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.

Erratum: Droplet pattern and condensation gradient around a humidity sink [Phys. Rev. E 89, 012402 (2014)].

This corrects the article DOI: 10.1103/PhysRevE.89.012402.

Thermovibrational instability in supercritical fluids under weightlessness.

Low amplitude, high frequency vibrations can induce in fluids under weightlessness behaviors that resemble those induced by gravity. Supercritical fluids (above their gas-liquid critical point) are used in the space industry and also display universal behavior. They are particularly sensitive to gravity effects. When submitted to vibration (typically 0.1 to 0.5mm amplitude, 10 to 50Hz frequency), a Rayleigh-Bénard-like instability is observed in experiments with H2 and CO2 under weightlessness. The thermal boundary layer created during a temperature change displays periodic fingering perpendicular to the vibration direction. A systematic two-dimensional numerical study by the finite volume method is performed in CO2 that shows that the fingering pattern is due to a thermovibrational instability, characterized by a vibrational Rayleigh number. The simulation and a simplified dimensional analysis show that the fingering wavelength and the vibrational Rayleigh number decrease as a power law with the distance in temperature to the critical point. However, due to the oversimplification of the analysis, the exponent in the simulation is found to be somewhat different than in the theoretical approach, calling for a more complete investigation of the problem.

Liquid-vapor rectilinear diameter revisited.

In the modern theory of critical phenomena, the liquid-vapor density diameter in simple fluids is generally expected to deviate from a rectilinear law approaching the critical point. However, by performing precise scannerlike optical measurements of the position of the SF_{6} liquid-vapor meniscus, in an approach much closer to criticality in temperature and density than earlier measurements, no deviation from a rectilinear diameter can be detected. The observed meniscus position from far (10K) to extremely close (1mK) to the critical temperature is analyzed using recent theoretical models to predict the complete scaling consequences of a fluid asymmetry. The temperature dependence of the meniscus position appears consistent with the law of rectilinear diameter. The apparent absence of the critical hook in SF_{6} therefore seemingly rules out the need for the pressure scaling field contribution in the complete scaling theoretical framework in this SF_{6} analysis. More generally, this work suggests a way to clarify the experimental ambiguities in the simple fluids for the near-critical singularities in the density diameter.

Thermoconvectional phenomena induced by vibrations in supercritical SF6 under weightlessness.

The effect of a linear harmonic vibration on heat propagation is investigated in near-critical SF6 under weightlessness conditions in space. Heat was issued from a pointlike source (thermistor), a situation representative of an industrial use of pressurized supercritical fluid storage. Two kinds of vibrations were used, large amplitude (64 mm) at 0.2 Hz and low amplitude (0.8 mm) at 1.6 Hz, with temperatures from 5 K to 20 mK from the critical temperature. The vibrations are seen to strongly affect the evolution and shape of the hot boundary layer (HBL), the heat exchange between the heat source and the fluid, and the bulk thermalization process by the adiabatic piston-effect process. The HBL is initially convected as symmetrical plumes over a distance that only depends on the vibration velocity and which corresponds to a Rayleigh-Bénard-like instability where the vibration acceleration acts as the earth gravity. Then the extremities of the plumes are convected perpendicularly to the direction of oscillation as two "pancakes," a process encountered in the vibrational Rayleigh-Bénard instability. When the vibration velocity is small, only one pancake centered at the hot source is observed. Temperature evolutions of the hot source and the fluid are studied in different locations. Convection flows and adiabatic piston effect compete to determine the thermal dynamics, with the latter being the most efficient near the critical point. The experimental results are compared with a two-dimensional numerical simulation that highlights the similarities and differences between the very compressible van der Waals gas and an ideal gas.

Band instability in near-critical fluids subjected to vibration under weightlessness.

Periodical patterns (bands) developing at the interface of two immiscible fluids under vibration parallel to interface are observed under zero-gravity conditions. Fluids are slightly below their liquid-vapor critical point where they behave in a scaled, universal manner. In addition, liquid and vapor densities are close and surface tension is very low. Linear stability analyses and direct numerical simulation show that this instability, although comparable to the frozen wave instability observed in a gravity field, is nonetheless noticeably different when gravity becomes zero. In particular, the neutral curve minimum corresponds to the long-wave perturbations with k=0 and zero dimensionless vibrational parameter, corresponding to no instability threshold. The pattern wavelength thus corresponds to the wavelength of the perturbations with maximal growth rate. This wavelength differs substantially from the neutral perturbations wavelength at the same vibrational parameter value. The role of viscosity is highlighted in the pattern formation, with a critical wavelength dependence on vibration parameters that strongly depends on viscosity. These results compare well with experimental observations performed in the liquid-vapor phases near the critical point of CO_{2} (in weightlessness) and H_{2} (under magnetic levitation).

Contact line dynamics in the late-stage coalescence of diethylene glycol drops.

We investigated the contact line dynamics of a composite drop formed as a result of the coalescence during the condensation of two diethylene glycol (DEG) drops at -4 degrees C on a silicon surface. The composite drop relaxes exponentially toward equilibrium with a typical relaxation time, tc, which depends on the equilibrium radius, R, of the composite drop. The value of tc is found to be in the range of 10-100 s for R approximately 1-4 microm. The relaxation dynamics is found to be larger by 6 orders of magnitude than that predicted by bulk hydrodynamics because of high dissipation in the contact line vicinity. Similar to low viscous liquids (water), this high dissipation can be attributed to an Arrhenius factor resulting from the phase change in the contact line vicinity and to the influence of surface defects that pin the contact line.

Piston-effect-induced thermal jets in near-critical fluids.

In very compressible fluids, such as fluids near their critical point, the bulk fluid is adiabatically thermalized by the expansion of a hot boundary layer. Thanks to this thermomechanical process (the so-called piston effect) the fluid velocity at the edge of the boundary layer can become very high when the heating power is concentrated in a fissure. Spectacular jets are then observed in SF6 and CO2. Data obtained under weightlessness (in order to remove convection) and data obtained under earth gravity are compared and analyzed. They emphasize the key role of the boundary layer expansion for thermal phenomena in compressible fluids and the hydrodynamic nature of the piston effect.

Projected climate change impacts upon dew yield in the Mediterranean basin.

Water scarcity is increasingly raising the need for non-conventional water resources, particularly in arid and semi-arid regions. In this context, atmospheric moisture can potentially be harvested in the form of dew, which is commonly disregarded from the water budget, although its impact may be significant when compared to rainfall during the dry season. In this study, a dew atlas for the Mediterranean region is presented illustrating dew yields using the yield data collected for the 2013 dry season. The results indicate that cumulative monthly dew yield in the region can exceed 2.8mm at the end of the dry season and 1.5mm during the driest months, compared to <1mm of rainfall during the same period in some areas. Dew yields were compared with potential evapotranspiration (PET) and actual evapotranspiration (ET) during summer months thus highlighting the role of dew to many native plants in the region. Furthermore, forecasted trends in temperature and relative humidity were used to estimate dew yields under future climatic scenarios. The results showed a 27% decline in dew yield during the critical summer months at the end of the century (2080).

Criticality in the slowed-down boiling crisis at zero gravity.

Boiling crisis is a transition between nucleate and film boiling. It occurs at a threshold value of the heat flux from the heater called CHF (critical heat flux). Usually, boiling crisis studies are hindered by the high CHF and short transition duration (below 1 ms). Here we report on experiments in hydrogen near its liquid-vapor critical point, in which the CHF is low and the dynamics slow enough to be resolved. As under such conditions the surface tension is very small, the experiments are carried out in the reduced gravity to preserve the conventional bubble geometry. Weightlessness is created artificially in two-phase hydrogen by compensating gravity with magnetic forces. We were able to reveal the fractal structure of the contour of the percolating cluster of the dry areas at the heater that precedes the boiling crisis. We provide a direct statistical analysis of dry spot areas that confirms the boiling crisis at zero gravity as a scale-free phenomenon. It was observed that, in agreement with theoretical predictions, saturated boiling CHF tends to zero (within the precision of our thermal control system) in zero gravity, which suggests that the boiling crisis may be observed at any heat flux provided the experiment lasts long enough.

Weightless experiments to probe universality of fluid critical behavior.

Near the critical point of fluids, critical opalescence results in light attenuation, or turbidity increase, that can be used to probe the universality of critical behavior. Turbidity measurements in SF6 under weightlessness conditions on board the International Space Station are performed to appraise such behavior in terms of both temperature and density distances from the critical point. Data are obtained in a temperature range, far (1 K) from and extremely close (a few µK) to the phase transition, unattainable from previous experiments on Earth. Data are analyzed with renormalization-group matching classical-to-critical crossover models of the universal equation of state. It results that the data in the unexplored region, which is a minute deviant from the critical density value, still show adverse effects for testing the true asymptotic nature of the critical point phenomena.

Dew harvesting efficiency of four species of cacti.

Four species of cacti were chosen for this study: Copiapoa cinerea var. haseltoniana, Ferocactus wislizenii, Mammillaria columbiana subsp. yucatanensis and Parodia mammulosa. It has been reported that dew condenses on the spines of C. cinerea and that it does not on the spines of F. wislizenii, and our preliminary observations of M. columbiana and P. mammulosa revealed a potential for collecting dew water. This study found all four cacti to harvest dew on their stems and spines (albeit rarely on the spines of F. wislizenii). Dew harvesting experiments were carried out in the UK, recording an increase in cacti mass on dewy nights. By applying a ranking relative to a polymethyl methacrylate (Plexiglas) reference plate located nearby, it was found that C. cinerea collected the most airborne moisture followed by M. columbiana, P. mammulosa and F. wislizenii respectively, with mean efficiency ratio with respect to the Plexiglas reference of 3.48 ± 0.5, 2.44 ± 0.06, 1.81 ± 0.14 and 1.27 ± 0.49 on observed dewy nights. A maximum yield of normalized performance of 0.72 ± 0.006 l/m(-2) on one dewy night was recorded for C. cinerea. Removing the spines from M. columbiana was found to significantly decrease its dew harvesting efficiency. The spines of three of the species were found to be hydrophilic in nature, while F. wislizenii was hydrophobic; the stems of all four species were hydrophilic. The results of this study could be translated into designing a biomimetic water collecting device that utilizes cactus spines and their microstructures.

Thermalization of a two-phase fluid in low gravity: heat transferred from cold to Hot

We present an experimental study of the thermal response to a positive temperature quench in two-phase fluid SF6 in low gravity for temperature ranging from 10.1 to 0.1 K from the critical temperature. The temperature was measured simultaneously in the gas, the liquid, and the cell wall by thermistors and the density distribution was observed by interferometry. During the quench the gas temperature considerably exceeded the temperature of the heating walls (overheating up to 23%). This striking observation is discussed in the light of the adiabatic heat transfer in this highly compressible fluid while the key role of the localization in low gravity of the gas and liquid phases is revealed.

Fast heat transfer calculations in supercritical fluids versus hydrodynamic approach.

This study investigates the heat transfer in a simple pure fluid whose temperature is slightly above its critical temperature. We propose an efficient numerical method to predict the heat transfer in such fluids when the gravity can be neglected. The method, based on a simplified thermodynamic approach, is compared with direct numerical simulations of the Navier-Stokes and energy equations performed for CO2 and SF6. A realistic equation of state is used to describe both fluids. The proposed method agrees with the full hydrodynamic solution and provides a huge gain in computation time. The connection between the purely thermodynamic and hydrodynamic descriptions is also discussed.

Gas spreading on a heated wall wetted by liquid.

This study deals with a simple pure fluid whose temperature is slightly below its critical temperature and whose density is nearly critical, so that the gas and liquid phases coexist. Under equilibrium conditions, such a liquid completely wets the container wall and the gas phase is always separated from the solid by a wetting film. We report a striking change in the shape of the gas-liquid interface influenced by heating under weightlessness where the gas phase spreads over a hot solid surface showing an apparent contact angle larger than 90 degrees. We show that the two-phase fluid is very sensitive to the differential vapor recoil force and give an explanation that uses this nonequilibrium effect. We also show how these experiments help to understand the boiling crisis, an important technological problem in high-power boiling heat exchange.

Simple, reliable, and sensitive interferometer for the measurement of the refractive index of liquids as a function of temperature.

We describe a very simple and inexpensive interferometer setup which provides high accuracy measurements of the refractive index (n) of liquids with temperature. A standard deviation sigma (n) =5.4x10(-7) has been observed in isooctane liquid when fitting n to a linear temperature variation.

Dynamic equilibrium under vibrations of H2 liquid-vapor interface at various gravity levels.

Horizontal vibration applied to the support of a simple pendulum can deviate from the equilibrium position of the pendulum to a nonvertical position. A similar phenomenon is expected when a liquid-vapor interface is subjected to strong horizontal vibration. Beyond a threshold value of vibrational velocity the interface should attain an equilibrium position at an angle to the initial horizontal position. In the present paper experimental investigation of this phenomenon is carried out in a magnetic levitation device to study the effect of the vibration parameters, gravity acceleration, and the liquid-vapor density on the interface position. The results compare well with the theoretical expression derived by Wolf [G. H. Wolf, Z. Phys. B 227, 291 (1969)].

Frozen-wave instability in near-critical hydrogen subjected to horizontal vibration under various gravity fields.

The frozen-wave instability which appears at a liquid-vapor interface when a harmonic vibration is applied in a direction tangential to it has been less studied until now. The present paper reports experiments on hydrogen (H2) in order to study this instability when the temperature is varied near its critical point for various gravity levels. Close to the critical point, a liquid-vapor density difference and surface tension can be continuously varied with temperature in a scaled, universal way. The effect of gravity on the height of the frozen waves at the interface is studied by performing the experiments in a magnetic facility where effective gravity that results from the coupling of the Earth's gravity and magnetic forces can be varied. The stability diagram of the instability is obtained. The experiments show a good agreement with an inviscid model [Fluid Dyn. 21 849 (1987)], irrespective of the gravity level. It is observed in the experiments that the height of the frozen waves varies weakly with temperature and increases with a decrease in the gravity level, according to a power law with an exponent of 0.7. It is concluded that the wave height becomes of the order of the cell size as the gravity level is asymptotically decreased to zero. The interface pattern thus appears as a bandlike pattern of alternate liquid and vapor phases, a puzzling phenomenon that was observed with CO2 and H2 near their critical point in weightlessness [Acta Astron. 61 1002 (2007); Europhys. Lett. 86 16003 (2009)].

Faraday instability in a near-critical fluid under weightlessness.

Experiments on near-critical hydrogen have been conducted under magnetic compensation of gravity to investigate the Faraday instability that arises at the liquid-vapor interface under zero-gravity conditions. We investigated such instability in the absence of stabilizing gravity. Under such conditions, vibration orients the interface and can destabilize it. The experiments confirm the existence of Faraday waves and demonstrate a transition from a square to a line pattern close to the critical point. They also show a transition very close to the critical point from Faraday to periodic layering of the vapor-liquid interface perpendicular to vibration. It was seen that the Faraday wave instability is favored when the liquid-vapor density difference is large enough (fluid far from the critical point), whereas periodic layering predominates for small difference in the liquid and vapor densities (close to the critical point). It was observed for the Faraday wave instability that the wavelength of the instability decreases as one approaches the critical point. The experimental results demonstrate good agreement to the dispersion relation for zero gravity except for temperatures very close to the critical point where a transition from a square pattern to a line pattern is detected, similarly to what is observed under 1g conditions.

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.