Electrical conduction in granular media:
From Branly Effect to intermittency!

The Branly Effect finally reveals its secrets

Discovered in 1890 by Edouard Branly, "the coherer effect" or "Branly effect" is an electrical transition from an insulating to a conductive state of an oxidized metallic powder when an electromagnetic wave is emitted in its vicinity.  Such a wave detector was at the origin of the first wireless radio transmissions at the end of the 19th century.  However, the physical origin of this phenomenon still remains not well understood.  A similar phenomenon of conduction transition is also observed when a DC voltage is directly applied to the sample and exceeds a certain value (we will call it herafter "DC Branly effect"). 

More than 110 years after the Branly discovery, various experiments (1D or 2D network of balls, and metallic powder) were performed at the Ecole Normale Supérieure de Lyon, in order to understand the origin of this electrical conduction transition.  These studies were notably rewarded by the 2004 Branly Prize.

chain

By means of a model experiment with a chain of metal balls, we show for the first time that the mechanism of the electrical conduction transition (DC Branly effect) results from the local heating of the microcontacts between each ball where microwelding occurs [1, 2, 1v, 2v, 4v, 1p].  The increase in temperature reached 1050°C for an applied voltage as low as 0.4 V!  The electrical conduction transition is related to the local properties of the contact between two grains.  It constitutes a first step towards more realistic granular media such as a 2D network of ordered balls (including the disorder of the contacts), or a metal powder sample (including the disorder of position) as discussed below.

In 2007, we studied the role of nearby electromagnetic waves (sparks) on the electrical conductivity of the chain of beads [3]. The effect of the spark is to lower the values of the most resistive contact, the most conductive contacts remaining unchanged. We show that the spark remotely generates within the chain a strong enough current to create microwelds between beads. This explains why the resistance of a granular medium is so sensitive to the electromagnetic waves produced in its neighborhood (Branly Effect).

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Electrical noise, intermittency in a metallic powder

We apply a DC voltage to a metal powder.  Under certain conditions, the temporal evolution of the current is then very noised.  We show that this electric noise has interesting properties of scale invariance (over 4 decades in time) and of intermittency which come from thermal expansions locally creating or destroying the electrical contacts [4, 3v, 1p, 2p].  These expansions can take place on various scales (size of the grain, size of force network, size of the sample):  The small scales depend on the large ones with similarities and differences with hydrodynamic turbulence.  These astonishing phenomena of self-similarity are connected to the collective effects of the granular matter.  Finally, with an AC applied voltage, we highlighted that the electric conduction of a metallic powder results from the wide distribution of contact resistances between grains, and does not bring in microscopic model of conduction, nor parameter related to the disorder of the medium [6].

PoudreA PoudreB PoudreC

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Does the dimension matter for electrical conduction in granular media?

We have reported measurements of the electrical conductivity in granular systems of various dimensions (1D, 2D or 3D). A new measurement technique provides a map of the electrical current paths within a 2D triangular lattice of metallic beads (2800 beads of mm size) [5]. We found that the current paths are localized in few discrete linear paths, and showed the quasi-1D feature of the electric conductivity in this 2D system can be easily explained.

2DExp
2DMap



Réferences

International peer-reviewed journals:

Branly effect:
[1] E. Falcon,  B. Castaing and M. Creyssels, European Physical Journal B 38, 475 - 483 (2004)
     Nonlinear electrical conductivity in a 1D granular medium

[2] E. Falcon and  B. Castaing,  American Journal of Physics 73, 302 - 307 (2005)
      Electrical conductivity in granular media and Branly's coherer: A simple experiment
[3] S. Dorbolo, A. Merlen, M. Creyssels, N. Vandewalle, B. Castaing and E. Falcon, EPL (Europhysics Letters) 79, 54001 (2007)
     Effects of electromagnetic waves on the electrical properties of contacts between grains

Electrical conductivity in granular media:
[4] E. Falcon, B. Castaing and C. Laroche, Europhysics Letters, 65, 186 - 192 (2004)
     Turbulent electrical transport in Copper powders
[5] M. Creyssels, S. Dorbolo, A. Merlen, C. Laroche, B. Castaing and E. Falcon, European Physical Journal E 23, 255 (2007)
      Some aspects of electrical conduction in granular systems of various dimensions

[6] M. Creyssels, E. Falcon and B. Castaing, Physical Review B 77, 075135  (2008)
      Scaling of AC electrical conductivity of powders under compression

Popularization articles:

[1v] E. Falcon and B. Castaing, Pour La Science 340, 58 - 64, Février 2006 (French edition of Scientific American)
       L'effet Branly livre ses secrets

[2v] E. Falcon and B. Castaing, Bulletin de la Société Française de Physique, 148, 9 - 12 (2005) (in french)
       Transport électrique dans les milieux granulaires : L'effet Branly continu

[3v] E. Falcon, B. Castaing and M. Creyssels Bulletin de la Société Française de Physique, 149, 6 - 9 (2005) (in french)
       Transport électrique dans les milieux granulaires : Bruit et Intermittence
[4v] E. Falcon, and B. Castaing, Investigacion Y Ciencia 404, pp. 80 - 85, Maio 2010 (Spain edition of Scientific American)
       El efecto Branly
           
Proceedings:

[1p] E. Falcon and B. Castaing, in Powders & Grains 2005, R.García-Rojo, H.J. Herrmann & S. McNamara, Eds. A.A.Balkema, pp. 323 - 327 (2005)
       Electrical properties of granular matter: From Branly Effect to intermittency
[2p] M. Creyssels, E. Falcon and B. Castaing, in Ribotta R. (Ed.), Rencontre du Non-Linéaire 2005, Non Linéaire Pub., Orsay, pp. 55 - 60 (2005) (in french)
        Bruit et intermittence du transport électrique dans les milieux granulaires
[3p] M. Creyssels, E. Falcon and B. Castaing, AIP Conference Proceedings 1145, pp. 123 – 126 (2009)
       Experiment and theory of the electrical conductivity of a compressed granular metal
        

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