Nature offers exciting examples for functional wetting properties based on superhydrophobicity, such as the self-cleaning surfaces on plant leaves and trapped air on immersed insect surfaces allowing underwater breathing. They inspire biomimetic approaches in science and technology. Superhydrophobicity relies on the Cassie wetting state where air is trapped within the surface topography. Pressure can trigger an irreversible transition from the Cassie state to the Wenzel state with no trapped air-this transition is usually detrimental for nonwetting functionality and is to be avoided. Here we present a new type of reversible, localized and instantaneous transition between two Cassie wetting states, enabled by two-level (dual-scale) topography of a superhydrophobic surface, that allows writing, erasing, rewriting and storing of optically displayed information in plastrons related to different length scales.micropillars | silicone nanofilaments | optical data storage | bistable | two-tier

Constantly increasing demand of renewable and nonpolluting energy production methods has made solar cells one of today's hottest research areas. Developing more cost-effective fabrication methods that enable production of extremely non-refl ecting surfaces is one of the key issues in solar cell research. [ 1 , 2 ] Many other applications, such as miniaturized chemical analysis systems, would also benefi t greatly from low-cost surfaces with low and uniform refl ectivity. [ 3 ] Typically, suppression of Fresnel refl ection has been achieved by antirefl ective coatings, but they suppress refl ection effi ciently only in a narrow wavelength range. Suppression of refl ection over a broad spectral range can be achieved by using nanotextured surfaces that form a graded transition of the refractive index from air to the substrate. [ 1 , 2 , 4-12 ] Here, we present a scalable, high-throughput fabrication method for such non-refl ecting nanostructured surfaces. Original nanostructures are etched on a silicon wafer and replication methods enable their transfer into polymeric materials. Previously, transfer of non-refl ecting structures into polymeric materials has not received enough attention. The fabrication starts with a maskless plasma-etching step, which forms nanosized spikes on a silicon substrate. Using the Taguchi method, [ 13 ] we show that the sidewall angles and heights of the nanospikes are controlled by the plasma-etching parameters. A silicon surface with pyramidshaped nanospikes serves as a template in the fabrication of an elastomeric stamp, which enables replication of the original nanospike pattern into polymeric materials. Denser nanospike arrays with steeper sidewalls suppress the refl ection of light most effi ciently, but they are not well-suited for replication. The refl ection measurements show that all implemented nanostructured surfaces greatly reduce the refl ection of light over a broad spectrum and that the size of the nanospikes contributes substantially to the antirefl ection properties. Our application for non-refl ecting surfaces is laser desorption ionization mass spectrometry (LDI-MS), which is a common technique in chemical analysis. [ 3 , 14 ] As a consequence of suppressed light refl ection, lower laser fl uence is enough to desorb and ionize the analytes from a nanostructured surface. We also make the surfaces self-cleaning by coating them with a low surface energy fl uoropolymer. High-throughput fabrication of low-cost self-cleaning surfaces, which suppress the refl ection of light over a wide spectral range, is expected to have applications ranging from chemical analysis of drugs and biomolecules to photovoltaics.

We demonstrate a simple yet efficient approach for droplet transport, in which the droplet is moving on a superhydrophobic surface, using gravity or electrostatic forces as the driving force for droplet transportation and using tracks with vertical walls as gravitational potential barriers to design trajectories. We further demonstrate splitting of a droplet using a superhydrophobic knife, and drop‐size selection using superhydrophobic tracks with a local widening.

Dropping in. Chemically modified silicon nanograss combines nanoscale topography with lithographically defined chemical patterns. The material exhibits a high wettability gradient that allows tailoring of complex droplet shapes. The oxidized nanograss is completely wetting, while the polymer‐coated nanograss has a contact angle of ca. 170° and is ultrahydrophobic. Novel droplet behavior on these surfaces is studied experimentally.

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In this study, a method for fabrication of high aspect ratio silicon nanopillars is presented. The method combines liquid flame spray production of silica nanoparticle agglomerates with cryogenic deep reactive ion etching. First, the nanoparticle agglomerates, having a diameter of about 100 nm, are deposited on a silicon wafer. Then, during the subsequent cryogenic deep reactive ion etching process, the particle agglomerates act as etch masks and silicon nanopillars are formed. Aspect ratios of up to 20:1 are demonstrated. The masking process is rapid, cheap and has the potential to be scaled up for large areas. Three other structured silicon surfaces were fabricated for comparison. All four surfaces were utilized as desorption/ionization on silicon (DIOS) sample plates. The mass spectrometry results indicate that nanopillar surfaces masked with the liquid flame spray technique are well suited as DIOS sample plates.

Articles you may be interested inCryogenic silicon etching in inductively coupled SF 6 /O 2 plasma has been studied, especially the behavior of mask materials. Suitability of eight different mask materials for cryogenic silicon deep reactive ion etching has been investigated. Three of the five photoresists suffered from cracking during cryogenic etching. We clarified the stages of the etching process and identified two mechanisms behind the cracking: thermal expansion mismatch and mechanical deformation from wafer clamping and backside helium pressure. Also thickness of the photoresist plays a role in cracking, but, contrary to common conception that all thick resists suffer from cracking in cryogenic etching, we found that SU-8 negative resist did not crack, even for very thick layers. This is explained to be due to its high cross-linking density. All three hard mask materials had high selectivities and were free of cracking problems. However, aluminum mask resulted in poor surface quality, while thermally grown SiO 2 and amorphous Al 2 O 3 deposited by atomic layer deposition showed smooth surfaces and sidewalls. Silicon dioxide had selectivity of 150:1, while Al 2 O 3 selectivity was 66 000:1. This extreme selectivity of Al 2 O 3 mask, combined with good surface quality, is shown to be highly beneficial in both shallow and through-wafer etching.

We have developed a lidless micropillar array electrospray ionization chip (microPESI) combined with mass spectrometry (MS) for analysis of drugs and biomolecules. The microPESI chip, made of silicon, contains a sample introduction spot for a liquid sample, an array of micropillars (diameter, height, and distance between pillars in the range of 15-200, 20-40, and 2-80 microm, respectively), and a sharpened tip for direct electrospray formation. The microchips were fabricated using deep reactive ion etching (DRIE) which results in accurate dimensional control. The chip, providing a reliable open-channel filling structure based on capillary forces and a electrospray emitter tip for ionization, allows an easy operation and reliable, non-clogging liquid transfer. The microPESI chip can be used for a fast analysis using single sampling or for continuous infusion measurements using a syringe pump for sample introduction. The microPESI-MS shows high sensitivity, with limit of detection 30 pmol/L (60 amol or 28 fg) for verapamil measured with tandem mass spectrometry (MS/MS) and using a sample volume of 2.5 microL. The system shows also good quantitative linearity (r2 > 0.99) with linear dynamic range of at least six orders of magnitude and good ion current stability (standard deviation <5%) in 1-h continuous flow measurement. The microPESI-MS is shown to be a very potential method for direct analysis of drugs and biomolecules.