The Theory of Bodily Tides. The Models and the Physics
  Description of tides is based on the form of dependence of the geometric lag on the tidal frequency. Some authors assume the lag angle to be constant, others set it to be linear in the frequency. The actual dependence of the lag on the frequency is complicated and is determined by the planet's rheology. A particular form of this dependence will fix the form of the frequency dependence of the tidal quality factor Q. Since at present we know the frequency-dependence of the quality factor, we can reverse our line of reasoning and obtain the appropriate frequency-dependence of the lag. Employment of a realistic frequency-dependence for Q renders considerable changes in timescales defined by tidal dynamics. 
Michael Efroimsky