Ville Jokinen

Non-Reflecting Silicon and Polymer Surfaces by Plasma Etching and Replication

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 ]

Microstructured Surfaces for Directional Wetting

Adv. Mater. 2009, 21, 4835–4838 2009 WILEY-VCH Verlag G IC A T IO N The wetting of topographically and chemically structured solid surfaces by liquids has attracted a lot of interest due to its significance in both nature and engineering applications. Following the pioneering work of Wenzel and Cassie, the field has branched into several different areas of research, including water-repellent, superhydrophobic surfaces consisting of chemically hydrophobic rough structures, patterned hydrophilic and hydrophobic domains for defined droplet shapes, study of droplet morphologies in chemical and physical surface features, and droplet behavior on surfaces with regular arrays of chemical and topographical features. Overall, the research activity of the field has been high, as summarized in a number of recent reviews. Recently, Courbin et al. and Extrand et al. reported how the geometry of a surface structured with circular micropillars in a regular square lattice could be used to control the spreading shapes of droplets in the partial wetting regime. In their work, the different spreading shapes resulted from the geometry of the array, leading to shapes with at least fourfold symmetry with respect to the initial droplet. Here, we show how the wetting behavior and available shapes can be enriched by utilizing the shapes of individual pillars in addition to the geometry of the lattice. We focus on directional wetting, where capillary imbibition from a reservoir droplet proceeds to only a limited sector of the surface. We demonstrate surfaces, where a droplet only spreads to a 908 sector, and surfaces where the droplet spreads to a 1808 sector in channel-like surface features that only fill in one direction. Directional wetting properties can be achieved by chemical patterning of the surface, where the shapes of the predetermined hydrophilic areas determine the shapes of the droplets. On chemically homogeneous surfaces, elongated droplet shapes have been demonstrated on microwrinkled poly(dimethylsiloxane) (PDMS), and anisotropic wetting has been observed on regular micropost arrays, where the rate and extent of imbibition depends on the inter-post distance to the given direction. On superhydrophobic surfaces, anisotropic rolling off droplets has been observed on feathers of waterfowl. In closed microchannels, capillarity has been used in creating channels that fill to a single direction by utilizing capillary geometrical valves or capillarity based ratchet structures in combination with external actuation. However, surfaces where the capillary imbibition proceeds from the initial contact spot to only a limited sector either on a uniform surface or in channel-like surface features have not been reported before. Our surfaces exhibiting directional wetting are based on an asymmetry in the reaches of liquid menisci leaning on the tips and bases of triangular micropillars placed in rectangular lattices. The geometry and parameters used throughout the paper are explained in Figure 1. Experimentally, we utilize lithographically defined SU-8 epoxy polymer microstructures (see Experimental Section), which are inherently somewhat rounded, thus avoiding possible pinning by geometrical valving effects. The water contact angle (u) of the structures was modified by oxygen plasma and hydrophobic recovery, similar to Extrand et al. (see Experimental Section). More permanent contact angles, likely required for applications, could be achieved either by stable chemical modification of the microstructure surfaces or by tailoring the surface tension of the liquid. Figure 2 presents the test structure used to study the reach of the liquid meniscus from the tips and bases of rows of triangle-like pillars. The test structure consists of a liquid introduction area, an auxiliary structure for measuring the reach of the liquidmeniscus, and twomicropillar arrays oriented so that rtips is measured from one array and rbases from the other. The reach-measurement structure consists of an additional set of micropillars that are positioned at lithographically determined distances away from the triangular-pillar arrays, and the reach of the menisci is measured by observing the contact or lack of contact of themenisci with these pillars. The contact is easily seen

Surface assisted laser desorption/ionization on two-layered amorphous silicon coated hybrid nanostructures

Matrix-free laser desorption/ionization was studied on two-layered sample plates consisting of a substrate and a thin film coating. The effect of the substrate material was studied by depositing thin films of amorphous silicon on top of silicon, silica, polymeric photoresist SU-8, and an inorganic-organic hybrid. Des-arg9-bradykinin signal intensity was used to evaluate the sample plates. Silica and hybrid substrates were found to give superior signals compared with silicon and SU-8 because of thermal insulation and compatibility with amorphous silicon deposition process. The effect of surface topography was studied by growing amorphous silicon on hybrid micro- and nanostructures, as well as planar hybrid. Compared with planar sample plates, micro- and nanostructures gave weaker and stronger signals, respectively. Different coating materials were tested by growing different thin film coatings on the same substrate. Good signals were obtained from titania and amorphous silicon coated sample plates, but not from alumina coated, silicon nitride coated, or uncoated sample plates. Overall, the strongest signals were obtained from oxygen plasma treated and amorphous silicon coated inorganic-organic hybrid, which was tested for peptide-, protein-, and drug molecule analysis. Peptides and drugs were analyzed with little interference at low masses, subfemtomole detection levels were achieved for des-arg9-bradykinin, and the sample plates were also suitable for ionization of small proteins.

Controlled communication between physically separated bacterial populations in a microfluidic device

The engineering of microbial systems increasingly strives to achieve a co-existence and co-functioning of different populations. By creating interactions, one can utilize combinations of cells where each population has a specialized function, such as regulation or sharing of metabolic burden. Here we describe a microfluidic system that enables long-term and independent growth of fixed and distinctly separate microbial populations, while allowing communication through a thin nano-cellulose filter. Using quorum-sensing signaling, we can couple the populations and show that this leads to a rapid and stable connection over long periods of time. We continue to show that this control over communication can be utilized to drive nonlinear responses. The coupling of separate populations, standardized interaction, and context-independent function lay the foundation for the construction of increasingly complex community-wide dynamic genetic regulatory mechanisms.Ekaterina Osmekhina et al. report a microfluidic device that allows complete control over growth and communication via quorum sensing between bacterial populations separated by a thin nano-cellulose filter. This enables the functionalization of multicellular populations, a strategy that could be used to construct complex genetic regulatory mechanisms