Theo Nash
Surface tension is the tendency of a liquid's surface to minimize its surface area when at rest. This phenomenon is caused by the greater attraction of liquid molecules to each other (cohesion) compared to the attraction to air molecules (adhesion). Water, due to its polar nature and hydrogen bonds, has a higher surface tension than most other liquids. However, when comparing water and alcohol, the results may vary. While water has a higher surface tension than ethanol, it is not clear if this relationship extends to other types of alcohol. The shape and polarity of alcohol molecules allow them to interact with water in ways that can alter its surface tension, as seen in the tears of wine phenomenon.
| Characteristics | Values |
|---|---|
| Surface tension | The tendency of liquid surfaces at rest to shrink into the minimum surface area possible |
| Reasons for surface tension | The greater attraction of liquid molecules to each other (due to cohesion) than to the molecules in the air (due to adhesion) |
| Mechanisms | An inward force on the surface molecules causing the liquid to contract; a tangential force parallel to the surface of the liquid |
| Surface tension and water | Water has a higher surface tension (72.8 mN per meter at 20 °C) than most other liquids due to the relatively high attraction of water molecules to each other through a web of hydrogen bonds |
| Surface tension and alcohol | Alcohol has a lower surface tension than water due to its different size and shape, with its polar part on one end. Alcohol molecules can meet at areas where they do not attract as strongly |
| Experiment | Place two pennies on a flat surface and add drops of water to one penny and alcohol to the other penny. Water beads up on the penny and the alcohol spreads out flat |
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What You'll Learn
Water has a higher surface tension than alcohol
The surface tension of water is influenced by the polarity of water molecules, which are very attracted to each other due to their polar nature. This polarity is also responsible for the strong surface tension of water. In contrast, alcohol has a different size and shape, with its polar part at one end. This results in areas where alcohol molecules do not attract each other as strongly, leading to a lower surface tension compared to water.
The difference in surface tension between water and alcohol can be observed through a simple experiment. By placing a drop of water and a drop of alcohol on separate pennies, it can be seen that water beads up on the penny while alcohol spreads out flat. This is because water is more attracted to itself than to the metal of the penny, whereas alcohol is less attracted to itself and thus spreads more. This experiment demonstrates the higher surface tension of water compared to alcohol.
Furthermore, the "tears of wine" phenomenon also illustrates the difference in surface tension between water and alcohol. When wine, which contains alcohol, is poured into a glass, drops and rivulets form on the side of the glass due to the interaction between the differing surface tensions of water and ethanol. This occurs as ethanol modifies the surface tension of water, and its faster evaporation rate compared to water also contributes to the effect. Thus, the "tears of wine" provide a visual representation of the higher surface tension of water relative to alcohol.
Overall, the higher surface tension of water compared to alcohol is a result of the strong attraction between water molecules due to their polarity, while alcohol molecules have a different structure that leads to weaker attraction in certain areas. This difference in surface tension can be observed through experiments and natural phenomena, such as the "tears of wine," highlighting the distinct behaviours of these two liquids.
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The polarity of water molecules
Water is a polar molecule, which means it has an uneven distribution of electrical charge across its structure. This polarity is due to the sharing of electrons between the oxygen and hydrogen atoms that make up water (H2O). Oxygen has a higher electronegativity than hydrogen, meaning it attracts electrons more strongly. This results in an uneven distribution of electrons, with oxygen holding a partial negative charge and hydrogen a partial positive charge. This polarity gives rise to water's unique properties, such as its high surface tension, heat capacity, and ability to act as a solvent.
Water's polarity leads to the formation of hydrogen bonds between water molecules. The partially positive hydrogen of one water molecule is attracted to the partially negative oxygen of another, creating a strong intermolecular force known as a hydrogen bond. This force is weaker than a covalent bond but stronger than other intermolecular forces, allowing water molecules to stick together or exhibit cohesion. The cohesive force is so strong that it can support the weight of small objects, such as a paper clip, on its surface through surface tension.
The polarity of water also enables it to act as a solvent for polar and ionic substances. Polar molecules, such as salts, amino acids, and sugars, can easily dissolve in water. These molecules are attracted to the polar water molecules and become evenly dispersed. This property of water is vital in chemical reactions within living organisms, as it facilitates reactions by dissolving reactants and even participating as a reactant or product.
Additionally, water's polarity contributes to its high heat capacity. The hydrogen bonds between water molecules allow it to absorb large amounts of heat energy without a significant increase in temperature. This property helps regulate the temperatures of living organisms and the environment.
In summary, the polarity of water molecules, resulting from the uneven distribution of electrons between oxygen and hydrogen atoms, gives rise to water's unique properties. These include its high surface tension, heat capacity, solvent abilities, and cohesive behaviour. Water's polarity leads to the formation of hydrogen bonds, which are responsible for many of its essential characteristics.
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Surface tension and the shape of liquid droplets
Surface tension is the tendency of liquid surfaces at rest to minimise their surface area. This is caused by the inward force on the surface molecules causing the liquid to contract, and the tangential force parallel to the surface of the liquid, which is generally referred to as surface tension. Liquids with strong cohesive bonds and weaker adhesive forces will tend to bead up or form droplets when in contact with another material.
The shape of liquid droplets is determined by surface tension. They tend to be pulled into a spherical shape by the cohesive forces of the surface layer. This is because a sphere has the smallest possible surface area to volume ratio. The Plateau-Rayleigh instability is an example of this phenomenon, where a stream of water breaks up into droplets due to the effects of surface tension.
The surface tension of water is more than twice that of ethanol. This can be demonstrated by placing drops of water and alcohol on a penny. The water beads up on the penny, while the alcohol spreads out flat. This is because water molecules are very attracted to each other, and the water is more attracted to itself than to the metal of the penny.
Surface tension can be measured in several ways. One method involves measuring the resonant frequency of spherical and hemispherical pendant droplets driven in oscillations by a modulated electric field. Another method involves levitating a droplet and allowing it to fall onto a platform, where it will bounce and oscillate in mid-air as it tries to minimise its surface area.
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Surface tension and capillary waves
Water molecules are polar, meaning they are very attracted to each other. This polarity helps explain why water has a strong surface tension. Surface tension is derived from cohesive forces of molecules pulled equally in every direction by neighbouring liquid molecules so that the net force is zero (equilibrium).
A demonstration of this can be observed by placing a paper clip on the surface of water. The paper clip, which is denser than water, normally sinks. However, due to the surface tension of the water, the paper clip can float.
Another demonstration involves placing a drop of water on the surface of a penny. Water beads up on the penny, whereas alcohol spreads out flat. This is because water is more attracted to itself than to the metal of the penny, whereas alcohol is less attracted to itself and thus spreads more.
Capillary waves are small, free, surface-water waves with short wavelengths. Their restoring force is the water's surface tension, which causes the wave to have a rounded crest and a V-shaped trough. The maximum wavelength of a capillary wave is 1.73 centimetres; longer waves are controlled by gravity and are called gravity waves. Capillary waves are fully dominated by the effects of surface tension, whereas gravity-capillary waves are also affected by gravity.
The velocity of capillary waves increases as the wavelength decreases, with a minimum velocity of 23.1 centimetres per second at a wavelength of 1.73 cm. Capillary waves can be used to measure the surface tension of fluids, such as water and blood, and may have potential biomedical applications.
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The tears of wine phenomenon
The phenomenon known as "tears of wine" has been observed by wine drinkers for centuries. It is manifested as a ring of clear liquid near the top of a glass of wine, from which droplets continuously form and fall back into the wine. This phenomenon is also referred to as wine legs, fingers, curtains, church windows, or feet. It is most noticeable in wines with a high alcohol content and can be eliminated by covering the glass, which prevents the evaporation of alcohol.
The "tears of wine" phenomenon is the result of a difference in surface tension between alcohol and water. Wine is a mixture of alcohol and water, and when it is swirled in a glass, a thin film of this mixture creeps up the sides of the glass. However, the alcohol in the wine evaporates faster than the water due to its higher vapour pressure. This results in a decrease in the concentration of alcohol in the thin film, leading to an increase in surface tension. The surrounding wine, which has a higher alcohol content and lower surface tension, is then pulled upwards to replace the evaporated liquid. This flow of liquid is known as the Marangoni effect or the Gibbs-Marangoni effect.
The phenomenon was first correctly explained by physicist James Thomson, the elder brother of Lord Kelvin, in 1855. However, it was British physicist C.V. Boys who suggested that the biblical reference in Proverbs 23:31, "Look not thou upon the wine when it is red, when it giveth his colour in the cup, when it moveth itself aright", may be referring to this effect. This suggests that observing the "tears of wine" was a way to identify wines with high alcohol content and thus, wines that should be avoided for sobriety.
In recent years, researchers at UCLA, led by Andrea Bertozzi, have contributed new insights into the phenomenon. They identified shock waves that help explain why thin drops of liquid creep up the side of wine glasses. Bertozzi and her team developed a theory based on differential equations to describe the tiny shock waves and the role of thermal effects induced by evaporative cooling in the Marangoni flow. Their work highlights the delicate interplay of interfacial and bulk forces that result in the "tears of wine" phenomenon.
Thus, the "tears of wine" phenomenon is a fascinating example of fluid dynamics and the behaviour of alcohol-water mixtures. It has captured the curiosity of wine drinkers, scientists, and mathematicians alike, leading to a deeper understanding of the complex behaviour of liquids at the molecular level.
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Frequently asked questions
No, water has a higher surface tension than alcohol. Water molecules are very attracted to each other, and this polarity is why water has a strong surface tension.
You can compare the surface tension of water and alcohol by placing two pennies on a flat surface and slowly adding drops of water and alcohol to each penny. You will notice that the water beads up on the penny, and the alcohol spreads out flat.
Water molecules are polar and are very attracted to each other. Alcohol has a different size and shape, and its polar part is on one end. Alcohol molecules can meet at areas where they do not attract as strongly.
