The Texas horned lizard as model for robust capillary structures for passive directional transport of cooling lubricants

Abstract

Moisture-harvesting lizards, such as the Texas horned lizard Phrynosoma cornutum, have remarkable adaptations for inhabiting arid regions. Special skin structures, in particular capillary channels in between imbricate overlapping scales, enable the lizard to collect water by capillarity and to transport it to the snout for ingestion. This fluid transport is passive and directional towards the lizard's snout. The directionality is based on geometric principles, namely on a periodic pattern of interconnected half-open capillary channels that narrow and widen. Following a biomimetic approach, these principles were transferred to technical prototype design and manufacturing. Capillary structures, 50 μm to 300 μm wide and approx. 70 μm deep, were realized by use of a pulsed picosecond laser in hot working tool steel, hardened to 52 HRC. In order to achieve highest functionality, strategies were developed to minimize potential structural inaccuracies, which can occur at the bottom of the capillary structures caused by the laser process. Such inaccuracies are in the range of 10 μm to 15 μm and form sub-capillary structures with greater capillary forces than the main channels. Hence, an Acceleration Compensation Algorithm was developed for the laser process to minimize or even avoid these inaccuracies. The capillary design was also identified to have substantial influence; by a hexagonal capillary network of non-parallel capillaries potential influences of sub-capillaries on the functionality were reduced to realize a robust passive directional capillary transport. Such smart surface structures can lead to improvements of technical systems by decreasing energy consumption and increasing the resource efficiency

Passive water collection with the integument: mechanisms and their biomimetic potential

“…However, inspired by the Texas horned lizard, surface structures have been fabricated for passive, directional transport . Channel asymmetry in the longitudinal direction as well as specific interconnections have been abstracted and transferred to materials such as polymethyl methacrylate (PMMA) and steel, maintaining directional transport of water and lubricants, even against gravity for a few centimetres (Comanns et al, , 2016b. Lately, other surface structures for passive, directional fluid transport have been derived from transport of the oily defensive liquid by the flat bug D. lunatus (Plamadeala et al, 2017) and from the morphology of the spermatheca of fleas (Buchberger et al, 2018).…”

“…Lately, other surface structures for passive, directional fluid transport have been derived from transport of the oily defensive liquid by the flat bug D. lunatus (Plamadeala et al, 2017) and from the morphology of the spermatheca of fleas (Buchberger et al, 2018). All such surface structures for passive, directional liquid transport have been considered to be of particular interest for similar applications, such as in fields of lubrication (Comanns et al, 2016b). Here, the aspect of abrasion plays an important role (Uddin and Liu, 2016).…”

“…Here, the aspect of abrasion plays an important role (Uddin and Liu, 2016). Further considerations have been made regarding heat exchangers, microfluidics, distilleries, e-ink displays and hygiene products Comanns et al, 2015Comanns et al, , 2016bBuchberger et al, 2018). In general, directional wetting phenomena or transportation have been found to result from directional surface structures or their orientation (Hancock et al, 2012;Xia et al, 2012;Chen et al, 2016).…”

“…Transferring abstracted functional principles of wetting phenomena, water collection or water transport to artificial materials commonly requires fabrication of rather filigree structures in the micrometre range. This is non-trivial, but several technologies have been used to fabricate with sufficient accuracy; for example, direct laser structuring (see Glossary) (Klocke, 2007;Poprawe, 2011;Chen et al, 2013;Comanns et al, 2016b). For somewhat larger structures, micromilling (see Glossary) might be considered (König and Klocke, 2002;Biermann and Krebs, 2011), whereas large areas could require methods such as replication, embossing, injection moulding and additive manufacturing (Abbott and Gaskell, 2007).…”

“Fluidic diode” for passive unidirectional liquid transport bioinspired by the spermathecae of fleas

“…In the literature, various concepts for passive liquid transport with [10][11][12][13][14][15][16] or without [17][18][19][20] unidirectionality have been reported. Asymmetric "sawtooth" or "ratchet-like" features on the surface of capillaries give rise to directionally biased fluid transport which is either passive [18] or powered by external energy sources [3] .…”