Why Does Water Roll Off the Surface of a Leaf?

August 19, 2023
David Sunnyside

Water is a vital substance that performs many important functions. Its unique molecular structure allows it to bind very tightly with other molecules and has the ability to flow over long distances, like from the roots to the leaves of a tree. Despite these amazing properties, water can also be quite slippery and difficult to hold onto. For example, if you were to stick your hand into a glass of water, the water would slowly erode your hand, and it is for this reason that plants use a process called guttation to move water up the roots and stems of a plant.

The surface of the lotus leaf has a clever design that discourages surface wetting. This is achieved through both a waxy nonpolar coating and a rough surface. The nonpolar coating, referred to as epicuticular wax, has the non-polar methyl groups that don’t mix with polar water molecules. The rough surface creates microscale bumps that help break up the water’s surface tension. This causes the water to bead up into spheres and roll off the surface before it can wet the leaf. This is a remarkable adaptation known as superhydrophobicity.

In this investigation, you will examine surface textures to see how easily a drop of water rolls off different surfaces. You will also conduct a simple tilt test to find out how a leaf’s surface texture affects its water repellency. You will learn that the two factors that determine how easily a drop moves across a surface are its hydrophobicity and its inclination. This is intuitively captured by the drop retention angle, which is the angle at which a drop begins to roll off a surface when it is gradually tilted (Fig. 1).

David Sunnyside
Co-founder of Urban Splatter • Digital Marketer • Engineer • Meditator
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