ABSTRACT

We are fascinated by the complex fluid beauty and the challenges underlying the design and use of plant-based foods and sustainable formulations, including paints and coatings, fibers, cosmetic creams, nail lacquers, and personal care products like hand soaps, shampoos, and conditioners. The need to provide food and better living to the exploding world population promises to exacerbate climate change due to the high carbon cost per calorie and non-biodegradable products. Plant-based foods and formulations offer promising alternatives; however, their suitability as sustainable substitutes will require emulating properties, processing, and functionalities of the animal- and fossil-fuel-based products. Flow behavior, stringiness, spinnability, and printability of formulation are often assessed qualitatively by dripping from a nozzle or a ladle or by stretching a liquid bridge between two surfaces (thumb and forefinger or between parallel plates). The handy tests examining the pinching time of a columnar neck undergoing spontaneous capillarity-driven pinching and extensional flows correlate better with stringiness and dispensing behavior than shear rheology characterized using torsional rheometers. However, well-documented challenges arise for the quantitative characterization of extensional rheology response, leaving unanswered questions about the influence of macromolecular properties on rheology, processing, and sensory perception, thus hampering the search for sustainable alternatives. Here we show that the dripping-onto-substrate (DoS) rheometry protocols we developed enable the characterization of extensional rheology of the polymer solutions and multicomponent formulations. We investigate the influence of polysaccharide thickeners on the rheology and fizzics of plant-based milk emulsions, real and vegan mayo, and model paints and cosmetics. We find that the pursuit of practically motivated problems concerning rheology, shelf-life, and consumer perception of sustainable formulations involve fundamental problems in soft matter physics and fluid mechanics. We probe the influence of ingredient-specific interactions and dynamics by elucidating the impact of significant stretching,   unraveling, and orientation of polymers and proteins (or drop/bubble deformation and breakup) in response to nonlinear extensional flows. A deeper understanding of pinching dynamics and the governing mechanical quantities compels exploration and advances into the physics and mathematics of self-similarity, finite-time singularity, coalescence and spreading kinetics,  stretched polymer physics, and scaling or dimensional analysis.