Extensional flow is often encountered in real-world liquid flows such as spraying, necking, and droplet formation. The behavior of polymer solutions under elongational flow cannot be predicted from the solution’s response to shear flow; thus the elongational properties need to be measured directly. The dripping-onto-substrate (DOS) rheology method pioneered by Sharma and coworkers is capable of measuring a vast array of fluids under elongational flow by filming the thinning of a liquid bridge with a high-speed camera. DOS is particularly well-suited for systems with high elongational viscosity and low shear viscosity, such as dilute solutions of long flexible polymers, where analysis of the elastocapillary regime yields the extensional relaxation time and extensional viscosity. In prior work, the determination of the critical time, which signals the onset of the elastocapillay regime, was done manually. Additionally, researchers have identified the distance between the substrate and nozzle as a key factor in the experimental setup. Both the identification of the critical time and the selection of the correct distance are key to generating consistent DOS results. Here, we describe protocols that facilitate identification of the regime of substrate-nozzle distance and provide an objective basis for determining critical time. We find the substrate-nozzle distance depends on the solvent and identify features in the collected data which indicate an unsatisfactory height was used. We present automated techniques that process the high-speed videos in an unbiased manner and without researcher input. This software performs a background subtraction, binarization based on a threshold that is evaluated for each individual frame, and generates a radius-time dataset. Using the dataset, we automatically identify the critical time and use it to find the elongational viscosity of the polymer solution. Through the improvements detailed above, we are able to dramatically increase the throughput of our DOS instrument. We use the improved dripping-onto-substrate methods to study aqueous solutions of poly(ethylene oxide) and poly(acrylamide). We find that poly(acrylamide) has a stronger molecular weight scaling than poly(ethylene oxide), indicating these solutions are more sensitive to the length of the polymer chain.

Investigation in experimental conditions and automation of dripping-onto-substrate rheology

ACS Spring 2022

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