(Top left) Kelvin-Helmholtz instability that develops when two fluids of different densities slip past each other. (Top right) Plateau-Rayleigh instability that causes a fluid stream to break up into drops. (Bottom) Laboratory experiments mimicking a vertical counterflow in which oil is flowing from below and a mock fluid is injected at the top. (Left) Oil (light) and another Newtonian solution (green) mix; the green fluid is not able to descend. (Right) Oil and a shear-thickening, viscoelastic solution (e.g., cornstarch); the Newtonian fluid and the cornstarch do not mix, and the green fluid is able to descend.