Philipp Comanns

Directional, passive liquid transport: the Texas horned lizard as a model for a biomimetic ‘liquid diode’

Moisture-harvesting lizards such as the Texas horned lizard (Iguanidae: Phrynosoma cornutum) live in arid regions. Special skin adaptations enable them to access water sources such as moist sand and dew: their skin is capable of collecting and transporting water directionally by means of a capillary system between the scales. This fluid transport is passive, i.e. requires no external energy, and directs water preferentially towards the lizards snout. We show that this phenomenon is based on geometric principles, namely on a periodic pattern of interconnected half-open capillary channels that narrow and widen. Following a biomimetic approach, we used these principles to develop a technical prototype design. Building upon the Young–Laplace equation, we derived a theoretical model for the local behaviour of the liquid in such capillaries. We present a global model for the penetration velocity validated by experimental data. Artificial surfaces designed in accordance with this model prevent liquid flow in one direction while sustaining it in the other. Such passive directional liquid transport could lead to process improvements and reduction of resources in many technical applications.

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

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 lizards 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.

Cutaneous water collection by a moisture-harvesting lizard, the thorny devil (Moloch horridus)

ABSTRACT Moisture-harvesting lizards, such as the Australian thorny devil, Moloch horridus, have the remarkable ability to inhabit arid regions. Special skin structures, comprising a micro-structured surface with capillary channels in between imbricate overlapping scales, enable the lizard to collect water by capillarity and transport it to the mouth for ingestion. The ecological role of this mechanism is the acquisition of water from various possible sources such as rainfall, puddles, dew, condensation on the skin, or absorption from moist sand, and we evaluate here the potential of these various sources for water uptake by M. horridus. The water volume required to fill the skin capillary system is 3.19% of body mass. Thorny devils standing in water can fill their capillary system and then drink from this water, at approximately 0.7 µl per jaw movement. Thorny devils standing on nearly saturated moist sand could only fill the capillary channels to 59% of their capacity, and did not drink. However, placing moist sand on skin replicas showed that the capillary channels could be filled from moist sand when assisted by gravity, suggesting that their field behaviour of shovelling moist sand onto the dorsal skin might fill the capillary channels and enable drinking. Condensation facilitated by thermal disequilibrium between a cool thorny devil and warm moist air provided skin capillary filling to approximately 0.22% of body weight, which was insufficient for drinking. Our results suggest that rain and moist sand seem to be ecologically likely water sources for M. horridus on a regular basis. Highlighted Article: Desert lizards such as thorny devils harvest moisture from different sources using their skin surface. Moist sand seems to be the most routine water source to meet their water demand.

Adsorption and movement of water by skin of the Australian thorny devil (Agamidae Moloch horridus)

Moisture-harvesting lizards, such as the Australian thorny devil Moloch horridus, have remarkable adaptations for inhabiting arid regions. Their microstructured skin surface, with channels in between overlapping scales, enables them to collect water by capillarity and passively transport it to the mouth for ingestion. We characterized this capillary water transport for live thorny devils using high-speed video analyses. Comparison with preserved specimens showed that live lizards are required for detailed studies of skin water transport. For thorny devils, there was no directionality in cutaneous water transport (unlike Phrynosoma) as 7 µl water droplets applied to the skin were transported radially over more than 9.2 mm. We calculated the total capillary volume as 5.76 µl cm−2 (dorsal) and 4.45 µl cm−2 (ventral), which is reduced to 50% filling by the time transportation ceases. Using micro-computed tomography and scanning electron microscopy of shed skin to investigate capillary morphology, we found that the channels are hierarchically structured as a large channel between the scales that is sub-divided by protrusions into smaller sub-capillaries. The large channel quickly absorbs water whereas the sub-capillary structure extends the transport distance by about 39% and potentially reduces the water volume required for drinking. An adapted dynamics function, which closely reflects the channel morphology, includes that ecological role.

Moisture-Harvesting Reptiles: A Review

Reptiles can live in arid environments due to special adaptations of their integument to such habitats. So called moisture-harvesting reptiles show behavioral and morphological adaptations, as their diet often does not cover the complete water demand and rain is scarce. The collection of water from various sources by moisture-harvesting reptiles is often accompanied by a stereotypical behavior: snakes coil up in the open and show a dorso-ventral flattening of their body to increase the surface area. Lizards also show a flattening of their body, but additionally raise their abdomen by splaying and extending their legs and lowering their head and tail. A similar behavior is observed in tortoises. Though there are several behavioral descriptions of moisture-harvesting reptiles, there are only few investigations about the physical principles enabling a passive collection of water. Special skin structures, comprising a micro structured surface with capillary channels in between imbricate overlapping scales, enable lizards to collect water efficiently. In some lizards, such as the Texas horned lizard Phrynosoma cornutum, water droplets applied to their body surface show a preferred spreading direction, transporting the water towards their mouth for ingestion. This passive directional transport is enabled by asymmetric and interconnected channels between the scales. Elucidation of the physical principles behind the directional water spreading has inspired a biomimetic transfer to optimize future applications in liquid handling, e.g. in fields of microfluidics.

Correction: Passive water collection with the integument: mechanisms and their biomimetic potential (doi:10.1242/jeb.153130)

There was an error published in J. Exp. Biol. (2018) 221 , [jeb153130][1] ([doi:10.1242/jeb.153130][2]). The corresponding authors email address was incorrect. It should be Philipp.Comanns{at}rwth-aachen.de. This has been corrected in the online full-text and PDF versions. We apologise to authors

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

ABSTRACT Several mechanisms of water acquisition have evolved in animals living in arid habitats to cope with limited water supply. They enable access to water sources such as rain, dew, thermally facilitated condensation on the skin, fog, or moisture from a damp substrate. This Review describes how a significant number of animals – in excess of 39 species from 24 genera – have acquired the ability to passively collect water with their integument. This ability results from chemical and structural properties of the integument, which, in each species, facilitate one or more of six basic mechanisms: increased surface wettability, increased spreading area, transport of water over relatively large distances, accumulation and storage of collected water, condensation, and utilization of gravity. Details are described for each basic mechanism. The potential for bio-inspired improvement of technical applications has been demonstrated in many cases, in particular for several wetting phenomena, fog collection and passive, directional transport of liquids. Also considered here are potential applications in the fields of water supply, lubrication, heat exchangers, microfluidics and hygiene products. These present opportunities for innovations, not only in product functionality, but also for fabrication processes, where resources and environmental impact can be reduced. Summary: Skin water collection has evolved in several animal genera, enabling access to differing water sources. Six mechanisms are presented and their innovation potential for technical applications is discussed.