Physicists get a perfect material for air filters

March 2, 2016

A research team from the Institute of Theoretical and Experimental Biophysics of the Russian Academy of Sciences has synthesized a 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 less than 15 nm beats any other similar material in terms of filtering and optical properties.

The scientists, whose work is published in the European Polymer Journal, characterize their material as lightweight (10-20 mg/m2), almost invisible (95 percent light transmission, more than that of window glass), showing low resistance to airflow and efficient interception of <1 micrometer fine particulate matter.

"Nanofibers" is more than a buzzword in the researchers' article. Previously, the same team demonstrated that reducing fiber diameter from 200 nm down to 20 nm decreased filter resistance to airflow by two-thirds, and that 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 is 65 nm.

The mean free path is the average distance one molecule covers before colliding with another. If all obstacles are larger than this value, the free stream coming at them can be considered a continuous medium.

The scientists used a technique called electrospinning in which a jet of a dissolved polymer is ejected through a special nozzle aimed at a target under the influence of an electric field. Ethanol is electrosprayed from the opposite side. 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 was developed back in the 1950s to purify air in the atomic industry. However, the researchers introduced an important improvement. Instead of obtaining nanomats on a solid conducting substrate, the new technology produced a free filter covering a 55 mm hole in a non-conductive polycarbonate screen.

The published work completes the cycle of the authors' papers devoted to development of the manufacturing technology and studies of nanofilters manufactured using this new process. The unique optical and filtering properties originate from a special mechanism of "healing" holes and defects in free-standing filters. Such holes literally attract fibers landing on the filter surface. As a result, a good filter without big holes can be obtained from a minimum quantity of nanofibers, and accordingly, with a minimum resistance to airflow. Moreover, active healing of big holes between threads provides the filters with the properties inherent in filters with calibrated pores, so-called track-etched membranes (nuclepores). The scientists have also demonstrated that the "healing" mechanism does not work in the conventional electrospinning technique in which nanofibers are deposited onto a conducting substrate completely at random.

The testing of nylon-4,6 electrospun films demonstrated that nearly weightless and invisible fabrics trap no less than 98 percent of airborne dust particles. For testing, the scientists used particles from 0.2 to 0.3 microns in diameter. This roughly corresponds to the amount of 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 previously described fabric. In terms of the interception extent to filter weight ratio and the interception resistance to airflow ratio, the new filtering material beats any existing equivalents by several times.

Discussing possible applications of this material, the scientists claim 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. For example, after pumping air or water through the new filter, intercepted microorganisms may be directly observed on the transparent filter under a microscope. Again, this effect is due to ultra-fine threads. Their thickness is significantly less than even the visible light wavelength.

Explore further: Material may offer cheaper alternative to smart windows

More information: Andrei Y. Mikheev et al. Filtering and optical properties of free standing electrospun nanomats from nylon-4,6, European Polymer Journal (2016). DOI: 10.1016/j.eurpolymj.2016.01.001

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