Alex Wu

Bactericidal activity of black silicon

Black silicon is a synthetic nanomaterial that contains high aspect ratio nanoprotrusions on its surface, produced through a simple reactive-ion etching technique for use in photovoltaic applications. Surfaces with high aspect-ratio nanofeatures are also common in the natural world, for example, the wings of the dragonfly Diplacodes bipunctata. Here we show that the nanoprotrusions on the surfaces of both black silicon and D. bipunctata wings form hierarchical structures through the formation of clusters of adjacent nanoprotrusions. These structures generate a mechanical bactericidal effect, independent of chemical composition. Both surfaces are highly bactericidal against all tested Gram-negative and Gram-positive bacteria, and endospores, and exhibit estimated average killing rates of up to ~450,000 cells min−1 cm−2. This represents the first reported physical bactericidal activity of black silicon or indeed for any hydrophilic surface. This biomimetic analogue represents an excellent prospect for the development of a new generation of mechano-responsive, antibacterial nanomaterials.

Air-directed attachment of coccoid bacteria to the surface of superhydrophobic lotus-like titanium

Superhydrophobic titanium surfaces fabricated by femtosecond laser ablation to mimic the structure of lotus leaves were assessed for their ability to retain coccoid bacteria. Staphylococcus aureus CIP 65.8T, S. aureus ATCC 25923, S. epidermidis ATCC 14990T and Planococcus maritimus KMM 3738 were retained by the surface, to varying degrees. However, each strain was found to preferentially attach to the crevices located between the microscale surface features. The upper regions of the microscale features remained essentially cell-free. It was hypothesised that air entrapped by the topographical features inhibited contact between the cells and the titanium substratum. Synchrotron SAXS revealed that even after immersion for 50 min, nano-sized air bubbles covered 45% of the titanium surface. After 1 h the number of cells of S. aureus CIP 65.8T attached to the lotus-like titanium increased to 1.27 × 105 mm−2, coinciding with the replacement of trapped air by the incubation medium.

Diatom attachment inhibition: limiting surface accessibility through air entrapment

Surfaces consisting of sub micron holes (0.420-0.765 μm) engineered into nanoparticle (12 nm) coatings were examined for marine antifouling behaviour that defines early stage settlement. Immersed surfaces were found to be resistant to a 5-hour attachment assay of Amphora coffeaeformis, a marine organism commonly found in abundance on fouled substrates such as foul-releasing paints and self-polishing coatings. Attachment inhibition was attributed to the accessibility of diatoms to the surface. This was governed by the size and morphology of trapped interfacial air pockets measured in-situ using synchrotron small angle x-ray scattering. Surfaces containing larger pores (0.765 μm) exhibited the highest resistance. Macroscopic wettability via contact angle measurements however remained at 160° and sliding angle of < 5° and was found to be independent of pore size and not indicative of early stage fouling behaviour. The balance of hierarchical nano/micro length scales was critical in defining the early stage stability of biofouling character of the interface.

Marine antifouling from thin air.

The dynamic relationship between the settlement behaviour of marine biota (cells, spores, larvae) and the longevity of an entrapped air layer (plastron) on submersed superhydrophobic surfaces was systematically investigated. Plastron lifetime decreased with increasing hydrophobic polymer loadings, and was correlated with the settlement rate of a range of fouling species of varying length scale, motility and hydrophobic/hydrophilic surface preference. The results show that the level of fouling on immersed superhydrophobic surfaces was greater when plastron lifetimes were minimal, regardless of the length scale, motility and the surface preference of the organisms. This is the first direct demonstration of the broad-spectrum attachment-inhibiting properties of a plastron on an immersed superhydrophobic surface.

Hierarchical surfaces: an in situ investigation into nano and micro scale wettability

Two scales of roughness are imparted onto silicon surfaces by isotropically patterning micron sized pillars using photolithography followed by an additional nanoparticle coating. Contact angles of the patterned surfaces were observed to increase with the addition of the nanoparticle coating, several of which, exhibited superhydrophobic characteristics. Freeze fracture atomic force microscopy and in situ synchrotron SAXS were used to investigate the micro- and nano-wettability of these surfaces using aqueous liquids of varying surface tension. The results revealed that scaling different roughness morphologies result in unique wetting characteristics. It indicated that surfaces with micro, nano or dual scale roughness induced channels for the wetting liquid as per capillary action. With the reduction of liquid surface tension, nano-wetting behaviour differed between superhydrophobic and non-superhydrophobic dual-scale roughness surfaces. Micro-wetting behaviour, however, remained consistent. This suggests that micro- and nano-wetting are mutually exclusive, and that the order in which they occur is ultimately governed by the energy expenditure of the entire system.

Effect of surface wettability on carbon nanotube water-based nanofluid droplet impingement heat transfer

Recent studies into droplet impingement heat transfer have demonstrated that it has great potential for providing high heat flux cooling in areas such as thermal management of electronics. The wettability of the surface affects the flow dynamics of the impingement process and the resulting heat transfer. In this study, the effect of surface wettability on carbon nanotube water-based nanofluid droplet impingement heat transfer has been studied and compared with water. Superhydrophobic or hydrophilic coatings are applied on one face of monocrystalline silicon wafers (the drop impinges on this face) while the other face is painted matt black to permit infrared thermography. The silicon wafer is preheated to 40 °C and a single droplet impinges normally on the top facing coated surface of the monocrystalline silicon wafer. The inverse heat conduction problem has been solved using the measured black face temperature. For both the water and nanofluid droplets, the convective heat transfer coefficient reduces with the decrease in surface wettability. It is found that the nanofluid produce a significantly higher convective heat transfer coefficient during droplet impingement than water, with the enhancement increasing with increasing wettability.

Near-edge X-ray absorption fine structure studies of Cr1-xMxN coatings

M. Mahbubur Rahman, Alex Duan, Zhong-Tao Jiang, Zonghan Xie, Alex Wu, Amun Amri, Bruce Cowie, Chun-Yang Yin

Time-resolved SANS analysis of micelle chain exchange behavior: thermal crosslink driven by stereocomplexation of PLA–PEG–PLA micelles

Time-resolved small-angle neutron scattering (TR-SANS) was used to study the dynamic chain exchange behavior of the micelle mixture from poly(L-lactide)-b-poly(ethylene glycol)-b-poly(L-lactide), PLLA–PEG–PLLA, and its enantiomeric copolymer, PDLA–PEG–PDLA. The mixture of enantiomeric micelle solutions underwent a sol-to-gel transition as a consequence of temperature increase to yield a thermo-responsive hydrogel. The mechanism of gelation is hypothesized as a chain exchange process between L-micelles and D-micelles followed by the formation of stereocomplex crystals from PLLA and PDLA blocks. Stereocomplex crystals that have different physicochemical properties from PLLA or PDLA crystals restrain further chain exchange ability, which results in a network structure. We investigated the changes in micelle re-organization given by the SANS intensity as a function of q-range over time ranging from one to twenty minutes. TR-SANS data supported the hypothesis that single and stereo-mixed micelle systems undergo equilibrium and non-equilibrium chain exchange behaviors, respectively. We observed considerable differences in the scattering profile for the stereo-mixed micelles due to the structural re-organization from the original micelle to a bridged micelle network. The molecular weight of the hydrophilic PEG block was shown to influence the intermicellar interaction and chain exchange rate; short PEG micelles exhibited faster chain exchange behavior which was correlated to the lower sol-to-gel transition temperature observed.

In Situ SAXS Analysis of Interfacial Wetting on Nanorough Surfaces

Superhydrophobic surfaces were fabricated through a nanoparticle sol-gel process in the presence of a mono-disperse latex particle. By varying precursor nanoparticle size, surfaces of varying degrees of nanoroughness but controlled macro-roughness were produced, all of which exhibited superhydrophobic properties (θwater >160°, sliding angle <10°). These were immersed in water and studied in situ using synchrotron small angle X-ray scattering where the percentage interface under wetting (in contact with liquid) was directly quantified and found to agree well with traditional Cassie equations. Wetting studies in sodium dodecyl sulphate solutions of decreasing surface tension highlighting surfaces of increased hierarchical roughness (pseudo-fractal dimension ~2.5) contained significant quantity of entrapped air even at fluid surface tensions down to 37 mN m–1.

Near-edge X-ray absorption fine structure studies ofCr1−xMxN coatings

M. Mahbubur Rahman, Alex Duan, Zhong-Tao Jiang, Zonghan Xie, Alex Wu, Amun Amri, Bruce Cowie, Chun-Yang Yin