Inhibited pattern formation by asymmetrical high voltage excitation in   nematic fluids
  In contrast to the predictions of the standard theory of electroconvection (EC), our experiments showed that the action of superposed ac and dc voltages rather inhibits pattern formation than favours the emergence of instabilities; the patternless region may extend to much higher voltages than the individual ac or dc thresholds. The pattern formation induced by such asymmetrical voltage was explored in a nematic liquid crystal in a wide frequency range. The findings could be qualitatively explained for the conductive EC, but represent a challenging problem for the dielectric EC. 
Peter Salamon, Nandor Eber, Balazs Fekete and Agnes Buka
cond-mat.soft