On stretching of plasma parameters and related open issues for the study of dust-ion-acoustic and dust-acoustic shock waves in dusty plasmas

To study the properties of the shock structures associated with dust-ion-acoustic (DIA) and dust-acoustic (DA) waves, the stretching of the plasma parameters [viz., kinematic or longitudinal viscosity coefficient eta(i) (eta(d)) for DIA (DA) waves and the plasma parameter delta(i) (delta(d)) associated with the dust charge fluctuation for DIA (DA) waves] has been used by many authors. It is argued that the stretching of such plasma parameters is not usually valid. The valid stretching coordinates for deriving the Burgers equation, which leads to the formation of DIA and DA shock waves, are provided. A few open issues related to the sources of dissipation [viz., different viscous forces giving rise to eta(i) and eta(d) and dust charge fluctuation giving rise to delta(i) and delta(d)] are also pinpointed. To remove the stretching of plasma parameters, which is not usually valid, from our future research work, and to address some related open issues will be able not only to enhance the quality of our future research work but also to help us in working on some challenging research problems in dusty plasma physics.