A sample of the new fabric in reflected light. Credit: The picture is courtesy of the researchers
Scientists have synthesized the material that is perfect for protection of respiratory organs, analytical research and other practical purposes. An almost weightless fabric made of nylon nanofibers with a diameter <15 nm beats any other similar materials in terms of filtering and optical properties. The material is lightweight (10-20 mg/m2), almost invisible (95% light transmission: more than that of a window glass), showing low resistance to airflow and efficient interception of <1 micrometer fine particulate matter.
Previously, the same team showed reducing fiber diameter from 200 nanometers down to 20 decreased filter resistance to airflow by 2/3s, and this effect could no longer be explained by classical aerodynamics. When an obstacle size is smaller than the free path of gas molecules, the standard methods estimating aerodynamic resistance based on the continuum theory no longer work. In normal conditions, the mean free path of air molecules makes 65 nanometers. The mean free path is the average distance one molecule manages to cover before colliding with another. Only if all obstacles are much bigger than this value, the free stream coming at them can be rightfully considered a continuous medium.
The scientists used electrospinning: a jet of a dissolved polymer is ejected through a special nozzle aiming at a target under action of an electric field. From the other side, ethanol is electrosprayed. The polymer jet and the alcohol ions take the opposite electric charges. Colliding in the air, they form ultra-thin fibrous films. Electrospinning technology as a way to produce nonwoven fibrous filters has been developed back in the 50s to purify air in atomic industry. However, the researchers introduced an important improvement: instead of obtaining nanomats on a solid conducting substrate like before, now with the new technology, a free filter is formed covering a 55-mm hole in a non-conductive polycarbonate screen.
The testing of nylon-4,6 electrospun films showed that the almost weightless and invisible fabrics trap no less than 98% of airborne dust particles. For testing, they used particles from 0.2 to 0.3 micron in diameter. This roughly corresponds to the dust that is not caught by the nasal pharynx, and penetrates the lungs causing a number of dangerous medical conditions. Submicron particles (< 1 micrometer in diameter) are the ones also used to test industrial and medical filters. To assess performance, resistance to airflow is tested as well.
Experiments to measure resistance have been made on singular samples so far. In real filters a multi-layer surface with a complex configuration is normally used. The experiments showed that the nylon-4,6 filtering material had the best properties out of all types of fabric previously described . In terms of interception extent to filter weight ratio and interception-resistance to airflow ratio, the new filtering material beats any existing equivalents by several times.
At first it looks like a ring sitting on a sheet of paper. In reality, it is a sample of the new fabric for filters. The small red spot on top of “l” letter is a spot wherea laser beam penetrates the film. Low intensity of dispersed light demonstrates high transparency of the new material.
APPS: it is more than the obvious air and water purification from particulate matter. Since the material surpasses glass in transparency, it can be used in biological research, Eg. after pumping air or water through the new filter intercepted microorganisms may be directly observed right on the transparent filter under a microscope. Again, this effect is due to ultra-fine threads. Their thickness is significantly less than even visible light wavelength. http://www.eurekalert.org/pub_releases/2016-03/miop-pga030116.php
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