Thermocapillary fluid and adiabatic waves near the critical point

Isothermal interfacial zones are investigated starting from a local energy which can be considered as the sum of two terms: one corresponding to a medium with a uniform composition equal to the local one and a second one associated with the non-uniformity of the fluid. In an extended van der Waals theory, the volume internal energy is proposed with a gradient expansion depending not only on the gradient of density but also on the gradient of entropy. We obtain the equation of conservative motions for non-homogeneous fluids near its critical point. For such a medium, it is not possible to obtain shock waves. The waves are tangential to the interface and the wave celerity is expressed depending on thermodynamic conditions at the critical point.