Lifting Dust Aggregates in Electric Fields

In eolian events, where large amounts of dust are carried through an atmosphere, strong electric fields can be generated above ground. Electric charge is transmitted via tribocharging during inter-particle collisions which can have a great impact on further particle transport and sedimentation. If different grain sizes, for example, charge differently, this might lead to size dependent particle and charge separation [1]. It could also promote particle lifting [2,3]. This shows, that understanding the charging behavior of particles and aggregates in strong electric fields is important in the context of particle transport in atmospheres. Especially on Mars, any kind of support for particle lifting might be crucial.

We investigate the charging behavior of mm-sized basaltic dust aggregates with the help of microgravity experiments at Bremen drop tower. Our setup consists of a 50 x 50 x 110 mm chamber which we operate in an air environment. The sides of the chamber are copper plates which function as electrodes. At the bottom of the chamber, the sample is placed inside a cylindric aluminum container, which is also coated with basalt dust. The dust grains making up the agglomerates are in the µm size range. The aggregates themselves range from 0.4 – 2.2 mm in diameter.

Before the microgravity phase, we shake the aggregates for 15 minutes in order to electrically charge them. As soon as the sample is ejected into an 8 kV DC field, the aggregates are accelerated towards one of the electrodes. Through this acceleration, we can estimate the charge of the individual agglomerates. This way, we observe initial charges up to 10⁵­ e, both negative and positive without an obvious bias in polarity. Once the aggregates reach an electrode, they either instantly stick to it or bounce off, but eventually cling to the copper plate. Most agglomerates larger than 0.4 mm do, however, recharge while sticking on the electrode until the repelling Coulomb force outweighs the adhesive sticking force. The sticking time is on the order of 0.05 – 0.5 s. The agglomerates charge up to 10⁷ e until they are accelerated to the opposite electrode and recharge again. This charge gained on the electrodes is up to two orders of magnitudes higher than the initial charge. When agglomerates bounce on an electrode, no significant charge is transmitted. The experiments are in agreement with a model where conductive grains on a conductor in an electric field charge, increasing the repulsive force until the different contacts can no longer oppose lifting with their adhesive forces [4].

Our results show that the basaltic dust aggregates are moderately electrically conductive. This presumably is caused by water clinging to the surface and the inside of the agglomerates, making the impact of an electric field on particle transport dependent on the humidity of the ambient atmosphere. In any case, these measurements allow us to quantify the charges and the lifting forces within a given field. If electric fields were present on Mars, electrostatic repulsion might support reducing the threshold friction velocity for saltation.

 

References:

[1]          K.M. Forward, D.J. Lacks, R.M. Sankaran, 2009, Geophys. Res. Lett.,36, p. L13201
[2]          Renno N. O., Kok J. F., 2008, Space Sci. Rev., 137, 419
[3]          von Holstein-Rathlou C., Merrison J. P., Brædstrup C. F., Nørnberg P., 2012, Icarus, 220, 1
[4]          F. C. Onyeagusi, F. Jungmann, J. Teiser, G. Wurm, (in prep).