Liquid microchannels on structured surfaces are built up using a wettability pattern consisting of hydrophilic stripes on a hydrophobic substrate. These channels undergo a shape instability at a certain amount of adsorbed volume, from a homogeneous state with a spatially constant cross section to a state with a single bulge. This instability is quite different from the classical Rayleigh Plateau instability and represents a bifurcation between two different morphologies of constant mean curvature. The bulge state can be used to construct channel networks that could be used as fluid microchips or microreactors.

The propensity of liquid films to bead off poorly wettable substrates leads to a wide variety of liquid structures via mechanisms which are far from being fully understood. In particular, dewetting via unstable surface waves may be driven at least by dispersion forces, electrostatic forces, or by Marangoni-type transport. A hierarchy of dynamical instabilities finally transforms the initial homogeneous film into the final state, consisting of an ensemble of individual, isolated droplets. While these processes of self-organized structure formation are interesting in themselves, it may also be desirable to generate liquid structures in a more well-defined and predictable way. We have therefore investigated experimentally the behaviour of various liquids on substrates, the wettability of which has been laterally structured. The resulting artificial liquid objects display several remarkable properties, both statically and dynamically. Aside from potential applications as `liquid microchips', it is shown how fundamental quantities can be extracted from the shapes of the liquid surfaces, as determined by scanning force microscopy. The three-phase contact line tensions obtained in this way are in fair agreement with theoretical predictions and might help to resolve long-standing debates on the role of wetting forces on the nanometre scale.

Optical experiments on the wetting properties of liquid 4He and molecular hydrogen are reviewed. Hydrogen films on noble metal surfaces serve as model systems for studying triple point wetting, a continuous transition between wetting and non‐wetting. By means of optically excited surface plasmons, the adsorbed film thickness for temperatures around, and far below, the bulk melting temperature is measured, and the physical mechanisms responsible for the transition are elucidated. Possible applications for other experiments in pure and applied research are discussed. Thin films and droplets of liquid helium are studied on cesium surfaces, on which there is a first order wetting transition. Our studies concentrate on dynamical observations via surface plasmon microscopy, which provide insight into the morphology of liquid helium droplets spreading at different temperatures. Features corresponding to pinning forces, the prewetting line, and the Kosterlitz‐Thouless transition are clearly observed.

We have investigated the ripening of breath figures with variable initial order. A dramatic impact of the degree of order on the coalescence behavior is observed. As opposed to the two-droplet coalescence events common to the usual disordered droplet arrays, four-droplet coalescence cascades predominate in a perfectly hexagonal breath figure. Upon introduction of disorder, a gradual transition to a regime dominated by three-droplet cascades is observed. The statistics of coalescence cascades allows for detailed conclusions on the microscopic droplet dynamics.

We are all accustomed to the idea that electronic apparatuses that would have required huge buildings for housing a few decades ago now fit nicely onto fingernail-size silicon chips. It is natural to contemplate the possibilities of doing similar things with chemical synthesis plants, or complicated chemical or biological analysis equipment, by managing a similar degree of miniaturization and control with liquids instead of electrically conducting solids. The conducting lines on the chip would then be replaced by microscopic liquid channels.A common approach to achieving this goal is to guide the liquids in narrow channels etched into solid substrates.[1] This suffers, however, from a number of drawbacks, the most important one being the fact that such channels are readily blocked by a single dust particle. It is therefore interesting to contemplate other possibilities of generating and manipulating liquid microstructures. One way to achieve this is to use substrates that provide a spatial pattern of wettability to which the liquid is then expected to adjust. The liquid forms a structure which is not in grooves etched into the substrate, but upon the substrate. Possible contaminants, e.g., dust, would then be easily washed off without destroying the device. Developments in this direction have mainly concentrated on the generation of the wettability pattern by micro contact printing of self-assembled monolayers of reactive molecules. However, a particularly reliable way to achieve such a structure, and one that is perfectly sufficient for studying the dynamic properties of the emerging liquid structures, is to evaporate a poorly soluble salt through a suitable mask, which renders the surface hydrophilic but does not appreciably dissolve in the adsorbed liquid. For demonstration, we have generated parallel wettable stripes on an elsewhere hydrophobic substrate by evaporating magnesium fluoride onto a silicone rubber surface, using as a mask a metal grid as is commercially available for transmission electron microscopy purposes.In order to deposit a liquid (water, in this case) onto this sample, we have mounted it onto a Peltier element and cooled it below the dew point. The result is shown in Figure 1 (left), where one can see parallel cylindrical channels of condensed water that have formed on the hydrophilic stripes. Virtually no droplets grew on the hydrophobic regions, which is a result of the proximity of the cylindrical channels: the latter have a much smaller curvature and thus a smaller saturation pressure. Hence they win the competition for the condensing water against the small droplets nucleating between them, thus keeping the latter from growing. It is therefore essential that the wettability pattern be microscopic; trying this with millimeter-sized structures would result in quite a mess. It is interesting to note that the channels displayed in Figure 1 are stable, even though they represent cylindrical liquid surfaces: from the well-known Rayleigh±Plateau instability, we know that liquid cylinders are...