Optimal operating conditions for capillary breakup rheometry based on half-times of liquid bridges

McDonnell et al. [1] recently proposed a novel microfluidic capillary breakup extensional rheometer. In this device, the time taken for the mid filament radius of a liquid bridge to thin to half its initial value is used with a calibration curve to obtain the extensional viscosity of nanolitresized sample drops. In this study, we use simulations of liquid bridges to show that the bridge aspect ratio and sample volume strongly influence bridge half-times and the shapes of calibration curves. It is observed that the breakup slows down considerably as these parameters approach the region for stable liquid bridges. In addition to this, satellite drops can begin to form for very slender bridges, in which case a halftime can be difficult to define. We find the optimal operating conditions for the halftime method that avoid these difficulties.