Every time we tap or swipe our touch screens, we leave a bit of ourselves behind. Our fingerprints are mostly sebum, an oily mixture of fatty acids, ester waxes, and squalene. Drew Evans, University of South Australia, Mawson Lakes, and colleagues compared fingerprints to deposits of artificial sebum, a mixture of triglycerides and free fatty acids.

Artificial sebum exhibits shear thinning behavior. Thus, rapidly moving a cloth over a device screen should clean a screen more effectively than moving it slowly. Artificial sebum does not exhibit thixotropy; that is, the viscosity does not change with the shear rate.

The team found a more complex situation for fingerprints: Although sebum is hydrophobic, fingerprints swell in high ambient humidity. Electron microscopy and Energy-dispersive X-ray (EDX) spectroscopy confirmed that salt residue from sweat is bound to the sebum and attracts water.

Fingerprints exhibit gel-like behavior, keeping their resting geometry in the absence of shear. Raising the temperature from 25 to 35 °C made the sebum deposits more likely to liquefy. They were easier to wipe off.

The researchers note that more study is required to determine adhesion effects, and to determine whether shear banding (strain localization) could result in only the top layer of the fingerprint being removed. This knowledge could be useful in developing better smudge-resistant screen coatings.

• Unusual Nature of Fingerprints and the Implications for Easy-to-Clean Coatings,
  Bastian Stoehr, Stuart McClure, Alexander Höflich, Mohammad Al Kobaisi, Colin Hall, Peter J. Murphy, Drew Evans,
  Langmuir 2016.
  DOI: 10.1021/acs.langmuir.5b04138