Model-independent measurement of strain , stress-to-viscosity ratio and relaxation time τ. The wound margin position (ellipse semi-axis) is plotted versus time after severing; data from figure 1b–d, see electronic supplementary material, movie S3, along the y-axis. The difference between the initial (L) and final () positions (blue dashed lines) directly yields the value of The initial velocity (slope of the orange dashed line) yields an estimate of . Inset: velocity, estimated by finite differences of successive positions, versus the position during the first 30 s. An arrow indicates the direction of increasing time t. The slope of a linear fit (purple line) yields the inverse of the relaxation time, (see equation (3.2)).