DOI 10.1140/epje/i2014-14109-y

Regular Article

Eur. Phys. J. E (2014) 37: 109 THE EUROPEANPHYSICAL JOURNAL E

Dew condensation on desert beetle skin

J. Guadarrama-Cetina1,a, A. Mongruel2, M.-G. Medici2,3, E. Baquero4, A.R. Parker5, I. Milimouk-Melnytchuk2,6,W. Gonzalez-Vinas1,b, and D. Beysens2,6,7,c

1 Dept. Physics and Appl. Math., University of Navarra, Pamplona, Spain2 Laboratoire de Physique et Mecanique des Milieux Heteroge`nes, Unite Mixte de Recherches 7636 Centre National de la

Recherche Scientique - Ecole Superieure de Physique et Chimie Industrielles - Universite Pierre et Marie Curie - UniversiteParis Diderot, 10 rue Vauquelin, 75231 Paris, France

3 Universite de Nice, LPMC-CNRS-UMR 7336, 06108 Nice Cedex 2, France4 Dept. of Enviromental Biology, University of Navarra, Pamplona, Spain5 Department of Life Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK6 OPUR, 60 rue Emeriau, 75015 Paris, France7 Service des Basses Temperatures, Commissariat a` lEnergie Atomique-Grenoble & Universite Joseph Fourier, 17, rue des

Martyrs, 38504 Grenoble, France

Received 13 August 2014 and Received in nal form 20 October 2014Published online: 20 November 2014 c EDP Sciences / Societa` Italiana di Fisica / Springer-Verlag 2014

Abstract. Some tenebrionind beetles inhabiting the Namib desert are known for using their body to collectwater droplets from wind-blown fogs. We aim to determine whether dew water collection is also possiblefor desert insects. For this purpose, we investigated the infra-red emissivity, and the wetting and structuralproperties, of the surface of the elytra of a preserved specimen of Physasterna cribripes (Tenebrionid)beetle, where the macro-structure appears as a series of bumps, with valleys between them. Dewformation experiments were carried out in a condensation chamber. The surface properties (infra-redemissivity, wetting properties) were dominated by the wax at the elytra surface and, to a lower extent, itsmicro-structure. We performed scanning electron microscope on histological sections and determined theinfra-red emissivity using a scanning pyrometer. The emissivity measured (0.95 0.07 between 814m)was close to the black body value. Dew formation occurred on the insects elytra, which can be explainedby these surface properties. From the surface coverage of the condensed drops it was found that dew formsprimarily in the valleys between the bumps. The dierence in droplet nucleation rate between bumps andvalleys can be attributed to the hexagonal microstructure on the surface of the valleys, whereas the surfaceof the bumps is smooth. The drops can slide when they reach a critical size, and be collected at the insectsmouth.

1 Introduction

Harvesting water in arid or semi-arid regions is a chal-lenge for life. Some insects such as beetles living in theNamib desert have developed a strategy to collect waterdrops [1,2]. When air is humid enough at night such thatfog (and/or dew) can form, the beetle tilts its body for-wards into the wind to collect the fog water in a mannerthat is well-known within some Tenebrionid, and termedfog-basking [1]. Droplets form on the upper surface of thefused fore-wings (elytra) and roll down the beetles sur-face to its mouth parts. The elytr of these water-col-lecting insects show surfaces that are either smooth orwith grooves or bumps (g. 1). However, in addition to fog

a Present address: Facultad de Ciencias, Universidad Na-cional Autonoma de Mexico, Mexico.

b e-mail: wens@unav.esc e-mail: daniel.beysens@espci.fr

droplets striking the elytra, the other possible mechanismby which water is extracted from the air to form largedroplets, is dew droplets nucleation and growth on theelytr. Laboratory studies have focused so far on the col-lection of fog droplets, either by spraying water or produc-ing fog [35]. In a seminal paper [3], Parker and Lawrencehighlighted the possible role of hydrophilic and hydropho-bic microstructures to improve water drop collection. Thedroplets observed on Physasterna cribripes elytr have adiameter of 140m and the water collection is assumedto be due to the hydrophilic property of the bumps andthe hydrophobic characteristics of the valleys in between.Water accumulates on the top of the bumps and when itsvolume is suciently large, the droplets roll down alongthe hydrophobic surface towards the insects mouth. Theirstudy was the starting point for further investigation fornanomaterials with mixed wetting properties for improvedwater collection eciency (see e.g. [4,6,7]).

Page 2 of 6 Eur. Phys. J. E (2014) 37: 109

Fig. 1. Physasterna cribripes (Tenebrionid), female. Thescale bar is 4mm.

However, further studies on living beetles [5] haveshown that tenebrionids that exhibit a fog-basking be-haviour rather possess a uniform hydrophobic and smoothsurface of their elytr. Moreover, fog-collecting eciencyon elytr (of preserved beetles) showing dierent surfacestructures (smooth or with bumps), is not signicantly in-uenced by these dierences in hydrophobicity. Thus, themicrostructure of the elytr may not be the primary pa-rameter for fog collecting but rather is secondary to theposture of the insect. Here, the beetle assumes a constantangle with the horizontal (23). This angle is necessary forthe fog drops to strike the surface and also to collect dewor fog water by gravity.

In all the above studies, there is some confusion be-tween fog and dew phenomena [4]. We note that fog isformed of liquid water droplets with a diameter in therange of 10m. In contrast, dew involves the condensationof water vapour on a surface following a process of hetero-geneous nucleation, which depends on the vapour super-saturation and on the wetting properties and the chemicaland geometrical heterogeneities of the substrate, and fur-ther growth. One can make, however, the assumption thatthe collection of water drops is the same for both capturedfog drops and condensed dew drops. It happens often thatdew and fog form during the same night. It is a matterof fact that during dewy nights, dew starts to form rst,with fog appearing only later, in the morning, when theatmosphere is at its coolest. The dew point temperatureis reached when air is saturated at 100% relative humid-ity. Therefore it is reasonable to question whether dewcan also form on an insects back, since insects have notemperature regulation and the elytr are disconnectedfrom the insects body by a layer of air that favours somethermal insulation. The climate where such insect lives(Gobabeb, Namibia from ref. [5]) is characterised for year2013 [8] by clear sky (only were recorded six rainy days, inMarch, September and December) and two seasons of high(6080%, September to March) and small (4556%, Aprilto August) mean nocturnal humidity. The mean noctur-nal temperature is about 18 C when air is humid and15 C when it is dry. The number of days where dew can

form corresponds typically to a temperature dierence be-tween air and dew point that does not exceed 10K, cor-responding to relative humidity larger than 55% [9]. Itshould correspond for year 2013 to more than 60% of dewynights. During condensation, the substrate temperature istypically between 1K and 7K below the dew point tem-perature [10].

It is the object of this study to determine whetherdew can form on the elytr of Physasterna cribripes.The elytr of these water-collecting insects show surfacesthat are either smooth or with grooves or bumps 0.51.5mm apart, each about 0.5mm in diameter and around0.4mm high (g. 1). In particular, we address the ques-tion of whether the particular infra-red emissivity, andthe wetting and geometrical properties of the elytr ofPhysasterna cribripes, (i.e. its very characteristic mor-phology), are of particular interest for dew harvestingwhen compared to regular, smooth surfaces.

2 Experimental investigations

An elytra from a preserved specimen of Physasterna crib-ripes collected from the Skeleton Coast, Namibia, was dis-sected to provide two sections, each about 3mm 7mm.Half of one section was studied in a Zeiss DSM 940A scan-ning electron microscope (SEM), and the remaining sec-tions were used as a surface for dew condensation experi-ments.

2.1 Elytra histology

A schematic representation of a transverse section of ely-tra is shown in g. 2. For water condensation, the outersurface of the elytra (valleys and bumps) is most impor-tant. From the electron micrographs, the surface of thevalleys only reveals a hexagonal periodic patterning withapproximately 6m period, and both the valleys and thebumps show some microset (g. 3). The periodic struc-ture is similar to the structure already reported by Parkerand Lawrence [3]. The surface is coated with wax, howeverit is dicult to estimate whether the layer thickness of thewax varies from valleys to bumps, which would alter thesurface energy and consequently the wetting properties.The wax on the exoskeleton of insects serves a protectivefunction and reduces desiccation. The amount of wax islarger in animals which live in dry areas. The wettabil-ity properties depends on both the chemical properties ofthe wax and the geometrical (roughness) properties of thesurface. The thickness of the layer of wax in Physasternacribripes is approximately 15 nm.

2.2 Infra-red emissivity

In nature, surface cooling is insured by infra-red (IR) ra-diative cooling. The IR emissivity of the elytra was de-termined in the wavelength domain between 814m (theatmospheric window [11]) with a scanning pyrometer with

Eur. Phys. J. E (2014) 37: 109 Page 3 of 6

Exocut

icle

Exocu

ticle

Epidermis

Cement + wax (~15 nm)

1-4 m

Outer epi

cuticle

Inner ep

icuticle

Exocutic

le

Wax canal filament

~200 nm

pore

can

al

cutic

le

(a) (b)(b)

Fig. 2. Sketch of the elytra structure. (a) General cross-sectional view. (b) Details of exocuticle and epicuticle (from (a)).

Fig. 3. SEM photos of elytra regions of Physasterna cribripes. (a) A bump and surrounding valleys. At the top of the bump,there is a small depression with a microseta. Another microseta is observed in the valley. The surface of the top of the bump issmooth; the surface of the valley contains a hexagonal structure (see (c)). (b) Transverse section through a bump. (c) Structuredsurface of the valley. (d) The smooth layer observed on the surface of the bump, which corresponds to the reticulated layer ofcement plus wax, above the outer epicuticle.

temperature resolution of 0.5 C. A section of cardboardwith the same shape as the elytra and with known emis-sivity 0.81 was used together with the second part of theelytra. In a rst set of experiments, the section of card-board (experiment type (1)) and the elytra (experimenttype (1)) were attached with heat conductive grease ontoa piece of glass with high emissivity (0.93). The glass it-self was attached with heat conductive grease onto a diskmade of electrolytic copper for high heat conduction. Thedisk made contact with the cold side of a Peltier element.The hot side of this Peltier element was connected to atemperature regulated water bath (0.1 C). The roomtemperature was set at 22 C and that of the disk at12 C. In this rst set of experiments, the pyrometer didnot detect any temperature dierence between the ely-tra and the glass, thus indicating that the elytra has an

emissivity close to that of the glass. In the further experi-ments, type (2) and (2), a section of aluminium foil withlow emissivity (0.25) was used instead of glass. Here, theelytra was well detected by the pyrometer, in agreementwith an emissivity much larger than the aluminium foil(g. 4).

The analysis of experiments type (1) and (2) in termsof emissivity gave the following values for the dierentsurfaces investigated. The uncertainty was 0.07. Glass:1; aluminium lm: 0.28; cardboard: 0.76; elytra: 0.95. Thegood agreement between the measured and expected emis-sivities for glass, cardboard and aluminium lm conrmsthe accuracy of the measurements. The high value of theelytra emissivity conforms to that reported for Schisto-cerca gregaria, a desert insect called the bird grasshop-per [12].

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Fig. 4. Scanning pyrometer study on a section of elytra ap-proximately 3mm 7mm attached to aluminium lm (seetext).

2.3 Condensation

The condensation experiments have to reproduce condi-tions typical of the desert conditions. However, for practi-cal reasons, the experiment time has to be reduced, whichcorresponds to somewhat larger degree of supersaturationthan found in an actual environment. The laboratory ex-periments will thus correspond to very favourable condi-tions and will somewhat emphasise the eects.

The second section of the elytra was attached withheat conductive grease onto the copper disk and placedinside the condensation chamber, which is cylindrical (di-ameter 2L = 10 cm, height: 1 cm). A ux of air saturatedwith water (35ml/min and 200ml/min) at room temper-ature (23 C 0.3 C), formed by bubbling air throughultra-pure water, was sent into the chamber through twoholes (nozzles) with 4.5mm diameter. They are set ata chamber perimeter and perfectly aligned on a cham-ber diameter right under the level of the plane surface.The latter is slightly hollowed out like a channel over alength of 1 cm. This considerably slows down the ow inthe ratio hole/chamber cross-section ( 1.6 102) evenbefore both ows meet at the expected stagnation point.Two other holes connected to the room are set on thechamber perimeter on a diameter perpendicular to theprevious one. The maximum velocity is um = 0.2m/s,corresponding to a nozzle output for the maximum owrate (200mL/min). The corresponding Reynolds numberRe = umL/ = 750 (with = 1.4 105 m2/s the airkinematic viscosity), a value much lower than the criticalReynolds number (5 105) where turbulence should oc-cur. Flow even at the nozzles output for the largest owrate is thus in the laminar regime. The elytra was posi-tioned in this central region of the chamber. The copperdisk temperature can be adjusted between 4 C and 14 Cto provide water vapour condensation. The temperaturemeasured on the elytra with a very thin thermocouple wasthe same than the copper disk temperature within at most1 C.

Note that here the cooling process (conduction) is notthe same as the radiative process that occurs during dewformation. However, the temperature is also nearly homo-geneous on the surface of the elytra in both cases.

The development of the condensed pattern was ob-served by optical microscopy (magnication of 20, res-olution of 2m) using a CCD camera. The images wererecorded on a computer for further analysis.

3 Observations and analyses

The development of water vapour condensation on the ely-tra was observed from the time the water vapour ux wassent into the chamber. We chose an area of observationwhere the two surface geometries (valley and bump) couldbe focused at the same time (g. 5). Applying the uxof water vapour, we found that water always condensedrstly on the most depressed parts of the elytra (valleys).This can be due to two reasons: i) a thermal eect, thevalleys being slightly colder due to their close vicinity withthe copper disk and exhibiting a lower thermal exchangewith the surrounding atmosphere that is at higher tem-perature; ii) a dierence in nucleation barrier (e.g. dueto a dierent wettability and/or dierent nucleation sitesand roughness); nucleation in the valleys can thus be pro-moted greater than on the bumps. The shapes of waterdrops in the valleys appear more irregular than on thebumps, corresponding to a smaller contact angle and/or alarger contact angle hysteresis [13]. Thermal and contactangle eects are investigated in the following experiments.

3.1 Growth, contact angle and roughness

The growth of water droplets on a plane surface obeysdierent regimes [14,15]. Nucleation of microdroplets pro-ceeds rst. The energy barrier for nucleation correspondsto the cost of formation of a vapour-liquid interface. Itthus depends on the water contact angle on the surface.The barrier is maximum for = 180 (very hydropho-bic surface) and minimum (zero) for = 0 (purely hy-drophilic surface). In addition, geometric defects, whichlower the vapour-liquid interface, also favour nucleation.Then nucleation proceeds preferentially on these nucle-ation sites that correspond to surface chemical and/orgeometric irregularities. After having nucleated, dropletsgrow with radius t1/3 (t is time) and the drop surfacecoverage 2 increases on the surface. The surface coverageis dened as the ratio of surface occupied by condenseddroplets to the total substrate surface. Droplets coales-cence and then growth is accelerated. As the coalescenceprocess lowers the surface coverage, a balance between theincrease (by droplet growth) and decrease (by coalescenceevents) follows, where the drop surface coverage becomesconstant (2 = 2), and droplet growth becomes self-similar (statistically scale invariant) with mean dropletradius t. The constant 2 depends on the wetting prop-erties (water contact angle and surface roughness) of thesubstrate [13]. These growth laws were indeed observed onboth valleys and bumps, for the two ow rates as reportedin gs. 6a-b.

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Fig. 5. Measurement of drop surface coverage (see text) in a valley (V) and a bump (B) of the elytra (ow rate: 200ml/min).(a) t = 5 s, (b) t = 297 s (image is focused on region V) and (c) t = 309 s (image is focused on region B).

Fig. 6. Development of drop radius and surface coverage ina valley (open triangles (red)) and on a bump (lled trian-gles (blue)) for two dierent elytr and ow rates. (a) Set 1(200ml/min); (b) Set 2 (35ml/min).

On a smooth surface with small contact angle hystere-sis, a mean contact angle (deg.) can then be obtained fromthe measurement of 2 in the self-similar, coalescence-limited stage of growth [13]

2 1 (/200). (1)The value of the contact angle should be between the

advancing contact angle (the drops are growing) and thereceding angle (drops that have coalesced). However, asthere are far fewer drops in the latter case, this angle be-comes close to the advancing contact angle. The analysiswas performed on the valley and bump areas of two dier-ent elytr exposed to the same room (23 C) and surface(10 C) temperatures, but to two dierent ow rates (set

1: 200ml/min; set 2: 35ml/min). The results for set 1 areshown in g. 5. The measurements of surface coverage 2in the valley and bump areas are reported in g. 6. Thedata can be tted to an empirical exponential function

2 = 2

[1 exp

(t t0

)]. (2)

Here 2 is the surface coverage limiting value in theself-similar growth regime, t0 is the nucleation time and is the typical time to reach the self-similar growth regime.t0 is contact angle and supersaturation dependent; de-pends on supersaturation and on water vapour ow rate.

The following values were found for the valleys:

Set 2 t0 (s) (s)

1 0.68 0.02 0 100 102 0.76 0.01 0 390 25

and for the bumps:

Set 2 t0 (s) (s)

1 0.36 0.01 97 2 63 52 0.40 0.01 120 20 400

The time t0 is smaller for valleys than for bumps( 100 s) in both sets 1 and 2, regardless of the ow rate.Thus, the nucleation time for valleys is taken as the originof time. This result indicates that nucleation is faster onthe valleys than on the bumps. The typical times arecomparable for both bumps and valleys (set 1: 80 s; set2: 400 s) but are larger in set 2 than in set 1 because ofthe lower ow rate.

The surface coverage limiting values were found to belarger in the valleys than on the bumps, for the two setsof data. Taking the mean values of 2 between set 1 andset 2, i.e. 0.720.04 for the valleys and 0.380.02 for thebumps, respectively, one obtains from eq. (1): (648)in the valleys and (124 4) on the bumps. The sur-face of the valleys thus appears more hydrophilic than thebumps. However, this dierence between valley and bumpcontact angles cannot be explained by a dierence in sur-face coating as the valleys and bumps are both coveredwith wax. The only dierence is that the surface of thebump is smooth while the valley surface exhibits a 6mperiod hexagonal patterning (g. 3). The patterning i)

Page 6 of 6 Eur. Phys. J. E (2014) 37: 109

Fig. 7. A valley and a bump where the gradient of tempera-tures has been reversed; the water vapour still condensed rstin the valley.

favours nucleation, ii) pins the droplets contact line, re-sulting in an increase of the contact angle hysteresis andan irregular droplet shape. The eects of patterning atthis spatial scale are thus similar to the eects of loweringthe water contact angle on a smooth surface.

3.2 Thermal eects

In order to test for dierential thermal eects betweenvalleys and bumps, we inverted the gradient of tempera-tures between the lowermost valley region and the highest(peak) part of the bump. To achieve this, we decreasedthe substrate temperature to 1 C while streaming dry air(23 C) to prevent water condensation. Then we rapidlyincreased the substrate temperature to 11 C while si-multaneously streaming humid air at the same tempera-ture. The surface temperature of the valley was then largerthan the bump temperature. However, as shown in g. 7,water vapour still began to condense in the valleys. Thiswas in accordance with the largest contact angle at thebump location, where nucleation requires further super-saturation than in the valleys.

4 Conclusions

This study reveals specic properties of the beetle skinconcerning dew water condensation. The high IR emis-sivity of the wax-coated elytra provides ecient radiativecooling. Higher droplet nucleation rate is found in valleys,where dew forms primarily. This is due to the hexagonalmicrostructure on the surface of the elytra. The latterprovides more nucleation sites than on the bumps whosesurface is smooth at the micron level. The role of thesebumps, which are found on the backs of many otherinsects (but not in the fog basking species Onymacris

unguicularis and Onymacris bicolor) remains thus un-clear. They could serve as wind shields for the valleys,increasing there the dew yields. They can also be used todeter possible predators.

The dew collection properties of Physasterna cribripesis basically due to the high infra-red emissivity of wax. Asinsects are wax coated, this property should be shared byall of them, in particular those which, like the Onymacrisunguicularis and Onymacris bicolor species, are known toharvest fog. Evidence that dew can be collected in addi-tion to fog water on the backs of these beetles has not beenexamined yet in eld experiments, where traditionally fogand dew phenomena have not been distinguished. Thisstudy reveals that dew collection is indeed possible on in-sect elytr when the external conditions are favourable:clear sky and relative humidity larger than 55%. These cli-matic conditions can be found in many coastal arid areasin the world.

The authors thank S. Zeine for help in the IR measurements.This work was partly supported by the Spanish MEC (Grantn. FIS2011-24642). JGC acknowledges nancial support fromthe Asociacion de Amigos de la Universidad de Navarra.

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