When fluctuating fields are confined between two surfaces, long range forces arise. A famous example is the quantum electrodynamical Casimir force that results from zero point vacuwn fluctuations confuted between two conducting metal plates'. A thermodynamic analogue is the critical Casimir force: it acts between surfaces immersed in a binary liquid mixture dose to its critical point and arises from the confiuement of con centra tion fluctuations within the thin film of fluid separating the sur faces 2 • So far, all ell:peri.mental evidence for the existence of this effect has been indirece 5 • Here we report the direct measurement of critical Casimir force between a single colloidal sphere and a flat silica surface immersed in a mixture of water and 2,6 lutidi.ne near its critical point. We use total internal reflection microscopy to detemti.ue in situ the forces between the sphere and the surface, with femtonewton resolution 6 • Depending on whether the adsorp tion preferences of the sphere and the surface for water and 2,6 lutidi.ne are identical or opposite, we measure attractive and repulsive forces, respectively, that agree quantitatively with theoretical predictions and exhibit exquisite dependence on the temperature of the system. We expect that these features of critical Casimir forces may result in novel uses of colloids as model systems.The simple act of confining a fluid can give rise to new ph en om en a not observed in the bulk. An intriguing example is the critical Casi.urir force predicted to occur in binary fluid mixtures close to their critical point; like other critical phenomena, it is characterized by universal scaling functions that depend only on the internal symmetries of the system rather than on its specific material properties' . Colloidal particles suspended in binary liquids offer a particularly interesting setting for the experintental observations of such forces. At suffi ciently small particle distances, concentration fluctuations of the solvent become confined between neighbouring colloidal surfaces and modify the pair interaction 8 • If tlte Casinlir interaction strengtlt is comparable to the thermal energy, drastic changes in the phase behaviour are expected. Reversible flocculation of silica colloids in water 2,6 lutidi.ue mixtures close to the critical point has in fact been observed 9 , and critical Casinlir forces may be invoked to explain tills phenomenon. However, flocculation was observed even far away from tlte critical point where critical fluctuations are negligible, so it cannot serve as conclusive evidence for the presence of Casi.utir forces 10 " 11 • Our experiments, ainted at directly measuring critical Casi.utir forces, use a single colloidal sphere and a planar surface immersed in a binary liquid mixture of water and 2,6 lutidine. Forces are deter mined using total internal reflection microscopy (TIRM, see Methods) 6 which allows itt situ measurements with femtonewton resolution 12 (Fig. 1). The binary liquid mixture bas a lower critical demixing point at Tc = ...

If a fluctuating medium is confined, the ensuing perturbation of its fluctuation spectrum generates Casimirlike effective forces acting on its confining surfaces. Near a continuous phase transition of such a medium the corresponding order parameter fluctuations occur on all length scales and therefore close to the critical point this effect acquires a universal character, i.e., to a large extent it is independent of the microscopic details of the actual system. Accordingly it can be calculated theoretically by studying suitable representative model systems. We report on the direct measurement of critical Casimir forces by total internal reflection microscopy with femtonewton resolution. The corresponding potentials are determined for individual colloidal particles floating above a substrate under the action of the critical thermal noise in the solvent medium, constituted by a binary liquid mixture of water and 2,6-lutidine near its lower consolute point. Depending on the relative adsorption preferences of the colloid and substrate surfaces with respect to the two components of the binary liquid mixture, we observe that, upon approaching the critical point of the solvent, attractive or repulsive forces emerge and supersede those prevailing away from it. Based on the knowledge of the critical Casimir forces acting in film geometries within the Ising universality class and with equal or opposing boundary conditions, we provide the corresponding theoretical predictions for the sphere-planar wall geometry of the experiment. The experimental data for the effective potential can be interpreted consistently in terms of these predictions and a remarkable quantitative agreement is observed.

We investigate the behavior of colloidal particles immersed in a binary liquid mixture of water and 2,6-lutidine in the presence of a chemically patterned substrate. Close to the critical point of the mixture, the particles are subjected to critical Casimir interactions with force components normal and parallel to the surface. Because the strength and sign of these interactions can be tuned by variations in the surface properties and the mixtures temperature, critical Casimir forces allow the formation of highly ordered monolayers but also extend the use of colloids as model systems.PACS numbers: 82.70. Dd, 68.35.Rh, 81.16.Dn Analogous to the geometrical confinement of quantumelectrodynamical (QED) vacuum fluctuations between two parallel metallic plates [1], the constraint of concentration fluctuations in fluid mixtures close to their critical point gives rise to critical Casimir forces acting on the confining surfaces [2]. The range of this interaction is set by the bulk correlation length ξ of the mixture which diverges when approaching the critical point. Therefore, critical Casimir forces are sensitive to minute changes in temperature. Despite several quantitative measurements of such forces [3,4], it was only recently when the amplitude of measured critical Casimir forces in quantumand classical liquids has been directly compared to theoretical predictions [5,6,7,8]. Direct force measurements of a single colloidal particle above a flat surface and immersed in a critical water-lutidine mixture demonstrated, that critical Casimir interactions can easily exceed multiples of the thermal energy k B T [8]. Accordingly, they offer a versatile opportunity to control the pair interaction in colloidal suspensions by weak temperature changes [9,10]. Apart from their exquisite temperature dependence, critical Casimir forces respond sensitively to the chemical properties of the confining surfaces. Depending on whether both surfaces preferentially attract the same mixture's component or not (symmetric or asymmetric boundary conditions), attractive or repulsive critical Casimir forces arise [8,11,12].So far, experimental investigations of critical Casimir interactions were limited to homogeneous surfaces (in contrast to QED Casimir forces [13]) where the corresponding forces act perpendicular to the confining walls. However, when one or both surfaces are chemically patterned, also lateral critical Casimir forces have been predicted [14] In this Letter we experimentally study the interaction between colloidal particles and chemically patterned substrates immersed in a binary critical mixture. Close to the critical point lateral critical Casimir forces lead to the formation of highly ordered colloidal assemblies whose structure is controlled by the underlying chemical pattern. This may suggest a novel route for templated growth of colloidal crystals. At higher particle concentrations, additional critical Casimir forces between nearby particle surfaces arise and eventually lead to the formation of three-dimensional, face...

We show that rain events are analogous to a variety of nonequilibrium relaxation processes in Nature such as earthquakes and avalanches. Analysis of high-resolution rain data reveals that power laws describe the number of rain events versus size and number of droughts versus duration. In addition, the accumulated water column displays scale-less fluctuations. These statistical properties are the fingerprints of a self-organized critical process and may serve as a benchmark for models of precipitation and atmospheric processes.Rainfall and rainfall-related quantities have been recorded for centuries [1,2]. All these measurements, however, have the disadvantage of low temporal resolution and low sensitivity. The rain measurements are based on the simple idea of collecting rain in a container and measuring the amount of water after a certain time. The time intervals between readings are typically hours or days. Even with the most sophisticated of these conventional methods, the fine details of rain events cannot be captured at all and very light rain might not be recorded due to evaporation or insufficient sensitivity of the instrument, making it impossible to address questions regarding single rain events.Recently, high-resolution data have been collected with a compact vertically pointing Doppler radar MRR-2, developed by METEK [3]. The instrument is operated by the Max-Planck-Institute for Meteorology, Hamburg, Germany at the Baltic coast Zingst (54 • 43'N 12 • 67'E) under the Precipitation and Evaporation Project (PEP) in BALTEX [4]. Rain rate, liquid water content, and drop size distribution were obtained from the radar Doppler spectra, based on a method described by Atlas [5][6][7]. At vertical incidence, the Doppler shift can be identified with the droplet fall velocity. As, in the atmosphere, larger drops fall faster than smaller drops, spectral bins can be attributed to corresponding drop sizes. For a given size, the scattering cross section of the droplets can be calculated by Mie theory [8]. This yields the number density of drops which is proportional to the spectral power divided by the corresponding cross section. The rain rate q(t) = i n i V i v i , where n i is the number density of drops of volume V i falling with velocity v i . The detection threshold for rain rates under the pertinent operation parameters was q min = 0.005 mm/h. Below this threshold, q(t) = 0 by definition.Precipitation profiles up to some thousand meters altitude can be observed. At present, the quantitative retrieval is restricted to rain. Snow and hail can be identified from the form of the Doppler spectra but have been excluded from the quantitative analysis. The analyzed data refer to 250m above sea level and have been collected from January to July 1999 with 1-min resolution.The processes that make a cloud release its water content are only very little understood. However, with the high temporal resolution of 1 min, single rain events can be identified and characterized. Previous work focused on the rainfall during a fixed ...