Due to the smoothness of the surfaces in surface micromachining, large adhesion forces between fabricated structures and the substrate are encountered. Four major adhesion mechanisms have been analysed: capillary forces, hydrogen bridging, electrostatic forces and van der Waals forces. Once contact is made adhesion forces can be stronger than the restoring elastic forces and even short, thick beams will continue to stick to the substrate. Contact, resulting from drying liquid after release etching, has been successfully reduced. In order to make a fail-safe device stiction during its operational life-time should be anticipated. Electrostatic forces and acceleration forces caused by shocks encountered by the device can be large enough to bring structures into contact with the substrate. In order to avoid in-use stiction adhesion forces should therefore be minimized. This is possible by coating the device with weakly adhesive materials, by using bumps and side-wall spacers and by increasing the surface roughness at the interface. Capillary condensation should also be taken into account as this can lead to large increases in the contact area of roughened surfaces. N Tas et al

The capillary filling speed of water in nanochannels with a rectangular cross section and a height on the order of 100nm has been measured over a length of 1cm. The measured position of the meniscus as a function of time qualitatively follows the Washburn model. Quantitatively, however, there is a lower than expected filling speed, which we attribute to the electro-viscous effect. For demineralized water in equilibrium with air the elevation of the apparent viscosity amounts up to 24±11% in the smallest channels (53nm height). When using a 0.1M NaCl (aq) solution the elevation of the apparent viscosity is significantly reduced.

We have found evidence that water plugs in hydrophilic nanochannels can be at significant negative pressure due to tensile capillary forces. The negative pressure of water plugs in nanochannels induces bending of the thin channel capping layer, which results in a visible curvature of the liquid meniscus. From a detailed analysis of the meniscus curvature, the amount of bending of the channel capping can be calculated and used to determine the negative pressure of the liquid. For water plugs in silicon oxide nanochannels of 108 nm height, a negative pressure of 17 ± 10 bar was found. The absence of cavitation at such large negative pressures is explained by the fact that the critical radius for seeding cavities (bubbles) is comparable to the channel height. Scaling analysis of capillarity induced negative pressure shows that absence of cavitation is expected at other channel heights as well.Nanofluidics, the study of fluid behavior in nanoconfinement, is still in its infancy. An important aspect of nanofluidics is the extremely large surface-to-volume ratios, leading to the prominence of surface forces. A clear example is the filling of nanochannels by capillarity, where the wetting properties of channel walls play a crucial role. Dujardin et al. studied this for carbon nanotubes and showed that they can be filled by low surface tension substances such as liquid sulfur, selenium, and cesium. 1 On a slightly larger scale, capillarity was studied by Sobolev et al., 2 who measured the capillary pressure of water in quartz capillaries with radii ranging from 200 to 40 nm. Their results indicate that the Young-Laplace equation is valid on 100 nm length scale, and that on this scale the surface tension of water is equal to its macroscopic value. Silicon micromachining techniques can be used to create nanochannels, which, due to the composition of the channel walls (silicon oxide, silicon nitride), are hydrophilic in nature. [3][4][5][6][7][8] We use these nanochannels to study capillarity and use surface tension effects to manipulate aqueous solutions on a picoliter scale. 9 In our study of capillarity, we observed a peculiar shape of the meniscus of water plugs in nanochannels (Figure 1), which we attribute to a downward bending of the (thin) channel capping under influence of the capillary forces (Figure 2).To obtain these water plugs, hydrophilic silicon oxide nanochannels 10 with an approximate height of 100 nm ( Figure 3) were filled with water, and the excess water was removed from the channel entrance. 11 Subsequently, the water plugs remaining in the channels during drying were observed. We have made a detailed analysis of the meniscus curvature, to estimate the pressure of the water plug in Figure 1. Interestingly, it shows that the tensile capillary forces are so strong on this scale that the water plugs are at a significant negatiVe pressure.The analysis is based on application of the YoungLaplace equation, which relates the pressure drop across a liquid meniscus to its curvature:

Substrates with arrays of silicon microwires (4 µ m diameter, 40 µ m length and 6 µ m pitch) with radial junctions (p-type base and 900 nm A solar-driven photoelectrochemical cell provides a promising approach to enable the large-scale conversion and storage of solar energy, but requires the use of Earth-abundant materials. Earth-abundant catalysts for the hydrogen evolution reaction, for example nickel-molybdenum (Ni-Mo), are generally opaque and require high mass loading to obtain high catalytic activity, which in turn leads to parasitic light absorption for the underlying photoabsorber (for example silicon), thus limiting production of hydrogen. Here, we show the fabrication of a highly efficient photocathode by spatially and functionally decoupling light absorption and catalytic activity. Varying the fraction of catalyst coverage over the microwires, and the pitch between the microwires, makes it possible to deconvolute the contributions of catalytic activity and light absorption to the overall device performance. This approach provided a silicon microwire photocathode that exhibited a near-ideal short-circuit photocurrent density of 35.5 mA cm −2 , a photovoltage of 495 mV and a fill factor of 62% under AM 1.5G illumination, resulting in an ideal regenerative cell efficiency of 10.8%.