This video shows instability in Taylor-Couette flow. In this video, the viscous fluid is between two concentric cylinders (an internal rotating cylinder and an external stationary cylinder). The fluid grabs onto the walls of the two cylinders and gets pulled as the inner cylinder rotates. The instability occurs when the inner cylinder rotates too quickly, causing the streaking pattern. I chose this as my Best of Web because the flow is very beautiful, and the science behind why this type of flow occurs is really interesting and makes the visual even cooler. Taylor-Couette flow is also an ultra-laminar flow and time-reversible. This means that if you are rotating slowly, you can reverse the rotation and restore the liquid to its original state before mixing.  

Credit: Chirag Kalelkar and Prasun Dey