How Tribo-Electric Charges Modify Powder Flowability

ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 25, 2017

How tribo-electric charges modify powder ﬂowability

Antonella Rescaglio1, Julien Schockmel2, Filip Francqui3, Nicolas Vandewalle1,2, and Geoffroy Lumay1,2

1CESAM-APTIS, University of Liege, Sart-Tilman, B-4000 Liège, Belgium 2CESAM-GRASP, University of Liege, B-4000 Liège, Belgium. 3GranuTools, Rue Jean-Lambert Defrêne 107, 4340 Awans, Belgium

ABSTRACT timization of processes or to avoid technical The presence of electrostatic charges inside a issues. In order to control and to optimize powder is known to influence drastically the processing methods, these materials have to material flowing properties. The triboelectric be precisely characterized. This characteriza- charges produced at the contacts between the tion methods is related either to the proper- grains and at the contacts between the grains ties of the grains or to the behavior of the and the container produces electrostatic forces. assembly of grains. Many advanced meth- On the one hand, the triboelectric effect is use- ods are available to measure physico-chemical ful for many applications, but on the other grain characteristics: laser diffraction to ob- hand, the triboelectrification causes complica- tain the grain size distribution, morphometer tions. Unfortunately, the triboelectric effect to measure grains shape, X-ray diffractometer is still poorly understood, even at the funda- to characterize crystallinity, ... During the past mental level. The difficulties are related to decades, some interesting techniques have been the non-equilibrium character of the tribolec- developed to measure powder flow like pow- tectric dynamic and to the variety of mech- der rheometers inspired by liquid rheometers,1 anisms behind this effect. Moreover, repro- shear cells2 and a variety of improved meth- ducible electrostatic measurements are difficult ods.3 However, too many techniques used in to perform. We developed an experimental de- R&D or control quality laboratories are still vice dedicated to the measurement of powder based on old measurement techniques.4 In par- triboelectric properties. This device measures ticular, the powder electrostatic properties are the ability of a powder to charge electrostati- still poorly understood, even at the fundamen- cally during a flow in contact with a selected tal point of view. material. This measurement is performed at controlled hygrometric conditions. We present When two materials are rubbed, electric the results of an experimental study involving charges are exchanged at the surfaces.5, 6 De- different powders and granular materials. The spite the numerous studies dedicated to this relation between the powder electrostatic prop- subject, the fundamental mechanisms behind erties, the hygrometry and the flowing behavior this triboelectric effect is not fully understood. is analyzed. In particular, the charging of objects composed by the same material and charging of pow- INTRODUCTION ders and granular materials are two examples Granular materials, fine powders and nanopow- of poorly understood subject. Even the basic ders are widely used in industrial applications. question related to the nature of the transferred Therefore, any progress in the understanding charges (electrons, ions or material) is still de- of powders behaviors could have huge conse- bated. In addition to the fundamental works quences for numerous industries for the op- needed to improve the understanding of the tri-

17 A. Rescaglio et al. boelectrification in powders and granular mate- GranuDrum rials, a precise and reproducible measurement 60 method is needed to quantify the ability of a 55 powder to produce electric charges. σ 50 When dealing with powder flow and powder electrostatic properties, it is well known 45 empirically that air moisture is an important pa- 40 7–9 index Cohesive rameter. Indeed, moisture influences surface 35 grains conductivity and capillary bridges for- 30 mation. For low relative air humidity, the elec- 0 20 40 60 80 100 trical conductivity necessary for charge dissipa- - + - - + + Relative humidity RH (%) tion is reduced. Therefore, the electric charges - + created by triboelectric effects induce forces Figure 1. Cohesive index s measured with between the grains and/or between the grains GranuDrum at a rotating speed W =8 RPM as and the container. For high relative air hu- a function of the relative humidity RH for a midity, the electrical conductivity increases and residence time in the conditioner fixed to liquid bridges may be formed at the contacts T = 150 minutes. between the grains. Therefore, the electrical charges are dissipated more easily. However, the apparition of liquid bridges induces cohe- der flowability measurement method based on 10, 11 sive forces inside the packing. At intermedi- the rotating drum principle. A horizontal ate relative humidity values the cohesion is ex- cylinder with vertical glass side walls called pected to decrease. drum is half filled with the sample of pow- In this paper, we show how the combined der (see insert of Figure 1). The drum rotates effect of electrostatic charges and capillary around its axis at an angular velocity W rang- bridges affects powder flowability. For that, ing from 2 RPM to 20 RPM. A CCD cam- the powder cohesiveness and the powder com- era takes snapshots (typically 50 images sep- paction dynamics is measured for different rel- arated by 0.5s) for each angular velocity. The ative air humidity conditions. Afterward, we air/powder interface is detected on each snap- focus on the effect of electrostatic charges on shot with an edge detection algorithm. Af- the flow at low humidity. The flow of a charged terwards, the average interface position and powder through the aperture of a silo is ana- the fluctuations around this average position lyzed. are computed. Then, for each rotating speed, the flow angle a f is computed from the av- MEASUREMENT METHODS erage interface position and the dynamic co- Four devices have been used to study the influ- hesive index s f is measured from the inter- ence of triboelectric charges on powder flow: face fluctuations. Indeed, interface fluctuations (i) GranuDrum to measure the powder cohe- are induced by the cohesive forces between the siveness, (ii) GranuPack to measure the com- grains. Therefore, the dynamic cohesive in- paction dynamics, (iii) a silo (GranuFlow) to dex s f is close to zero for non-cohesive pow- measure the flowability and (iv) a tribocharger ders and increases when the cohesive forces in- (GranuCharge) to measure the ability of a pow- crease. der to charge during a flow in contact with a Before the measurement in the rotating selected material. The set-ups used to perform drum, the powder is conditioned during a time the present study are the prototypes of the in- T in a slowly rotating reactor with an air flux struments presently commercialized by Granu- of controlled RH. To obtain more information Tools company. about this method, see.9 The GranuDrum3 is an automated pow- The GranuPack instrument3 is an auto-

18 ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 25, 2017

GranuPack created inside a granular material during a flow 18 in contact with a selected material. The sample is poured manually (the feeding could be 16 automated) in a V-tube and flows to a Fara- 14 day cup. The tube material can be selected for each tube. The faraday cup is connected to a 12 customized electrometer able to measure elec- 10 trostatic charges. At the end of the flow, the total value of the electric charge Q present in 8

Characteristic tap number n1/2 tap number Characteristic the powder is measured and the charge density 6 q = Q/m, where m is the sample mass, is com- 0 20 40 60 80 100 - + puted. The V-tube geometry has been selected - Relative humidity RH (%) -- +++ to combine the different mechanisms leading to tribo-electrification: (i) friction between the Figure 2. Number of taps n1/2 needed to reach grains, (ii) friction between the grains and the one half of the compaction process, i.e. needed wall and (iii) impacts of the grains on the wall ( + )/ to reach the density r0 rN 2, as a function at the connection between the two tubes. The of the relative air humidity RH. Faraday cup can be replaced by a silo with an aperture D and coupled with an electronic mated and improved tapped density measure- scale (see Figure 3 (right)) to measure the in- ment method. The behavior of the powder sub- fluence of the electrostatic charges on the flow mitted to successive taps is analyzed with an rate. This configuration with the flowmeter (the automated device (see insert of Figure 2). The silo) following directly the tribo-charger is nec- classical Hausner ratio Hr, the initial density r0 essary to minimize the charge decay between and the final density rN after N taps are mea- the tribo-charger and the flow measurement. sured precisely. Moreover, a dynamical param- eter n1/2 (the number of taps needed to reach RESULTS the density (r0 + rN)/2) can be extracted from Before presenting our very recent results link- compaction curves. The compaction curve is ing directly the total amount of electrostatic a plot of the bulk density as a function of the charge inside a powder and the flowability, we tap number. The powder is placed in a metal- show how the relative air humidity RH influ- lic tube with a rigorous initialization process. ences the flowability. The RH is expected Afterwards, a light hollow cylinder is placed to modify the amount of electrostatic charge on the top of the pile to keep it flat during inside the powder, modifying indirectly the compaction. A single tap consists in the tube flowability. containing the powder sample moving up to a To measure the influence of RH on pow- height of DZ = 1mm and then performing a free der cohesiveness, a powder conditioning at fall. The height h of the pile is measured au- fixed humidity during T = 150 minutes has tomatically after each tap. From the height h, been done before the measurements in the the volume V of the pile is computed. As the GranuDrum. Figures 1 shows the influence of powder mass m is known, the bulk density r is RH on powder cohesiveness for a lactose pow- evaluated and plotted after each tap. The bulk ders (Granulac 140 from the Meggle). The density is the ratio between the mass m and the cohesion is found to decrease slightly in the volume V of the powder. The relative air hu- range of RH from 0% to 50%. Afterward, the midity can be controlled inside the device dur- cohesion increase importantly from RH=50% ing the measurement. to RH=100%. For low RH, the cohesion is A triboelectric charger (see Figure 3 (left)) induced by the apparition of electric charges is used to measure the total electrostatic charge in the powder. For high humidity condi-

19 A. Rescaglio et al.

Tribo-charger + Silo Tribo-charger Tribo-charger + Silo 4.6

4.4

V-Tube 4.2

4.0 flow rate (g/s) flow 3.8

D 3.6 USB -2.5 -2 -1.5 -1 -0.5 0 0.5 Charge density q (uC/kg) Electro- meter Scale Figure 4. Flow rate through the silo aperture (see Figure 3 (right)) as a function of the Figure 3. (left) The powder tribocharger used charge density inside the powder. to measure the electrostatic charge created inside the powder after a flow in contact with a selected material. The device used to perform flowability. The flow rate through the aper- the present study is the prototype of the ture of a silo has been measured with a pow- GranuCharge instrument commercialized by der previously charged with the tribo-charger GranuTools company. (right) The tribocharger (see Figure 3 (right)). The silo aperture size is used to charge the powder before filling the has been fixed to D = 5mm and the powder is silo to measure the flow rate. The silo formed by glass beads of diameter d = 50µm. measurement system comes from the The charge density q expressed in µC/kg has GranuFlow instrument from GranuTools. been varied by changing the V-tube characteristics. The small glass beads has been selected tions, the condensation of water at the sur- to obtain a hight charge density. Figure 4 shows face of the grains leads presumably to the for- the evolution of the flow rate as a function of mation of capillary bridges at the contact be- this charge density q. The flow rate is found to tween the grains, increasing powder cohesive- decrease significantly when the absolute value ness. The same behavior is observed with glass of the charge density increases. Moreover, as beads during compaction measurements at con- shown by the error bars, the fluctuations of the trolled RH. The glass beads are spherical and flow rate from one measurement to the other monodisperse with a diameter d=1mm. For the (five repetitions) increases also with the abso- present study, the number of taps has been fixed lute value of the charge density q. Indeed, to N = 2000 in order to reach the saturation. As the cohesive forces related to the electrostatic shown by Figure 2 the compaction character- charges are inducing flow fluctuations. istic time n1/2 decreases between RH=20% to RH=45% and increases afterward. A low value CONCLUSION of the compaction characteristic time n1/2, i.e. We have shown that the relative air humidity a fast compaction dynamics, is associated to a affects strongly the flow of powders. The pow- good flowability.12 der cohesiveness increases for both dry and For low RH, the flowability is decreased wet conditions. When the relative air humid- due to the apparition of electrostatic charges. ity is low, grain-grain friction inside the flow For high RH, the flowability is decreased due induces the appearance of electrical charges in to capillary interactions. An optimal flowabil- the packing, inducing cohesive forces between ity is obtained around RH=45%. Now, we fo- the grains. The condensation in high humid- cus on low RH conditions to link the quantity ity conditions implies the formation of liquid of electrostatic charges in the powder and the bridges between contacting grains which in-

20 ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 25, 2017 duce also cohesive forces. Taking both ef- pharmaceutical powders”. Powder Technology, fects (triboelectricity and capillarity) into ac- 189, 409. count, the cohesion is minimized for intermedi- 9. Lumay, G., Traina, K., Boschini, F., ate values of the relative humidity, i.e. between Delaval, V., Rescaglio, A., Cloots, R., and Van- RH=30% and RH=50%. dewalle, N. (2016). “Effect of relative air hu- The flow rate through the aperture of a silo midity on the flowability of lactose powders”. has been measured with a powder previously Journal of Drug Delivery Science and Technol- charged with a tribo-charger. This flow rate is ogy, 35, 207e212. found to decrease when the absolute value of 10. Rajchenbach, J. (1990). “Flow in pow- the charge density q increases. Moreover, the ders: From discrete avalanches to continuous electrostatic charges are inducing flow fluctua- regime”. Phys. Rev. Lett., 65, 2221. tions. These first results are opening new per- spectives to understand the influence of electro- 11. M. A. S. Quintanilla, J.M.V. and Castel- static charges on powder flow. lanos, A. (2006). “The transitional behaviour of avalanches in cohesive granular materials”. ACKNOWLEDGEMENTS J. Stat. Mech., page P07015. We Thanks Walloon region (Fonds de matu- 12. G. Lumay, N. Vandewalle, C.B.L.D. and ration - convention 1318086 and BEWARE - Gerasimov, O. (2006). “Linking compaction convention 1410265) and F.R.S.-FNRS (Grant dynamics to the flow properties of powders”. PDR T.0043.14) for the financial support. Appl. Phys. Lett., 89, 093505.

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