A note on the stochastic nature of particle cohesive force and implications to threshold friction velocity for aerodynamic dust entrainment

Authors: SHAO, YAPING - University Of Cologne; KLOSE, MARTINA - New Mexico State University

Submitted to: Aeolian Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/11/2016
Publication Date: 9/1/2016
Citation: Shao, Y., Klose, M. 2016. A note on the stochastic nature of particle cohesive force and implications to threshold friction velocity for aerodynamic dust entrainment. Aeolian Research. 22:123-125.

Interpretive Summary: How much surface friction is needed to lift soil particles? It has been the long-standing view that particles in the dust-size range (smaller than approximately 60 µm) cannot be lifting by aerodynamic forces alone because of strong inter-particle cohesive forces holding the particles at rest. While this is the case on average, inter-particle cohesive forces are influenced by a large range of factors such as particle shape, mineralogy, surface roughness, etc., leading to a strong variation of the forces. When this variation is included in physics-based descriptions of the friction velocity needed to lift particles, this so-called “threshold friction velocity” is lower than previously thought for particles in the dust-size range. This suggests that dust can be lifted by aerodynamic forces alone.

Technical Abstract: There is considerable interest to determine the threshold for aeolian dust emission on Earth and Mars. Existing schemes for threshold friction velocity are all deterministic in nature, but observations show that in the dust particle size range the threshold friction velocity scatters strongly due to stochastic interparticle cohesion. In the real world, there always exists a certain amount of free dust which can be easily lifted from the surface by weak winds or even turbulence, as exemplified by dust devils. It has been proposed in the dust-devil research community, that the pressure drop at dust-devil center may be a major mechanism for dust-devil dust emission, known as the 'p effect. It is questioned here whether the 'p effect is substantial or whether the elevated dust concentration in dust devils is due to free dust emission. A simple analysis indicates that the 'p effect appears to be small and the dust in dust devils is probably due to free dust emission and dust convergence. To estimate free dust emission, it is useful to define a lower limit of dust-particle threshold friction velocity. A simple expression for this velocity is proposed by making assumptions to the median and variance of inter-particle cohesive force. The simple expression is fitted to the data of the Arizona State University Vortex Generator. While considerable uncertainty remains in the scheme, this note highlights the need for additional research on the stochastic nature of dust emission.