A Study On Charging Mechanism Of Sand Grains And Charge-to-mass Ratio In Wind-blown Saltation

In wind-blown saltation process, there will be different polarity and charge magnitude between large and small sand particles, and these charged saltating particles can form an electric field. Wind-blown sand electrification plays an important role in critical friction velocity, sand movement, sand transpoart rate, etc. Moreover, recent studies indicated that charged particles could attenuate electromagnetic wave propagation in sandstorms, and could form the geological patterns. Why sand particle charges, and how to predict the charge magnitude are the basic scientific problems in wind-blown sand electrification. Therefore, we investigate the contact charging mechanism of sand particles and the sand electrification in wind-blown saltation in this study.We systematically demonstrates experiments of contact charging on single collision processes of millimeter size glassy particles, including the headon collision of glassy particle-glassy particle and the headon collision of glassy particle-glassy plane. Due to the lacks of contact charging experiments, the existing contact charging models can be directly verified, therefore our experiments fills the gap. Moreover, we carry out the wind-tunnel&field experiments on the charge-to-mass ratio along the height and the averaged charge-to-mass ratio in wind-blown sand.There are electron trapping by point defects at silica surfaces, such as the nonbridging oxygen center, a three-coordinated silicon with a singly occupied dangling bond, the peroxy radical and the self-trapped hole. These defects can act as deep electron traps or trapped holes. Therefore, based on the high-energy trapped hole transferring, we proposed an asymmetric contact charging model to predict the charge magnitude, and the predictions agrees well with our experiments of contact charging in a single collision. The existing contact charging models cannot explain why the contact charge changes with impacting velocity, but based on sof-sphere contact our model can depict experiments well. Therefore our contact charging model can help us to understand the mechanisms of sand tribocharging phenomena in a more clear way, such as to the charge polarity, magnitude, and effects on discharge and attenuation of electromagnetic wave propagation.Saltation plays an important role in contact charging when sand particles impact the sand bed. Using the discrete element method (DEM), we simulate the sand-bed collision processes. Based on the asymmetric contac charging model, we analyze the contact charge of sand-bed collision processes, and demonstrate the probability density function (PDF) of the net charge transfer of the rebounded particle. The net charge transfer of the rebounded particle depends on impacting velocity, impacting angle, impacting radius, contact angle and contacting radius. Therefore we consider the probability density function of multi stochastic variables, such as two stochastic variables and five stochastic variables.We simulate the wind-blown saltation of uniform size and mixed size sand bed. In the case of uniform size sand bed, no charging mechanism is brought, and all saltating particles are assumed to be equally charged. The streamwise sand transport rate is related to the sand particle’s radius, the charge-to-mass ratio, and the friction velocity, then a general model is given for the streamwise sand transport rate with three variables. In the case of mixed size sand bed, a vertical turbulent velocity fluctuation is added to the wind field, and our contact charing model is introduced to the wind-blown saltation. The charge flux and the charge-to-mass ratio varyging with height, and the averaged charge-to-mass ratio can be obtained statistically.In summary, we investigate the contact charging in a single collision of identical insulating particles experimentally and theoretically, which can help us to understand sand electrification phenomena in wind-blown saltation in a more clear way. The general formula of streamwise sand transport rate can be easily applied to predict sand transport rate, and the charge-to-mass ratio varying with height and friction velocity can be numericall predicted, which provide certain theoretical guidance to sand electricfication in wind-blown sand saltation.