was observed that the breakdown did not always occur at the minimum gap and this was likely due to the actual location of the bubble when breakdown was initiated. For plane-plane geometry where bubbles were generated in the plane electrode, breakdown voltages dropped at a certain heater power and remained low thereafter. The heater power at which the drop occurred increased with pressure. Breakdown data for subcooled liquid nitrogen will also be presented.

Effect of Bubbles on Liquid Nitrogen Breakdown in Plane-Plane Electrode Geometry From 100–250 kPa

Liquid nitrogen (LN2) is used as the cryogen and dielectric for many high temperature superconducting, high voltage applications. When a quench in the superconductor occurs, bubbles are generated which can affect the dielectric breakdown properties of the LN2. Experiments were performed using plane-plane electrode geometry where bubbles were introduced into the gap through a pinhole in the ground electrode. Bubbles were generated using one or more kapton heaters producing heater powers up to 30 W. Pressure was varied from 100-250 kPa. Breakdown strength was found to be relatively constant up to a given heater power and pressure at which the breakdown strength drops to a low value depending on the pressure. After the drop the breakdown strength continues to drop gradually at higher heater power. This is particularly illustrated at 100 kPa. After the drop in breakdown strength the breakdown is believed to be due to the formation of a vapor bridge. Also the heater power at which the breakdown strength changes from that of LN2 to that of gaseous nitrogen increases with increasing pressure. The data can provide design constraints for high temperature superconducting fault current limiters (FCLs) so that the formation of a vapor bridge can be suppressed or avoided.

Low cost anti-soiling coatings for CSP collector mirrors and heliostats

Most concentrating solar power (CSP) facilities in the USA are located in the desert southwest where open land and sunshine are abundant, but airborne dust is prevalent. The accumulation of dust, sand and other natural pollutants on collector mirrors and heliostats presents a significant operational problem and M&O cost for the CSP facilities in this region. The optical performance of the CSP collectors is key to achieving low electricity costs, where a 1% decrease in reflectance directly leads to a 1% increase in the levelized cost of electricity (LCOE) generated by these facilities. In this paper we describe the development of low cost, easy to apply anti-soiling coatings based on superhydrophobic (SH) functionalized nano silica materials and polymer binders that possess the key requirements necessary to inhibit particulate deposition on, and adhesion to, CSP mirror surfaces, and thereby significantly reducing mirror cleaning costs and facility downtime. The key requirements for these coatings are excellent optical clarity with minimal diffuse reflectance, and coating mechanical and exposure durability in harsh desert environments while maintaining SH and dirt shedding properties. The coatings developed to date have excellent SH properties with water contact angles >165° and rolling angles <5°. The solar weighted optical reflectance of the anti-soiling coating over the wavelength range 250 nm to 3μm is >99% that of uncoated mirror surfaces with coating diffuse reflectance being <1% over this wavelength range. Ongoing mechanical and accelerated solar UVA exposures also indicate these coatings will meet the required durability goals.

Epoxy nanodielectrics fabricated with in situ and ex situ techniques

In this study, we report fabrication and characterisation of a nanocomposite system composed of a commercial resin and extremely small (several nanometres in diameter) titanium dioxide particles. Nanoparticles were synthesised in situ with particle nucleation occurring inside the resin matrix. In this nanodielectric fabrication method, the nanoparticle precursor was mixed to the resin solution, and the nanoparticles were in situ precipitated. Note that no high shear mixing equipment was needed to improve particle dispersion – nanoparticles were distributed in the polymer matrix uniformly since particle nucleation occurs uniformly throughout the matrix. The properties of in situ nanodielectrics are compared to the unfilled resin and an ex situ nanocomposite. We anticipate that the presented in situ nanocomposite would be employed in high-temperature superconductivity applications. In additions, the improvement shown in the dielectric breakdown indicates that conventional high-voltage components and systems can be reduced in size with novel nanodielectrics.

Colloidosome like structures: self-assembly of silica microrods

Self-assembly of one-dimensional structures is attracting a great deal of interest because assembled structures can provide better properties compared to individual building blocks. In the present work, silica microrod self-assembly has been demonstrated by exploiting Pickering emulsion based strategy. Micron-sized silica rods were synthesized employing previously reported methods based on polyvinylpyrrolidone/pentanol emulsion droplets. Rods self-assembled to make structures in the range of ≈10–40 μm. Smooth rods assembled better than segmented rods. The assembled structures were bonded by weak van der Waals forces.

Control of membrane permeability in air-stable droplet interface bilayers

Air-stable droplet interface bilayers (airDIBs) on oil-infused surfaces are versatile model membranes for synthetic biology applications, including biosensing of airborne species. However, airDIBs are subject to evaporation, which can, over time, destabilize them and reduce their useful lifetime compared to traditional DIBs that are fully submerged in oil. Here, we show that the lifetimes of airDIBs can be extended by as much as an order of magnitude by maintaining the temperature just above the dew point. We find that raising the temperature from near the dew point (which was 7 °C at 38.5% relative humidity and 22 °C air temperature) to 20 °C results in the loss of hydrated water molecules from the polar headgroups of the lipid bilayer membrane due to evaporation, resulting in a phase transition with increased disorder. This dehydration transition primarily affects the bilayer electrical resistance by increasing the permeability through an increasingly disordered polar headgroup region of the bilayer. Temperature and relative humidity are conveniently tunable parameters for controlling the stability and composition of airDIB membranes while still allowing for operation in ambient environments.