Delaying viscous fingering patterns during fluid detachment

We have studied viscous fingering instability patterns that frequently develop during thin-film detachment. An experimental investigation was conducted employing a thin layer of a simple yield stress fluid between two rigid circular parallel plates of a commercial rheometer operating in a probe-tack mode. We present a method to delay this type of interface instability by allowing an extra amount, a small external cap, of the mediating working fluid bulging out the contour boundary of the plates. When carefully prepared, this extra cap moves the air-fluid interface away from the high velocity flow field generated near the confines of the plates at the beginning of the detachment. A lower velocity flow field causes a delay on the instability growth which, within a given set of parameters, reduces or completely removes the fingering pattern. The effect of fingering instability in the presence of extra amounts of extra sample were investigated by monitoring the traction normal-force <i>versus</i> gap separation during detachment. Studies varying both the initial gap, the gap separation velocity, and the amount of a carefully prepared externally positioned sample layer were systematically conducted in order to determine the conditions where the instability delay was sufficient to remove fingering patterns. The force-decaying regimes were identified as well as their dependencies on the experiment control parameters. A proper adjustment of the extra cap resulted in complete removal of the fingering pattern while the peak of the normal traction force was unaffected. The absence of fingering both during and after the detachment was confirmed by videoing the process from underneath with a transparent glass bottom plate. Nevertheless, when the traction force profile was compared to the standard results obtained with trimmed samples, an additional stage with a levelled-off force interval, akin to those caused by cavitation, took place. Finally, we show the method is robust and may contribute as an alternative approach for delaying, and sometimes completely removing, the effect of fluid fingering on fluid-surface contact applications where one desires a final uniform sample distribution.