• Question: why does water and other liquids stick to solids? is it because of the meniscis

    Asked by Lucy M to Ahmed, Francesca, George, James, Nitheen on 17 Nov 2014.
    • Photo: Francesca Paradisi

      Francesca Paradisi answered on 17 Nov 2014:

      Hi Lucy, do you mean the liquids forming droplets on the surface of solids or being absorbed by them? Some solids are porous and water and other liquids can get into the pores of a rock for example, wetting it. The wetness will be retained till all of the water is dried out. Or if you are talking about non porous materials like plastics or glass and the fact that water sticks to it forming a droplet, that is due to a property called adhesion, which describes how there are forces that allow for interaction between water molecules and and a solid surface. It is all at molecular level. The droplet then has a roundish shape due to the fact that the other molecules not in direct contact with the surface are sticking to one another and this is called cohesion.

    • Photo: James Sullivan

      James Sullivan answered on 17 Nov 2014:

      Hi Lucy – great question –

      Water molecules are attracted to one another because the way their electrons are arranged makes them have polarity (one side of the molecule is positive and the other negative).
      Positive things attract negative things and that makes them stick together as drops (and this causes the meniscus to form also).

      Water sticks to other things because of similar forces (similar but not as strong).

    • Photo: George Dowson

      George Dowson answered on 17 Nov 2014:

      The meniscus is part of it!
      When a liquid contacts a surface or is dropped on it, it generates something called a contact angle depending on how favourable the interaction between the water and surface is. If the contact angle is 90 degrees the water droplet will sit on the surface like a hemisphere (half ball). If it is lower than 90 degrees it’ll spread out wider “wetting” the surface. If it’s higher than 90 the surface is hydrophobic so the water will avoid the surface as much as possible. At the extreme, 180 degrees, the water droplet will sit on the surface like a ball. These surfaces are called superhydrophobic and examples including lotus flowers and some special coatings that make water run off without wetting.

    • Photo: Ahmed Osman

      Ahmed Osman answered on 18 Nov 2014:

      Hi Lucy M
      you come again with brilliant question
      Water molecules attract other water molecules. In fact, every molecule of all ‘normal’ liquids attract each other. In general, every neutral atom or molecule attracts one another — more or less, when they are a little distance apart, and tend to repel each other when you push them too close together. The forces in action for this to happen are certainly electrical in nature — they are generally known as Van der Waals’ forces, and they are mainly due to forces between two dipoles (i.e. uneven charge distributions whose total charge is zero). However, a side note is that the molecules need not be polar on their own, they will induce a dipole moment when they are brought together, and thus attract each other.

      Water, if left alone in a zero-gravity space (or equivalently, when in free fall), will tend to form itself into a sphere. Since every molecule pulls on one another, and the molecules on the surface have no molecule to pull on them from the outside, this causes what is known as ‘surface tension’. This is the reason why one can overfill a glass with water, past the top, by a couple millimeters. Now, since there is tension in the surface, water tries to minimize the surface (it is like a balloon with extremely flexible surface), and since a sphere has the least surface area for a given volume, it forms a sphere.
      thanks
      Ahmed

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