Experimental Study of Dust Acoustic Waves in the Strongly Correlated   Regime
  Low frequency dust acoustic waves (DAW) were excited in a laboratory argon dusty plasma by modulating the discharge voltage with a low frequency AC signal. Metallic graphite particles were used as dust grains and a digital FFT technique was used to obtain dispersion characteristics. The experimental dispersion relation shows the reduction of phase velocity and a regime where $\partial \omega/\partial k < 0$. A comparison is made with existing theoretical model. 
P. Bandyopadhyay, G. Prasad and A. Sen
physics.plasm-ph