  There is mounting evidence indicating that relaxation dynamics in liquids
approaching their glass transition not only becomes increasingly cooperative
(1,2) but the relaxing regions also become more compact in shape(3-7). While
the surface tension of the interface separating neighboring relaxing regions is
thought to play a crucial role in deciding both their size and
morphology(8-10), owing to the amorphous nature of these regions, even
identifying these interfaces has not been possible in bulk liquids. Here, by
devising a scheme to identify self-induced disorder sites in bulk colloidal
liquids, we directly quantified the dynamics of interfaces delineating regions
of high and low configurational overlap. This procedure also helped unveil a
non-monotonicity in dynamical correlations that has never been observed in bulk
supercooled liquids. Using the capillary fluctuation method (11,12), we
measured the surface tension of amorphous-amorphous interfaces with
supercooling and find that it increases rapidly across the mode-coupling area
fraction. Remarkably, a similar growth in the surface tension is also seen in
the presence of a pinned amorphous wall. Our observations help prune theories
of glass formation and opens up new research avenues aimed at tuning the
properties of amorphous-amorphous interfaces, and hence the glass itself, in a
manner analogous to grain boundary engineering in polycrystals (13).
