We present an experimental study of the fingering patterns in a Hele-Shaw cell, occurring when a gel-like material forms at the interface between aqueous solutions of a cationic surfactant (cetyltrimethylammonium bromide) and an organic salt (salicylic acid), two solutions known to form a highly elastic wormlike micellar fluid when mixed homogeneously. A variety of fingering instabilities are observed, depending on the velocity of the front (the injection rate), and on which fluid is injected into which. We have found a regime of non-confined stationary or wavy fingers for which width selection seems to occur without the presence of bounding walls, unlike the Saffman-Taylor experiment. Qualitatively, some of our observations share common mechanisms with instabilities of cooling lava flows or growing biofilms.
The steady shape of a drop of dilute polymer solution falling through a quiescent viscous Newtonian fluid is considered. Experimentally, we find that an immiscible drop of 0.16% xanthan gum in 80:20 glycerol/water falling through 9.8 P polydimethylsiloxane oil may exhibit a stable dimple at its trailing edge. At higher volumes the dimple extends far into the interior of the drop, and pinches off via a Rayleigh-type instability, injecting oil droplets into the polymer drop. At even larger volumes, a toroidal shape develops. We show that the dimpled shape can be reproduced mathematically with axisymmetric solutions for Stokes flow past a non-Newtonian drop, using the constitutive equation for a Simple Fluid of Order Three.
We present experimental results on the dynamics of wormlike micellar filaments surrounded by an immiscible viscous bulk fluid. For certain concentrations, these filaments develop a beads-on-string structure previously observed only in polymer jets and filaments surrounded by air. By taking advantage of the longer time scales present in this experiment, we are able to quantify the evolution of individual beads. We also investigate the stability of these filaments and the robustness of the beads-on-string structure by stretching the filament within a rotating flow.
A solid foundation of chemistry principles is only gained through a true comprehension of the material as opposed to pure memorization. One of the most fundamental concepts in chemistry is that of determining the amount of product formed in a chemical reaction when one of the reactants is limiting. To increase students' comprehension of this important concept, a conceptual approach is presented that is tangible for limiting-reagent problems on the molecular level and extends directly to typical examples that would be encountered in a classroom or laboratory setting. To accommodate visual learners, a graphical methodology is incorporated that determines the limiting reagent as the reactant that is exhausted first while simultaneously finding the amounts of the excess reagents and products.
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