The Coandă Effect

Pour a thin stream of water near the back of a spoon held at an angle, and instead of falling straight down, the water will curve and cling to the spoon's surface for a moment before finally breaking away. This behavior, in which a moving stream of fluid tends to follow a nearby curved surface rather than continuing in a straight line, is known as the Coandă effect. The phenomenon takes its name from Romanian inventor Henri Coandă, who patented a device exploiting the effect in the 1930s, though the underlying physics was described earlier by British scientist Thomas Young. The mechanism depends on pressure differences created by the moving fluid itself. As the jet passes close to a curved surface, it pulls along, or entrains, some of the surrounding fluid, lowering the pressure in that narrow region compared to the opposite side of the stream. This pressure imbalance pushes the jet toward the surface, causing it to bend and follow the curve. Engineers have since put the effect to practical use. Aircraft designers have experimented with Coandă-effect wing flaps to increase lift, ventilation systems use it to distribute airflow evenly through ducts, and some hand dryers and small drones rely on it to direct airflow more efficiently than a straight jet could manage.