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Surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) t ...
is one of the areas of interest in
biomimetics Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from grc, βίος (''bios''), life, and μίμησ ...
research. Surface tension forces will only begin to dominate gravitational forces below length scales on the order of the fluid's
capillary length The capillary length or capillary constant, is a length scaling factor that relates gravity and surface tension. It is a fundamental physical property that governs the behavior of menisci, and is found when body forces (gravity) and surface forces ...
, which for water is about 2 millimeters. Because of this scaling, biomimetic devices that utilize surface tension will generally be very small, however there are many ways in which such devices could be used.


Applications


Coatings

A lotus leaf is well known for its ability to repel water and self-clean. Yuan and his colleagues fabricated a negative mold of alotus leaf from
polydimethylsiloxane Polydimethylsiloxane (PDMS), also known as dimethylpolysiloxane or dimethicone, belongs to a group of polymeric organosilicon compounds that are commonly referred to as silicones. PDMS is the most widely used silicon-based organic polymer, as its ...
(PDMS) to capture the tiny hierarchical structures integral for the leaf's ability to repel water, known as the
lotus effect The lotus effect refers to self-cleaning properties that are a result of ultrahydrophobicity as exhibited by the leaves of ''Nelumbo'', the lotus flower. Dirt particles are picked up by water droplets due to the micro- and nanoscopic architec ...
. The lotus leaf's surface was then replicated by allowing a copper sheet to flow into the negative mold with the assistance of
ferric chloride Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The col ...
and pressure. The result was a lotus leaf-like surface inherent on the copper sheet. Static water
contact angle The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liq ...
measurements of the biomimetic surface were taken to be 132° after etching the copper and 153° after a
stearic acid Stearic acid ( , ) is a saturated fatty acid with an 18-carbon chain. The IUPAC name is octadecanoic acid. It is a waxy solid and its chemical formula is C17H35CO2H. Its name comes from the Greek word στέαρ "''stéar''", which means ta ...
surface treatment to mimic the lotus leaf's waxy coating. A surface that mimics the lotus leaf could have numerous applications by providing water repellent outdoor gear. Various species of floating fern are able to sustain a liquid-solid barrier of air between the fern and the surrounding water when they are submerged. Like the lotus leaf, floating fern species have tiny hierarchical structures that prevent water from wetting the plant surface. Mayser and Barthlott demonstrated this ability by submerging different species of the floating fern salvinia in water inside a pressure vessel to study how the air barrier between the leaf and surrounding water react to changes in pressure that would be similar to those experienced by the
hull Hull may refer to: Structures * Chassis, of an armored fighting vehicle * Fuselage, of an aircraft * Hull (botany), the outer covering of seeds * Hull (watercraft), the body or frame of a ship * Submarine hull Mathematics * Affine hull, in affi ...
of a ship. Much other research is ongoing using these hierarchical structures in coatings on ship hulls to reduce viscous drag effects.


Biomedical

A lung is composed of many small sacks called
alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit. Uses in anatomy and zoology * Pulmonary alveolus, an air sac in the lungs ** Alveolar cell or pneumocyte ** Alveolar duct ** Alveolar macrophage * M ...
that allow oxygen and carbon dioxide to diffuse in and out of the blood respectively as the blood is passed through small capillaries that surround these alveoli. Surface tension is exploited by alveoli by means of a
surfactant Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsion#Emulsifiers , ...
that is produced by one of the cells and released to lower the surface tension of the fluid coating the inside of the alveoli to prevent these sacks from collapsing. Huh and his fellow researchers created a lung mimic that replicated the function of native
alveolar cells A pulmonary alveolus (plural: alveoli, from Latin ''alveolus'', "little cavity"), also known as an air sac or air space, is one of millions of hollow, distensible cup-shaped cavities in the lungs where oxygen is exchanged for carbon dioxide. Al ...
. An
extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...
of gel, human alveolar epithelial cells, and human pulmonary microvascular endothelial cells were cultured on a polydimethylsiloxane membrane that was bound in a flexible vacuum diaphragm. Pressurization cycles of the vacuum diaphragm, which simulated breathing, showed similar form and function to an actual lung. The type II cells were also shown to emit the same surfactant that lowered the surface tension of the fluid coating the lung mimic. This research will hopefully some day lead to the creation of lungs that could be grown for patients that need to have a transplant or repair performed.


Locomotion

Microvelia exploit surface tension by creating a surface tension gradient that propels them forward by releasing a surfactant behind them through a tongue-like protrusion. Biomimetic engineering was used in a creative and fun way to make and edible
cocktail A cocktail is an alcoholic mixed drink. Most commonly, cocktails are either a combination of spirits, or one or more spirits mixed with other ingredients such as tonic water, fruit juice, flavored syrup, or cream. Cocktails vary widely acr ...
boat that mimicked the ability of microvelia to propel themselves on the surface of water by means of a phenomenon called the
marangoni effect The Marangoni effect (also called the Gibbs–Marangoni effect) is the mass transfer along an interface between two phases due to a gradient of the surface tension. In the case of temperature dependence, this phenomenon may be called thermo-capill ...
. Burton and her colleagues used
3D printing 3D printing or additive manufacturing is the construction of a three-dimensional object from a CAD model or a digital 3D model. It can be done in a variety of processes in which material is deposited, joined or solidified under computer co ...
to make small plastic boats that released different types of
alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...
s behind the boat to lower the surface tension and create a surface tension gradient that propelled each boat. This type of propulsion could one day be used to make sea vessels more efficient.


Actuators

Fern
sporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cy ...
consist of hygroscopic ribs that protrude from a spine on the part of the plant that encapsulate spores in a sack (
diagram A diagram is a symbolic representation of information using visualization techniques. Diagrams have been used since prehistoric times on walls of caves, but became more prevalent during the Enlightenment. Sometimes, the technique uses a three ...
). A capillary bridge is formed when water condenses on to the surface of these spines. When this water evaporates, surface tension forces between each rib cause the spine to retract and rip open the sack, spilling the spores. Borno and her fellow researchers fabricated a biomimetic device from polydimethylsiloxane using standard
photolithography In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer, to protec ...
techniques. The devices used the same hygroscopic ribs and spine that resemble fern sporangia. The researchers varied the dimensions and spacing of the features of the device and were able to fine-tune and predict movements of the device as a whole in hopes of using a similar device as a microactuator that can perform functions using free energy from a humid atmosphere. A
leaf beetle The insects of the beetle family Chrysomelidae are commonly known as leaf beetles, and include over 37,000 (and probably at least 50,000) species in more than 2,500 genera, making up one of the largest and most commonly encountered of all beetle ...
has an incredible ability to adhere to dry surfaces by using numerous capillary bridges between the tiny hair-like
seta In biology, setae (singular seta ; from the Latin word for "bristle") are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Annelid setae are stiff bristles present on the body. ...
e on its feet. Vogel and Steen noted this and designed and constructed a switchable wet adhesion mechanism that mimics this ability. They used standard photolithography techniques to fabricate a switchable adhesion
gripper A ''gripper'' is something that grips things or makes it easier to grip things. It may refer to: * grippers, tools for building hand strength * a Robot end effector, the "hand" of a robot * a person working in a grip (job) In the United States, ...
that used a pump driven by electro-osmosis to create many capillary bridges that would hold on to just about any surface. The leaf beetle can also reverse this effect by trapping air bubbles between its setae to walk on wet surfaces or under water. This effect was demonstrated by Hosoda and Gorb when they constructed a biomimetic surface that could adhere objects to surfaces under water. Using this technology could help to create autonomous robots that would be able to explore treacherous terrain that is otherwise too dangerous to explore. Various life forms found in nature exploit surface tension in different ways. Hu and his colleagues looked at a few examples to create devices that mimic the abilities of their natural counterparts to walk on water, jump off the liquid interface, and climb menisci. Two such devices were a rendition of the
water strider The Gerridae are a family of insects in the order Hemiptera, commonly known as water striders, water skeeters, water scooters, water bugs, pond skaters, water skippers, or water skimmers. Consistent with the classification of the Gerridae as tr ...
. Both devices mimicked the form and function of a water strider by incorporating a rowing motion of one pair of legs to propel the device, however one was powered with elastic energy and the other was powered by electrical energy. This research compared the various biomimetic devices to their natural counterparts by showing the difference between many physical and dimensionless parameters. This research could one day lead to small, energy efficient water walking robots that could be used to clean up spills in waterways.


Environment

The Stenocara beetle, a native of the
Namib Desert The Namib ( ; pt, Namibe) is a coastal desert in Southern Africa. The name is of Khoekhoegowab origin and means "vast place". According to the broadest definition, the Namib stretches for more than along the Atlantic coasts of Angola, Nami ...
has a unique structure on its body that allows it to capture water from a humid atmosphere. In the Namib Desert, rain is not a very common occurrence, but on some mornings a dense fog will roll over the desert. The stenocara beetle uses tiny raised
hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are n ...
spots on its hydrophobic body to collect water droplets from the fog. Once these droplets are large enough, they can detach from these spots and roll down the beetle's back and into its mouth. Garrod et al. has demonstrated a biomimetic surface that was created using standard photolithography and plasma etching to create hydrophilic spots on a hydrophobic substrate for water collection. The optimal sizing and spacing of these spots that allowed the most water to be collected was similar to the spacing of the spots on the body of the stenocara beetle. Currently, this surface technology is being studied to implement as a coating on the inside of a water bottle the will allow the water bottle to self fill if left open in a humid environment, and could help to provide aid where water is scarce.


References

{{reflist Biotechnology Biophysics