  The physical aging of supercooled glycerol induced by upward temperature
steps of amplitude reaching 45 K was studied by a new method consisting in
heating a micrometer-thick liquid film at a rate of up to 60 000 K/s, holding
it at a constant high temperature for a controlled duration before letting it
quickly cool down to the initial temperature. By monitoring the final slow
relaxation of the dielectric loss, we were able to obtain quantitative
information on the liquid response to the initial upward step. The so-called
TNM (Tool-Narayanaswamy-Moynihan) formalism provided a good description of our
observations despite the large distance from equilibrium, provided that
different values of the nonlinearity parameter were used for the cooling phase
and for the (much further from equilibrium) heating phase. In this form, it
allowed to precisely quantify how to design an ideal temperature step, i.e.,
where no relaxation occurs during the heating phase. It helped bringing a clear
physical understanding of how the (kilosecond long) final relaxation is related
to the (millisecond long) liquid response to the upward step. Finally, it made
possible the reconstruction of the fictive temperature evolution immediately
following a step, evidencing the highly non-linear character of the liquid
response to such large amplitude temperature steps. This work illustrates both
the strengths and limitations of the TNM approach. This new experimental device
offers a promising tool to study far-from-equilibrium supercooled liquids
through their dielectric response.
