  We exploit the deformation of drops spinning in a denser background fluid to
investigate the effective interfacial tension (EIT) between miscible molecular
fluids. We find that for sufficiently low interfacial tension, spinning drops
develop dumbbell shapes, with two large heads connected by a thinner central
body. We show that this shape depends not only on the density and viscosity
contrast between the drop and background fluids, but also on the fluid
molecular structure, and hence on the stresses developing at their interface
due to different molecular interaction. We systematically investigate the
dynamics of dumbbell-shaped drops of water-glycerol mixtures spinning in a pure
glycerol reservoir. By developing a model for the deformation based on the
balance of the shear stress opposing the deformation, the imposed normal stress
on the drop and an effective interfacial tension, we exploit the time evolution
of the drop shape to measure the EIT. Our results show that the EIT in
water-glycerol systems is orders of magnitude lower than that reported in
previous experimental measurements, and in excellent agreement with values
calculated via the phase field model proposed in [Phys. Rev. X 6, 041057,
2016].
