Experimental observation of strong coupling effects on the dispersion of   dust acoustic waves in a plasma
  The dispersion properties of low frequency dust acoustic waves in the strong coupling regime are investigated experimentally in an argon plasma embedded with a mixture of kaolin and $MnO_2$ dust particles. The neutral pressure is varied over a wide range to change the collisional properties of the dusty plasma. In the low collisional regime the turnover of the dispersion curve at higher wave numbers and the resultant region of $\partial\omega/\partial k < 0$ are identified as signatures of dust-dust correlations. In the high collisional regime dust neutral collisions produce a similar effect and prevent an unambiguous identification of strong coupling effects. 
P. Bandyopadhyay, G. Prasad, A. Sen and P.K. Kaw
physics.plasm-ph