http://www.collembola.org/publicat/dewdrops.htm - Last updated on 2008.01.19 by Frans Janssens
Checklist of the Collembola: Note on the self-cleaning properties of the epicuticular surface of Collembola by removal of dirt by dew droplets flow

Frans Janssens, Department of Biology, University of Antwerp, Antwerp, B-2020, Belgium

Introduction

In November 2006, Brian Valentine keenly observed and recorded several Collembola specimens that were completely covered by small dew droplets (see Fig.1 and Fig.2). Also, Brian X. recorded in November 2007, a specimen of Allacma fusca covered in dew drops (see Fig.2a).


Fig.1. Entomobrya sp.
Specimen with dew droplets, 2006.11.18, UK, Worthing.
2006 © Valentine, B.

Fig.2. Dicyrtomina sp.
Specimen with dew droplets, 2006.11.19, UK, Worthing.
2006 © Valentine, B.

Fig.2a. Allacma fusca
Specimen with dew droplets, 2007.11.04, UK, Dudley.
2007 © Kilford, B.

In this short preliminary note, we will explore the possibility of the self-cleaning capabilities of the epicuticular ultrastructured surface by dew droplets flow.

Formation of dew droplets at the epicuticula

Fig.3. Basic hexagonal epicuticular ultrastructure.
2001 © Borensztajn, S.
Evaporation of water from the substrate soil surface, increases the moisture in the air. Dew is a form of dropwise condensation of evaporated water. Dew droplets form on a surface when the surface cools down to the dew point. This is the temperature at which the saturation of the evaporated water in the air occurs, resulting in condensation of water at nucleation sites of the cool surface.
The epicuticular ultrastructure forms a raster of open cells (see Fig.3) in which the moistured air is trapped and held in place easily. The cells are typically 150 nm high and about 700 nm in diameter. Due to the abscence of air flow in these epicuticular cells, the moisture level of the air in the cells will rise more quickly than the moisture level in the air outside the cells. Therefore, the epicuticular cells will act as active nucleation sites for the formation of dew droplets. The dew droplets grow from these active nucleation sites. Due to the non-wettable properties of the epicuticular pilars and rims of the epicuticular ultrastructure, the larger dew droplets eventually are lifted out of the epicuticular cells.

To be completed.

Non-wettable properties of the ultrahydrophobic epicuticular regular rough surface

Fig.4. Entomobrya sp.
Ball-shaped dew droplets, close-up.
2006 © Valentine, B.
Note the almost ball-shaped dew droplets on the non-wettable epicuticula of the Collembola specimens in Fig.1, Fig.2, and Fig.3. These ball-shaped droplets are a clear indication that the epicuticula of Collembola is an ultrahydrophobic surface. Noble-Nesbitt (1963 cited from Robson, 1964:296) has shown that the hydrofuge properties of Podura cuticle depend on the wax capping of the large number of small surface tubercles about 0.4 micron apart. The intervening surface is wettable. So long as the wax on the tubercles is intact, only fluids whose contact angle is less than critical will penetrate. (Robson 1964:296). But the ultrahydrophobic properties of the cuticula are not only due to the wax depositions in the epicuticular layer, but also due to the characteristic epicuticular ultrastructure (see Fig.3) (Janssens et al., 2004-). Non-wettability of the cuticle is ensured by the contours of microtubercles which cover the epicuticular surface. Similar structures arise in Onychophora and other terrestrial arthropods by convergence (modified after Robson 1964:281).

Note also that in many Poduromorpha, this epicuticular ultrastructure is superimposed on a secondary and even tertiary cuticular structure. Wenzel (1936) stated that wettability is improved by roughness for a hydrophilic surface, but gets worse for a hydrophobic surface. The non-wettable properties of ultrahydrophobic surfaces rely on the minuscule contact area of the droplets with these surfaces. The epicuticular ultrastructure reduces the contact area of the droplets with the epicuticular surface to a minimum.
Rough surfaces, which are, with the exception of mica and graphite, essentially all solid surfaces can be classified into three separate sets. A rough surface can be regular or irregular (random). Hierarchical rough surfaces are an intermediate case. The roughness structure of rough surfaces is on the submicrometre scale. The epicuticular ultrastructured surface can be compared with a regular hierachical rough surface. The regular ultrastructure of the epicuticula is clearly at submicrometre scale (see Fig.3). For such surfaces, Herminghaus derived an argument that a hierarchical structure of the roughness could render any surface non-wettable. (modified after Blossey, 2003).

Self-cleaning properties of the epicuticular ultrastructure

The ball-like equilibrium shapes of dew droplets on ultrahydrophobic surfaces are only half the story of self-cleaning: to clean the surface, hydrophilic material has to be transported along it - and best, off it.

The Lotus-effect. Interestingly, non-wettable plant leaf surfaces, such as those of the famous Lotus plant, have a built-in elementary cleaning mechanism. This was noticed in the mid-nineties by botanists studying plant surfaces. They observed that droplets running off the leaves can carry dry contaminants along - the origin for the Lotus leaf's status as a sacred object of purity. (after Blossey, 2003).
The secret of the Lotus leaf can be found in numerous tiny pillars with a wax layer on top. Water drops are lifted by these pillars, get into a spherical shape and can simply not cover the surface. Dirt gets no chance to stick to the surface via water. The spherical drops roll off and take dirt particles with them. (Science Daily, January 15, 2007).
Microfluidics can also be based on droplets on ultrahydrophobic surfaces alone: because the droplets have very low contact areas with the substrate, they are easy to move by external forces, such as gravity, air flow, specimen movements or vibrations. (after Blossey, 2003).
The dew droplets continue to grow, and when they touch, they merge with one another until one is large enough to be pulled away from its position, e.g. by gravity. It runs off, transporting any contamination encountered on its path, and merging with the smaller droplets in his path. New droplets immediately begin to grow at the nucleation sites in the cleaned path. (after Lienhard & Lienhard, 2006).

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