Breath figures of two immiscible substances on a repellent surface

The understanding of the competition between different substances while condensing on a cold surface is of high interest in situations in which it is desirable to control their condensation rates and the formed morphologies. We do the experiments for mixtures of water and hexamethyldisiloxane vapors at several concentrations. The dropwise condensation of the vapors forms breath figures on a substrate that is repellant to both substances. We report the average radius of the drops for each specie as a function of time. Also, we pay attention to the evolution of the corresponding morphologies and the appearance of hybrid clusters

Percolation-induced frost formation

We report the observation of an unconventional mechanism for frost formation. On a smooth hydrophobic surface cooled much below the water freezing temperature (−9 °C), we find that, instead of the classical freezing of individual supercooled condensed droplets, frost can occur through a multi-step 2-dimensional percolation-driven mechanism. This in-plane propagation process provides a model to investigate more complex bulk phase transformations such as those occurring in atmospheric supercooled clouds. It can also lead to a new method to control and design in-plane solidification at a nanoscale level

Droplet pattern and condensation gradient around a humidity sink

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 t5. 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

Dynamics of water condensation over arrays of hydrophilic patches

We report experimental results of drop-wise condensation on a wettability patterned substrate. It consists of a 2-d array of hydrophilic patches/spots on a macroscopically hydrophobic surface. We show that in this kind of system, there is not a relevant humidity sink, but the scale and the closeness of the different patches/spots affect the mechanisms which are important during the experiment. These results may provide clues to obtain higher dew yields in arid or semi-arid regions as a way to obtain potable water

Dew condensation on desert beetle skin

Some tenebrionind beetles inhabiting the Namib desert are known for using their body to collect water droplets from wind-blown fogs. We aim to determine whether dew water collection is also possible for desert insects. For this purpose, we investigated the infra-red emissivity, and the wetting and structural properties, of the surface of the elytra of a preserved specimen of Physasterna cribripes (Tenebrionidæ) beetle, where the macro-structure appears as a series of “bumps”, with “valleys” between them. Dew formation experiments were carried out in a condensation chamber. The surface properties (infra-red emissivity, wetting properties) were dominated by the wax at the elytra surface and, to a lower extent, its micro-structure. We performed scanning electron microscope on histological sections and determined the infra-red emissivity using a scanning pyrometer. The emissivity measured (0.95 ± 0.07 between 8–14 μm) was close to the black body value. Dew formation occurred on the insect’s elytra, which can be explained by these surface properties. From the surface coverage of the condensed drops it was found that dew forms primarily in the valleys between the bumps. The difference in droplet nucleation rate between bumps and valleys can be attributed to the hexagonal microstructure on the surface of the valleys, whereas the surface of the bumps is smooth. The drops can slide when they reach a critical size, and be collected at the insect’s mouth

Breath Figures Formation

We present experimental observations of Breath Figures (BF) which are formed by the dew of water when it condenses on a cold surface. The experiments were done in specific conditions and configurations of temperature, surfaces and mixes in controlled concentration of miscibles and immiscibles substances like the salt saturated solution, alcohol and silicon oil (C_6H_18O_2Si). The hydrophobic surfaces used on those observations are thin glasses coated with ITO (Indium Tin Oxide), 3M ECG-1700 which is repellent to the silicon oil too, and silane which is hydrophobic. Those observations let us to perform direct geometrical measurements of the droplets like radii r, areas a and with the values of contact angle θ the volume V that form the Breath Figures. Other indirect measurements obtained from the direct measurements are the factor occupation \epsilon^2, the values of concentration of water once it has condensed over a cold surface t_c^{−1}, and when it changes from liquid phase to solid phase and when the BF takes part in a process of deposition, also the fractal dimension of the BF pattern to characterize the geometrical properties in a liquid phase and solid phase. Once the average of direct measurements are obtained we extract laws of growth and the evolution of the phenomena a how outward factors modify the achievement of the process of condensation and what is the observable pattern result

Breath Figures Formation

We present experimental observations of Breath Figures (BF) which are formed by the dew of water when it condenses on a cold surface. The experiments were done in specific conditions and configurations of temperature, surfaces and mixes in controlled concentration of miscibles and immiscibles substances like the salt saturated solution, alcohol and silicon oil (C_6H_18O_2Si). The hydrophobic surfaces used on those observations are thin glasses coated with ITO (Indium Tin Oxide), 3M ECG-1700 which is repellent to the silicon oil too, and silane which is hydrophobic. Those observations let us to perform direct geometrical measurements of the droplets like radii r, areas a and with the values of contact angle θ the volume V that form the Breath Figures. Other indirect measurements obtained from the direct measurements are the factor occupation \epsilon^2, the values of concentration of water once it has condensed over a cold surface t_c^{−1}, and when it changes from liquid phase to solid phase and when the BF takes part in a process of deposition, also the fractal dimension of the BF pattern to characterize the geometrical properties in a liquid phase and solid phase. Once the average of direct measurements are obtained we extract laws of growth and the evolution of the phenomena a how outward factors modify the achievement of the process of condensation and what is the observable pattern result