Guillermo J. Amador

Eyelashes divert airflow to protect the eye

Eyelashes are ubiquitous, although their function has long remained a mystery. In this study, we elucidate the aerodynamic benefits of eyelashes. Through anatomical measurements, we find that 22 species of mammals possess eyelashes of a length one-third the eye width. Wind tunnel experiments confirm that this optimal eyelash length reduces both deposition of airborne particles and evaporation of the tear film by a factor of two. Using scaling theory, we find this optimum arises because of the incoming flows interactions with both the eye and eyelashes. Short eyelashes create a stagnation zone above the ocular surface that thickens the boundary layer, causing shear stress to decrease with increasing eyelash length. Long eyelashes channel flow towards the ocular surface, causing shear stress to increase with increasing eyelash length. These competing effects result in a minimum shear stress for intermediate eyelash lengths. This design may be employed in creating eyelash-inspired protection for optical sensors.

Cleanliness is next to godliness: mechanisms for staying clean

ABSTRACT Getting dirty is a fundamental problem, and one for which there are few solutions, especially across the enormous range of animal size. How do both a honeybee and a squirrel get clean? In this Review, we discuss two broad types of cleaning, considered from the viewpoint of energetics. Non-renewable cleaning strategies rely upon the organism as an energy source. Examples include grooming motions, wet-dog shaking or the secretion of chemicals. Renewable cleaning strategies depend on environmental sources of energy, such as the use of eyelashes to redirect incoming wind and so reduce deposition onto the eye. Both strategies take advantage of body hair to facilitate cleaning, and honeybees and squirrels, for example, each have around 3 million hairs. This hair mat increases the area on which particles can land by a factor of 100, but also suspends particles above the body, reducing their adhesion and facilitating removal. We hope that the strategies outlined here will inspire energy-efficient cleaning strategies in synthetic systems. Summary: This Review discusses how animals living in a world of microscopic debris, ranging from dust, pollen and dew to insidious parasites like mites and bacteria stay clean.

Honey bee hairs and pollenkitt are essential for pollen capture and removal

While insect grooming has been observed and documented for over one hundred years, we present the first quantitative analysis of this highly dynamic process. Pollinating insects, like honey bees, purposely cover themselves with millions of pollen particles that, if left ungroomed, would make sensing and controlled flight difficult. How do they get clean? We show that the hairs on insect eyes are tuned to the pollen they collect; namely, the hairs are spaced so that they suspend pollen above the body for easy removal by the forelegs. In turn, hair spacing on the foreleg dictates the legs ability to store the pollen removed during each swipe. In tests with wax-covered honey bees, we show that hairy forelegs are necessary for pollen removal. Moreover, the viscous fluid found on the surface of pollen grains, or pollenkitt, greatly enhances adhesion. We find that bees accumulate twice as much pollen if pollenkitt is present. This study may help further understand pollination, as well as inform designs for mechanically-sensitive functional surfaces with micro- and nano-structures that are easier to keep clean.

Splash-cup plants accelerate raindrops to disperse seeds

The conical flowers of splash-cup plants Chrysosplenium and Mazus catch raindrops opportunistically, exploiting the subsequent splash to disperse their seeds. In this combined experimental and theoretical study, we elucidate their mechanism for maximizing dispersal distance. We fabricate conical plant mimics using three-dimensional printing, and use high-speed video to visualize splash profiles and seed travel distance. Drop impacts that strike the cup off-centre achieve the largest dispersal distances of up to 1 m. Such distances are achieved because splash speeds are three to five times faster than incoming drop speeds, and so faster than the traditionally studied splashes occurring upon horizontal surfaces. This anomalous splash speed is because of the superposition of two components of momentum, one associated with a component of the drops motion parallel to the splash-cup surface, and the other associated with film spreading induced by impact with the splash-cup. Our model incorporating these effects predicts the observed dispersal distance within 6–18% error. According to our experiments, the optimal cone angle for the splash-cup is 40°, a value consistent with the average of five species of splash-cup plants. This optimal angle arises from the competing effects of velocity amplification and projectile launching angle.

A Compound Analogical Design for Low Cost Solar Panel Systems

In support of the Department of Energy Sunshot initiative target of

Soiled adhesive pads shear clean by slipping: a robust self-cleaning mechanism in climbing beetles

1.25 per watt photovoltaics systems for commercial applications, whole system designs were pursued using the analogical design methodology, an essential step in the bio inspired design approach. A functional decomposition of solar panel systems was conducted, and then key functions critical to system integrity and cost reduction were identified. Three sources of bio-inspiration were mainly used: hierarchical structures as a common design dimension exploited in natural systems, and leaves’ ability to maintain position through changes in shape and angle of attack when triggered by wind flow, and limpet shells’ reduction of hydrodynamic forces by way of natural geometrical features. The design team developed concepts with varying degrees of abstraction then attempted to reconcile them with other functional requirements. Variants that descended from the leaf concept were generally found to be biophilic and offer aesthetic value; however, presented shortcomings in electrical design and installation procedure (Kellert 2008). Alternatively, concepts inspired by hierarchical structures and limpet shells were found to have greater variability, enabling reconciliation with other functional requirements, resulting in a complete system solution capable of meeting the cost reduction objective. From the analysis of these design variants, we may conclude that transferring solution principles directly from nature is best done when there is small set of functional requirements that must be fulfilled and value in a biophilic design. However, in cases of significant system complexity, abstracted lessons from nature may be found to be more flexible and easily reconciled with multiple requirements.© 2012 ASME

Wrinkling Instability and Adhesion of a Highly Bendable Gallium Oxide Nanofilm Encapsulating a Liquid-Gallium Droplet

Animals using adhesive pads to climb smooth surfaces face the problem of keeping their pads clean and functional. Here, a self-cleaning mechanism is proposed whereby soiled feet would slip on the surface due to a lack of adhesion but shed particles in return. Our study offers an in situ quantification of self-cleaning performance in fibrillar adhesives, using the dock beetle as a model organism. After beetles soiled their pads by stepping into patches of spherical beads, we found that their gait was significantly affected. Specifically, soiled pads slipped 10 times further than clean pads, with more particles deposited for longer slips. Like previous studies, we found that particle size affected cleaning performance. Large (45 μm) beads were removed most effectively, followed by medium (10 μm) and small (1 μm). Consistent with our results from climbing beetles, force measurements on freshly severed legs revealed larger detachment forces of medium particles from adhesive pads compared to a flat surface, possibly due to interlocking between fibres. By contrast, dock leaves showed an overall larger affinity to the beads and thus reduced the need for cleaning. Self-cleaning through slippage provides a mechanism robust to particle size and may inspire solutions for artificial adhesives.

Numerical Study of Wind Loads on Residential Roof-Mounted Solar Photovoltaic Arrays

The wrinkling and interfacial adhesion mechanics of a gallium-oxide nanofilm encapsulating a liquid-gallium droplet are presented. The native oxide nanofilm provides mechanical stability by preventing the flow of the liquid metal. We show how a crumpled oxide skin a few nanometers thick behaves akin to a highly bendable elastic nanofilm under ambient conditions. Upon compression, a wrinkling instability emerges at the contact interface to relieve the applied stress. As the load is further increased, radial wrinkles evolve, and, eventually, the oxide nanofilm ruptures. The observed wrinkling closely resembles the instability experienced by nanofilms under axisymmetric loading, thus providing further insights into the behaviors of elastic nanofilms. Moreover, the mechanical attributes of the oxide skin enable high surface conformation by exhibiting liquid-like behavior. We measured an adhesion energy of 0.238 ± 0.008 J m-2 between a liquid-gallium droplet and smooth flat glass, which is close to the measurements of thin-sheet nanomaterials such as graphene on silicon dioxide.

Sticky Solution Provides Grip for the First Robotic Pollinator

Residential rooftops offer attractive options for solar arrays since it makes productive use of otherwise unused space and are co-located with residential demand. However, the current installation practice in the solar panel industry is based on code (ASCE-7) that is intended to estimate the design wind loads on buildings and roofs and is not intended to apply to roofmounted solar arrays. Conservative mounting approaches are likely to result in over designed and expensive mounting systems, while less conservative methods may jeopardize the integrity of the whole system and safety of the surrounding structure. One of the major challenges of producing affordable energy form solar photovoltaic arrays is the cost of the installation. Thus, understanding wind-induced aerodynamic loads in arrays of solar panels is an important part of designing appropriate mounting systems. This study addresses the wind load on a 1:12 scale model of a moderate (83.6 m 2 ) residential structure with a roof pitch of 26.5 o with two arrays of solar panels on one side. The wind angle is varied from 0 to 360 degrees to address front and back roof-mounted arrays. The flow is simulated using the incompressible Navier-Stokes equation and