FYFD

Tag: dissolution

  • Dissolving

    Dissolving

    It looks like the fiery edge of a star’s corona, but this photo actually shows a dissolving droplet. The droplet, shown as the lower dark region in this shadowgraph image, is a mixture of pentanol and decanol sitting in a bath of water. Pentanol is a type of alcohol that is fully miscible with decanol and is water soluble, so that it will dissolve into the surrounding water over time. Decanol, on the other hand, is immiscible with water, so that part of the droplet won’t mix with the surrounding water.

    The bright swirls along the droplet’s edge show areas with more pentanol. As the alcohol dissolves into the water, it forms a buoyant plume at the top of the droplet that rises due to pentanol’s lower density. That rising plume draws fresh water in from the sides, shown by the upper white arrows. Inside the droplet, flow moves in the opposite direction, from the top toward the outer edges. This is a result of uneven surface tension within the droplet. Scientists are interested in understanding the dynamics of these multiple component drops for applications like printing, where it’s desirable for pigments in an ink drop to be distributed evenly as the drop dries.  (Image credit: E. Dietrich et al.)

  • How Many Licks Does It Take to Get to the Center of a Lollipop?

    How Many Licks Does It Take to Get to the Center of a Lollipop?

    Many a child has wondered how many licks it takes to get to the center of a lollipop. Physically, this is a problem of a solid body dissolving in a flow, and it’s one scientists are interested in for its geological, industrial, and pharmacological applications.

    The animation above shows flow around a dissolving (candy!) body that was originally spherical. With both spheres and cylinders, the final shape the body takes is consistent – it has a front boundary with a curvature of nearly constant radius and a back face that is approximately flat. This creates a boundary layer of uniform thickness across the front face, and that uniform flow makes the surface dissolve steadily and evenly so that it maintains the same overall shape.

    With their model and experiments, researchers have even been able to tackle the classic question of how many licks it takes:

    “For candy of size 1 cm licked at a speed of 1 cm/s, we estimate a total of 1000 licks, a prediction that is notoriously difficult to test experimentally.”

    (Image credit: J. Huang et al., source, pdf)

  • Pinning a Drop

    Pinning a Drop

    The shape of a droplet sitting on a surface depends, in part, on its surface tension properties but also on the nanoscale roughness of the surface. Small variations in the height and shape of the surface will change the area a drop contacts as well as the contact angle the edge of the drop makes with the surface. If the contact line between the drop and surface stays the same as a droplet evaporates into the surrounding gas or dissolves into the surrounding liquid, then we say the drop is pinned. A pinned drop’s contact angle will decrease as the drop’s volume decreases. This strains the ability of the nanoscale roughness to keep the drop’s edge pinned. As individual points of contact fail, the drop’s edge may jump inward to a new contact point. This set of discrete jumps between pinned states is called a stick-jump or stick-slip mode. (Image credit: E. Dietrich et al., source; see also: E. Dietrich et al. 2015)

  • Icebergs and Caramel

    Icebergs and Caramel

    What do icebergs and caramel have in common? Both have similar scalloped erosion patterns as they dissolve. When caramel dissolves in water, the denser caramel sinks in the buoyant water. An initially smooth surface will first form lines, then the flowing caramel and the uneven surface interact, forming chevrons, followed by larger scallops. A similar process happens with melting icebergs. The meltwater from an iceberg is less dense than the surrounding seawater, so it will rise as it melts. This causes variations in the salt concentration and temperature near the iceberg, which cause it to melt differently in different spots, ultimately leading to the same scallop shapes observed in the caramel. Check out the full-size PDF of the poster here. (Image credit: C. Cohen et al.)