Surface tension and contact angle
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Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the imbalance in cohesive forces of the surface layer. In the absence of other forces, including gravitydrops of virtually all liquids would be approximately spherical.
The spherical shape minimizes the necessary "wall tension" of the surface layer according to Laplace's law. Water droplet lying on a damask.
Surface tension - Wikipedia
Surface tension is high enough to prevent floating below the textile Another way to view surface tension is in terms of energy. A molecule in contact with a neighbor is in a lower state of energy than if it were alone not in contact with a neighbor.Contact Angle and the Young Equation
The interior molecules have as many neighbors as they can possibly have, but the boundary molecules are missing neighbors compared to interior molecules and therefore have a higher energy. For the liquid to minimize its energy state, the number of higher energy boundary molecules must be minimized.
The minimized number of boundary molecules results in a minimal surface area. Since any curvature in the surface shape results in greater area, a higher energy will also result. Consequently, the surface will push back against any curvature in much the same way as a ball pushed uphill will push back to minimize its gravitational potential energy.
Effects of surface tension[ edit ] Water[ edit ] Several effects of surface tension can be seen with ordinary water: Beading of rain water on a waxy surface, such as a leaf. Water adheres weakly to wax and strongly to itself, so water clusters into drops. Surface tension gives them their near-spherical shape, because a sphere has the smallest possible surface area to volume ratio.
Formation of drops occurs when a mass of liquid is stretched.
The animation below shows water adhering to the faucet gaining mass until it is stretched to a point where the surface tension can no longer keep the drop linked to the faucet.
It then separates and surface tension forms the drop into a sphere. If a stream of water was running from the faucet, the stream would break up into drops during its fall.
Gravity stretches the stream, then surface tension pinches it into spheres. The nonwettability of the water strider's leg means there is no attraction between molecules of the leg and molecules of the water, so when the leg pushes down on the water, the surface tension of the water only tries to recover its flatness from its deformation due to the leg.
This behavior of the water pushes the water strider upward so it can stand on the surface of the water as long as its mass is small enough that the water can support it. Dynamic sessile drop method Main article: Sessile drop technique The sessile drop contact angle is measured by a contact angle goniometer using an optical subsystem to capture the profile of a pure liquid on a solid substrate.
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The angle formed between the liquid—solid interface and the liquid—vapor interface is the contact angle. Older systems used a microscope optical system with a back light. Current-generation systems employ high resolution cameras and software to capture and analyze the contact angle. Angles measured in such a way are often quite close to advancing contact angles. Equilibrium contact angles can be obtained through the application of well defined vibrations.
This complexity is further amplified when one attempts to incline the surface. Experimental apparatus to measure pendant drop contact angles on inclined substrates has been developed recently. An automated system allows for tilting the substrate and analysing the images for the calculation of advancing and receding contact angles. The dynamic sessile drop method[ edit ] Main article: Sessile drop technique The dynamic sessile drop is similar to the static sessile drop but requires the drop to be modified.
A common type of dynamic sessile drop study determines the largest contact angle possible without increasing its solid—liquid interfacial area by adding volume dynamically. This maximum angle is the advancing angle.