Shaun Atherton

An introduction to superhydrophobicity

This paper is derived from a training session prepared for COST P21. It is intended as an introduction to superhydrophobicity to scientists who may not work in this area of physics or to students. Superhydrophobicity is an effect where roughness and hydrophobicity combine to generate unusually hydrophobic surfaces, causing water to bounce and roll off as if it were mercury and is used by plants and animals to repel water, stay clean and sometimes even to breathe underwater. The effect is also known as The Lotus Effect(®) and Ultrahydrophobicity. In this paper we introduce many of the theories used, some of the methods used to generate surfaces and then describe some of the implications of the effect.

SU-8 Guiding Layer for Love Wave Devices

SU-8 is a technologically important photoresist used extensively for the fabrication of microfluidics and MEMS, allowing high aspect ratio structures to be produced. In this work we report the use of SU-8 as a Love wave sensor guiding layer which allows the possibility of integrating a guiding layer with flow cell during fabrication. Devices were fabricated on ST-cut quartz substrates with a single-single finger design such that a surface skimming bulk wave (SSBW) at 97.4 MHz was excited. SU-8 polymer layers were successively built up by spin coating and spectra recorded at each stage; showing a frequency decrease with increasing guiding layer thickness. The insertion loss and frequency dependence as a function of guiding layer thickness was investigated over the first Love wave mode. Mass loading sensitivity of the resultant Love wave devices was investigated by deposition of multiple gold layers. Liquid sensing using these devices was also demonstrated; water-glycerol mixtures were used to demonstrate sensing of density-viscosity and the physical adsorption and removal of protein was also assessed using albumin and fibrinogen as model proteins.

Plastron Respiration Using Commercial Fabrics

A variety of insect and arachnid species are able to remain submerged in water indefinitely using plastron respiration. A plastron is a surface-retained film of air produced by surface morphology that acts as an oxygen-carbon dioxide exchange surface. Many highly water repellent and hydrophobic surfaces when placed in water exhibit a silvery sheen which is characteristic of a plastron. In this article, the hydrophobicity of a range of commercially available water repellent fabrics and polymer membranes is investigated, and how the surface of the materials mimics this mechanism of underwater respiration is demonstrated allowing direct extraction of oxygen from oxygenated water. The coverage of the surface with the plastron air layer was measured using confocal microscopy. A zinc/oxygen cell is used to consume oxygen within containers constructed from the different membranes, and the oxygen consumed by the cell is compared to the change in oxygen concentration as measured by an oxygen probe. By comparing the membranes to an air-tight reference sample, it was found that the membranes facilitated oxygen transfer from the water into the container, with the most successful membrane showing a 1.90:1 ratio between the cell oxygen consumption and the change in concentration within the container.

Hydrophobic Smart Material for Water Transport and Collection

Stenocara Gracilipes (the Namib Desert beetle) is a desert dwelling beetle which has adapted to make use of fog as an alternative water source in an environment which receives little rain water. Using a combination of hydrophobic and hydrophilic areas on its carapace, the beetle is able to collect condensation on its back which is then channelled towards the mouth. In this paper we attempt to mimic this effect by selectively altering the hydrophobicity of a number of water repellent fabrics. Fabrics were treated using Granger’s Extreme Wash-in to make them hydrophobic and then laser etched to alter the hydrophobicity. We show a clear relationship between the hydrophobicity of the fabric and the laser energy applied to the surface. Laser etching was used to create a herring bone pattern of channels on the surface of the fabrics. Water sprayed onto the surface preferentially followed the channels into a collection vessel, giving a collection efficiency of 81%. To replicate real world conditions dry ice was used to create fog which was then blown, using an electric fan, onto the fabric at a speed of approximately 2.5 km/h. The water vapour condensed on the surface and then followed the channels into a collection vessel. It was found that the patterned fabrics achieved a collection rate of 0.31 l h−1 m−2.

Low-Cost QCM Sensor System for Screening Semen Samples

Artificial insemination is a well-established part of modern agricultural practice. A viable semen sample is judged by the total number of spermatozoa (sperm) in the sample and the motility of the sperm. In this paper, we report the development of a reusable measurement cell and electronics for screening semen samples based on the Quartz Crystal Microbalance (QCM) and Universal Frequency to Digital Converter (UFDC-1) to produce a low-cost sensor system. After introducing the semen sample at one end of the measurement cell, sperm swim down a channel before causing a frequency change on the QCM. Data is presented that shows the different frequency changes using a commercial frequency counter caused by porcine semen samples, one two days old and one twenty one days old. Similar data is presented for a motile semen sample measurement using the low-cost UFDC-1.

Assessing sperm motility using acoustic plate mode devices

In this work we report a simple time of flight technique for assessing the number and motility of pig sperm in a semen sample. Acoustic plate mode (APM) devices have been employed as the sensor element at the end of a channel down which sperm swim. Combining the APM data with conventional microscopy, an estimate of 18.9 plusmn4.8 pg for the pig sperm effective mass at 52 MHz APM devices is derived.

Layer guided surface acoustic wave sensors using langasite substrates

The use of acoustic wave sensors for industrial applications is widespread. At present there are few sensors for assessing fluid properties which are capable of operating at temperatures in excess of 500°C. In this work we present surface acoustic wave devices fabricated on Langasite substrates as possible candidates for such sensors. Two port delay line devices are produced and investigated in terms of temperature and their ability to measure viscosity-density properties of liquids. Single port resonator devices are fabricated and a polymer guiding layer applied to enhance sensitivity. A sharp resonance is seen for a guiding layer thickness of 4.2µm and the mass sensitivity is assessed by depositing layers of gold onto its surface. This sensitivity is found to 749 Hz·ng−1·cm−2 which is several orders of magnitude higher that that for a thickness shear mode device produced on the same substrate. By further developing these devices with particular focus on the reflector arrangement on the single port resonator devices, highly sensitive sensors for temperatures in excess of 900°C may be produced which will be suitable for use with automated data processing.