The growth dynamics of water drops condensed on a superhydrophobic geometrically patterned surface were studied. Drop size evolution at early and intermediate times is self-similar. Drop growth laws do not differ for a flat surface because of a reduction of both drop and substrate dimensionality. A striking observation is the instantaneous drying of the top surface of grooves at a point in time due to coalescence of the drops with a completely filled channel. At late times, only a few large drops grow connected to the channels, in a mixed Wenzel-penetration regime.

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Condensation-induced water drop growth was studied on a super-hydrophobic spike surface. The dynamics are described by three main stages depending on the size of the drop with respect to the different spike pattern length scales. The initial stage is characterized by nucleation of the drops at the bottom (cavities) of the spikes. During the intermediate stage, large drops are surrounded by smaller drops within the neighboring cavities in what is described as a "bright ring". This ring persists until coalescence occurs with the central drop. The last stage is characterized by Wenzel-type drops growing with scaling laws similar to that on a planar surface but with contact angle θ * ≈ 111 • , lower than for deposited drops (θ = 164 • ). Condensation on spike surfaces does not retain anything of super-hydrophobicity, in contrast to super-hydrophobic square and strip patterns.Introduction. -Understanding water drop behavior on super-hydrophobic surfaces is an exciting research area in basic sciences. When the contact angle of a liquid drop on a surface is large (>150 • ), the surface is usually known as super-hydrophobic. The ideal example, perhaps, may be a lotus leaf. As a result of the large contact angle, water drops do not adhere to the surface and simply roll off, removing dust from the leaf surface in the process. This natural self-cleaning phenomenon is known as the "Lotus effect" [1,2]. It is now clearly understood that the air trapped within the rough surface cavities reduces the wetting ability. Although research concerning wetting on such surfaces has been extensive, to date very few studies of the condensation process on super-hydrophobic surfaces have been carried out [3][4][5][6].The fundamental theories concerning wetting on a rough surface were proposed long ago by Wenzel [7] and . In Cassie-Baxter's model (air pocket model) the rough surface is considered as composed of solid and air. Water drops sit partially on solid surface and air trapped in the microstructures. The apparent contact angle θ * thus depends on the

The dynamics of coalescence of two water sessile drops is investigated and compared with the spreading dynamics of a single drop in partially wetting regime. The composite drop formed due to coalescence relaxes exponentially toward equilibrium with a typical relaxation time that decreases with contact angle. The relaxation time can reach a few tenths of seconds and depends also on the drop size, initial conditions, and surface properties (contact angle, roughness). The relaxation dynamics is larger by 5 to 6 orders of magnitude than the bulk hydrodynamics predicts, due to the high dissipation in the contact line vicinity. The coalescence is initiated at a contact of the drops growing in a condensation chamber or by depositing a small drop at the top of neighboring drops with a syringe, a method also used for the studies of the spreading. The dynamics is systematically faster by an order of magnitude when comparing the syringe deposition with condensation. We explain this faster dynamics by the influence of the unavoidable drop oscillations observed with fast camera filming. Right after the syringe deposition, the drop is vigorously excited by deformation modes, favoring the contact line motion. This excitation is also observed in spreading experiments while it is absent during the condensation-induced coalescence.

The early-stage coalescence of two sessile drops is investigated theoretically and experimentally. The coalescence of small sessile drops of diethylene glycol on silicon wafer is induced either by condensation or syringe deposition. The bridge geometry in directions parallel and perpendicular to the substrate, the bridge contact angle and the direct fluid velocity are simultaneously analyzed. The process is characterized by the nucleation and growth of a bridge between the two drops. Three stages are identified. i) An initial stage of dynamic drying where the capillary number Ca > 0.02 and the contact line does not move appreciably. The bridge does not wet the solid and its size grows as time t perpendicularly to the substrate and as t 1/2 parallel to the substrate. ii) A late stage where the contact line starts to move and where the bridge relaxes exponentially, making eventually the composite drop to be ellipsis like. This stage is followed by iii) a very slow evolution limited by the contact line motion where the drop relaxes to a circular shape with a dynamics that is 6 to 7 orders larger than bulk hydrodynamics predicts.

International audienceWe compare the characteristics of dew at nearly the same latitude (42–45°N) for the Mediterranean island of Corsica (Ajaccio, France) and two continental locations (Bordeaux, France, Atlantic coastal area; Grenoble, France, alpine valley). Dew amount was measured on a horizontal reference plate made of polymethylmethacrylate (PMMA) and placed at 1 m above the ground. Data are correlated with plate and air temperature, air relative humidity, wind speed and cloud cover during the period from 14-08-1999 to 15-01-2003. General features as well as particularities of the sampling sites are discussed. The average daily dew yield is higher for the island station at Ajaccio (0.070 mm) than the Bordeaux coastal area (0.046 mm) or the Grenoble valley (0.036 mm). However, the accumulated dew yield was highest for the coastal station (9.8 mm/year) as compared to the island (8.4 mm/year), and much larger than in the alpine valley (4 mm/year). The difference between cumulated and average dew yield stems from the greater number of dew days in the coastal area (58%) versus 33% for the island and 30% in the valley. The higher wind speeds at the island station (average wind during dew is 2 m/s) and lower relative humidity explain the smaller number of dew days. The dew rate seasonal variation is negligible in Bordeaux and exhibits during summer a maximum in Ajaccio and a minimum in Grenoble. A computer model that includes simple meteorological data (air temperature and relative humidity, wind speed, cloud cover) is used to determine the thermal balance and fit to dew mass evolution. Two parameters that account for heat and mass exchange can be adjusted. It was found that, within the uncertainties, these two numbers are the same for the two continental sites, thus allowing dew formation on plates to be evaluated from only simple meteorological measurements. Somewhat larger values are found for the island, due to limitations in the model, which are discusse

We describe the evolution of a water drop saturated with NaCl and the growth of pure water droplets in a breath figure pattern (BF) condensing around it. This salty drop acts as a humidity sink, inhibiting the BF inside a ring at a distance r = δ from the sink center and slowing down BF growth outside the ring. The initial salty drop is taken either from a salt-saturated solution (type I experiment) or by placing an NaCl crystal on the substrate (type II experiment). The results are similar, provided that the initial time for type II evolution is taken at the end of the crystal dissolution. The evolution of the salty drop radius R is deduced from the establishment of a three-dimensional hyperbolic concentration profile around the salty drop. This profile scales with r/δ. Accounting for the salt concentration decrease with salty drop growth, R is seen to grow as t 5 . In the region r > δ, water droplets nucleate and grow. The rate of evolution of the water droplets at constant r/δ can be used to determine the local water pressure. The corresponding data reasonably agree with a hyperbolic water vapor profile around the salty drop. These results can be applied to the growth of BF patterns to determine whether hyperbolic or linear water vapor profiles apply.

a b s t r a c tWater condensation, a complex and challenging process, is investigated on a metallic (Zn) surface, regularly used as anticorrosive surface. The Zn surface is coated with hydroxide zinc carbonate by chemical bath deposition, a very simple, low-cost and easily applicable process. As the deposition time increases, the surface roughness augments and the contact angle with water can be varied from 75 • to 150 • , corresponding to changing the surface properties from hydrophobic to ultrahydrophobic and superhydrophobic. During the condensation process, the droplet growth laws and surface coverage are found similar to what is found on smooth surfaces, with a transition from Cassie-Baxter to Wenzel wetting states at long times. In particular, it is noticeable in view of corrosion effects that the water surface coverage remains on order of 55%.