Researchers at the Harvard Medical School Department of Systems Biology have discovered that due to bacterial slime, bacterial biofilms could be the most liquid-repellent surfaces on Earth, even more than Teflon, which is one of the most widely used chemicals for keeping surfaces waterproof.
Biofilms are bacterial cells that stick together through bacterial slime, scientifically referred to as the extracellular matrix, which is a mixture of sugar and protein. Most bacteria have been studied since the seventeenth century as individual cells, but in the last couple decades biofilms have started being recognized as an important part of the bacterial ecosystem. The liquid resistance of biofilms was discovered as a byproduct of the research being done with them.
To measure how liquid-resistant a material is, a drop of liquid is placed on a surface. The angle between the surface and the opposing edge of the liquid drop determines how resistant a material is to liquid. If the contact angle is larger, the material is more resistant.
Teflon, or polytetrafluoroethylene, has a contact angle of 110 degrees. This is predominately because Teflon is so hydrophobic: when an equal ethanol/water mixture is used instead of water, the contact angle nearly halves. In other words, as more ethanol was added to the mixture, the liquid spread from its drop shape to the dome shape that most liquids have when in contact with materials.
Biofilms have a contact angle near 135 degrees when drops of water are added, and stay in that shape even when the liquid is up to 80-90% ethanol. Past 90% ethanol, the biofilms start becoming significantly wetter. Lysol, drain opener, acetone, methanol and isopropanol also create high contact angles with the biofilms; bleach is the only major exception.
The surface assists in keeping the material liquid-repellent. The matrix has a highly crinkled appearance on a 10 micrometre or 100 micrometre scale. This multi-layered crinkling has been demonstrated as a strong component in oil-resistant surfaces.
The researchers think that once they completely understand their molecular structure, biofilms could have significant uses not as a Teflon replacement but in other applications for materials. They believe the material matches the concept that protein and sugar is needed for the repelling property, and while they still don’t completely understand why the sugar is so dominant as a repellent, they have started doing follow-up research.
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