0
Surfaces on which water beads are called hydrophobic surfaces. That is, the water "fears" the surface. Water tends to bead up on these types of surfaces because it is more strongly attracted to itself (other water molecules) than the surface molecules.
Common hydrophobic surfaces include waxes, Teflon, and most plastics.
Water can bead up on surfaces with hydrophobic properties, such as waxed cars, non-stick pans, and some types of fabrics treated with water-repellent coatings. It can also bead up on surfaces with rough textures that create air pockets, such as certain types of leaves and some types of plastics.
Water beads best on non-polar surfaces, such as waxy or oily surfaces. This is because the water molecules are not attracted to the non-polar surface, but they are attracted to each other. Therefore, the molecules will pull together into an efficient shape, with little contact with the surface and more contact with other water molecules. A sphere, or bead, is a shape that meets these qualifications. The attraction of water molecules by water molecules is known as cohesion.
Short answer:
Water beads form on a surface when the cohesion of the water molecules with each other is much greater than the adhesion of the water to the surface.
This phenomena, or the lack there of, is called wetting and has been studied for centuries. The cohesion of a liquid is directly related to the surface tension of a liquid. Water, which has a high surface tension, readily forms beads on materials which are hydrophobic, i.e. materials with a small adhesion to water. The waxed surface of a car has small adhesion and so water beads form regularly. The raw painted surface of the car would have more adhesion with water and so greater wettability. The opposite extreme, the tendency of a liquid to wet a surface due to strong adhesion, is critical to a situation such as lubrication where the liquid should coat the solid surface. It is the two properties combined, adhesion and cohesion, which determine wettabilty and beading.
Long Answer:
When one speaks of water beading on a surface, one means the formation of small droplets of water on a surface. This phenomena depends not only on the properties of water and the surface, but most importantly on the property of the interaction of the water with the surface. In science, this is the topic of "wetting" and one describes the tendency in terms of the "wettability" of a particular combination of liquid and surface. Beading is a manifestation of low wettability or little wetting.
A liquid droplet on a surface tends to form a more nearly spherical shape if the forces of attraction (or cohesion) between the liquid molecules is strong and the forces between the liquid and and the solid (adhesion) are weak. We associate strong cohesive forces of a liquid with a high surface tension and a high surface tension opposes wetting. Water is an example of a liquid with high surface tension.
The interaction of water with a surface like wax or plastic or Teflon is an example weak adhesion or weak forces between the liquid molecules and solid surface. Thus, wetting is highly unfavorable and formation of nearly spherical droplets is favored and we say water beads. Surfaces like metal or glass have somewhat stronger adhesive forces so wetting is greater and droplets are less spherical.
Contact Angle:
Wetting is not an on or off phenomena but varies in degree. The characteristic that provides a quantitative measure of wetting is called the contact angle. When a liquid like water is placed on a surface like teflon, the water forms a flattened droplet. The edge of the droplet makes an angle with the surface at the point of contact. We call that the contact angle, i.e. the angle between the solid surface and a tangent drawn to the surface of the droplet at the point of contact. This angle is measured on the side of the tangent that touches the water and so measures how much one would have to raise a horizontal line to match the liquid surface as one approaches the droplet from outside. This angle can be between zero and 180 degrees. If the droplet is very flat, the contact angle is small. If the droplet remained nearly spherical, then the contact angle would be zero near 180 degrees. It is important to note that this angle does not depend on the size of the droplet and even though gravity flattens a large droplet in the middle, when one approaches the edge, the contact angle is the same whether it is a small or large droplet.
When wetting is very poor, the contact angle large. This is the case for water droplets on wax, plastic, teflon and other surfaces with low wettability and hence a greater tendency toward beading. Water will wet glass more easily as well as a metal surface.
The contact angle for water on teflon is about 107 to 110 degrees but water on is only glass 53 degrees. This is different for metals too, where for instance, water on gold is 65 degrees and, depending on the plastic a value around plastic 62 degrees may be expected. Obviously all this depends on factors like temperature and surface roughness. These variables technologically important have been studied extensively and tens of thousands of examples are known.
Applications:
Obviously, one wants one's raincoat to bead water as well as the wax on one's car. But the opposite is true for a lubricant. One wants oil to adhere to a surface of a bearing and have a small contact angle, ideally a zero contact angle. Coatings are designed with wettabilty as a primary consideration. Cloth may be treated with material to prevent wetting, but that is also an example where the surface texture or roughness plays a critical role. Paint is an example of a coating that would be carefully evaluated for wettability depending on the application.
Much of Biology occurs at the interface of a watery compartment. Without the surface properties of water as they are, life would not exist.
Finally, without high surface tension and low adhesion, all those little bugs that walk on water would have no place to go.
Short answer:
Water beads form on a surface when the cohesion of the water molecules with each other is much greater than the adhesion of the water to the surface.
This phenomena, or the lack there of, is called wetting and has been studied for centuries. The cohesion of a liquid is directly related to the surface tension of a liquid. Water, which has a high surface tension, readily forms beads on materials which are hydrophobic, i.e. materials with a small adhesion to water. The waxed surface of a car has small adhesion and so water beads form regularly. The raw painted surface of the car would have more adhesion with water and so greater wettability. The opposite extreme, the tendency of a liquid to wet a surface due to strong adhesion, is critical to a situation such as lubrication where the liquid should coat the solid surface. It is the two properties combined, adhesion and cohesion, which determine wettabilty and beading.
Long Answer:
When one speaks of water beading on a surface, one means the formation of small droplets of water on a surface. This phenomena depends not only on the properties of water and the surface, but most importantly on the property of the interaction of the water with the surface. In science, this is the topic of "wetting" and one describes the tendency in terms of the "wettability" of a particular combination of liquid and surface. Beading is a manifestation of low wettability or little wetting.
A liquid droplet on a surface tends to form a more nearly spherical shape if the forces of attraction (or cohesion) between the liquid molecules is strong and the forces between the liquid and and the solid (adhesion) are weak. We associate strong cohesive forces of a liquid with a high surface tension and a high surface tension opposes wetting. Water is an example of a liquid with high surface tension.
The interaction of water with a surface like wax or plastic or Teflon is an example weak adhesion or weak forces between the liquid molecules and solid surface. Thus, wetting is highly unfavorable and formation of nearly spherical droplets is favored and we say water beads. Surfaces like metal or glass have somewhat stronger adhesive forces wetting is greater and droplets are less spherical.
Contact Angle:
Wetting is not an on or off phenomena but varies in degree. The characteristic that provides a quantitative measure of wetting is called the contact angle. When a liquid like water is placed on a surface like teflon, the water forms a flattened droplet. The edge of the droplet makes an angle with the surface at the point of contact. We call that the contact angle, i.e. the angle between the solid surface and a tangent drawn to the surface of the droplet at the point of contact. This angle is measured on the side of the tangent that touches the water and so measures how much one would have to raise a horizontal line to match the liquid surface as one approaches the droplet from outside. This angle can be between zero and 180 degrees. If the droplet is very flat, the contact angle is small. If the droplet remained nearly spherical, then the contact angle would be zero near 180 degrees. It is important to note that this angle does not depend on the size of the droplet and even though gravity flattens a large droplet in the middle, when one approaches the edge, the contact angle is the same whether it is a small or large droplet.
When wetting is very poor, the contact angle large. This is the case for water droplets on wax, plastic, Teflon and other surfaces with low wettability and hence a greater tendency toward beading. Water will wet glass wets more easily as well as a metal surface.
The contact angle for water on Teflon is about 107 to 110 degrees but water on is only glass 53 degrees. This is different for metals too, where for instance, water on gold is 65 degrees and, depending on the plastic a value around plastic 62 degrees may be expected. Obviously all this depends on factors like temperature and surface roughness. These variables technologically important have been studied extensively and tens of thousands of examples are known.
Applications:
Obviously, one wants one's raincoat to bead water as well as the wax on one's car. But the opposite is true for a lubricant. One wants oil to adhere to a surface of a bearing and have a small contact angle, ideally a zero contact angle. Coatings are designed with wettabilty as a primary consideration. Cloth may be treated with material to prevent wetting, but that is also an example where the surface texture or roughness plays a critical role. Paint is an example of a coating that would be carefully evaluated for wettability depending on the application.
Much of biology occurs at the interface of a watery compartment. Without the surface properties of water as they are, life would not exist.
Finally, without high surface tension and low adhesion, all those little bugs that walk on water would have no place to go.
Add your answer:
Earn +20 pts
Why does water form spherical drops on some surfaces?
Water forms spherical drops on hydrophobic surfaces because the molecules have a tendency to minimize contact with the surface due to surface tension. This shape reduces the surface area in contact with the surface, allowing the water droplet to bead up into a more stable and energetically favorable shape.
Why does wet paint bead up on some shiny smooth surfaces and spread evenly over other surfaces?
Wet paint beads up on some shiny, smooth surfaces due to surface tension, where the paint molecules are more attracted to each other than to the surface. On other surfaces, the paint spreads evenly due to good wetting properties, where the paint molecules form strong bonds with the surface, allowing it to flow and cover the surface uniformly.
What is Ability to repel water?
The ability to repel water is known as hydrophobicity. It is a property of certain materials that causes them to repel water molecules, causing water to bead up and roll off the surface rather than soaking in. This property is often utilized in coatings, fabrics, and other products to make them water-resistant.
Water can climb up some surfaces by what?
Water can climb up some surfaces due to capillary action, where the water molecules are attracted to the surface and can overcome gravity to move against it. This phenomenon is caused by the adhesive forces between the water molecules and the surface material, allowing water to move upwards in narrow spaces like tubes or fibers.
Why does water bead up on a smooth surface?
Water beads up on a smooth surface due to surface tension, which is caused by the cohesive forces between water molecules. This surface tension minimizes the contact area between water and the surface, causing the water to form beads to reduce the energy required to maintain the surface tension.
Is copper surface hydrophobic?
No, copper surfaces are typically hydrophilic, meaning they have an affinity for water and will readily allow water to spread across the surface rather than bead up.
Why does water form spherical drops on some surfaces?
Water forms spherical drops on hydrophobic surfaces because the molecules have a tendency to minimize contact with the surface due to surface tension. This shape reduces the surface area in contact with the surface, allowing the water droplet to bead up into a more stable and energetically favorable shape.
Why does wet paint bead up on some shiny smooth surfaces and spread evenly over other surfaces?
Wet paint beads up on some shiny, smooth surfaces due to surface tension, where the paint molecules are more attracted to each other than to the surface. On other surfaces, the paint spreads evenly due to good wetting properties, where the paint molecules form strong bonds with the surface, allowing it to flow and cover the surface uniformly.
Why does water bead up on a waxed car but gasoline does not?
Water has a higher surface tension than gasoline, causing it to bead up on a waxed surface. The hydrophobic properties of the wax repel the water, leading to beading. Gasoline, on the other hand, has a lower surface tension than water and does not experience the same beading effect on a waxed surface.
Which statement best describes a difference between land and water surfaces?
Land surfaces heat up and cool down faster than water surfaces.
What are some things that can repel in water?
Some things that can repel in water include hydrophobic substances like oils, waxes, and certain plastics. These substances have a low surface energy that causes water to bead up and roll off instead of being absorbed. Additionally, certain coatings and treatments can be applied to surfaces to make them water-repellent.
What property of water makes it bead up on waxy surface?
The form of water you are seeing is water vapor turning to mist and then collecting on leaves.
Does water bead up on wax paper because its cohesive forces are greater than its adhesive forces?
Yes.
Why are land surfaces heated more quickly?
Land surfaces heat up more quickly than water surfaces because soil has a lower heat capacity than water, meaning it requires less energy to raise its temperature. Additionally, land surfaces have a lower albedo, meaning they absorb more sunlight and convert it into heat. This combination of lower heat capacity and higher absorption of sunlight leads to land surfaces heating up more rapidly than water surfaces.
What is Ability to repel water?
The ability to repel water is known as hydrophobicity. It is a property of certain materials that causes them to repel water molecules, causing water to bead up and roll off the surface rather than soaking in. This property is often utilized in coatings, fabrics, and other products to make them water-resistant.
Why does water bead up on freshly waxed car?
Wax is hydrophobic, meaning it repels water. Of course, this repulsion is not strong enough to make the water hover a millimeter above the surface, but it is strong enough to force the water to act in this way, rather than sit there in a thin sheet. The "surface tension" of the water itself also plays a role.
What is a solid that have six flat surfaces?
A cube has six flat surfaces, but so do other solids. Look up 'hexahedron' in wikipedia.
