The Forces behind Capillary Action

The seemingly unintuitive phenomena of capillary action, which enables liquids to rise up narrow tubes, even against external forces such as gravity, underpins the fundamental way in which nature and mechanisms in our everyday life operate.

Without it, the diverse and abundant plant-life that populates Earth could not thrive, our tear ducts would not be able to cleanse the eye and the body’s cells would not be capable of rehydrating. To understand how it works, we must first understand what is meant by surface tension.

What causes surface tension?

Water (H2O) molecules are polarised; the oxygen (O) molecule is highly electronegative, so electrons in the covalent bond between hydrogen (H) and O atoms are pulled closer to the O atom. This leaves a slightly negative charge on the O atom and positive charge on the H atom, polarising charge across the molecule. This gives rise to hydrogen bonding, an incredibly strong intermolecular force that causes the H20 molecules to form tight bonds with each other.

Intermolecular bonding between the positively charged hydrogen (H) atom of one water molecule and the negatively charged oxygen (O) atom of another water molecule.

Whilst molecules at the centre are equally attracted to other water molecules surrounding them, those at the surface can only bond with water molecules below them creating a net cohesive force downwards. This increased attraction causes surface molecules to be under tension, more resistant to breaking and to exhibit skin-like behaviour that is known as surface tension.

Water molecules in the centre of the liquid can form bonds with water molecules all around them, but water molecules can only form bonds with water molecules below them, so they experience a net force downwards.

Surface tension in Everyday Life

This surface tension is responsible for a vast range of phenomena: it allows water striders to walk on water, paperclips to float and the spherical shape of small liquid droplets. When surface tension is the dominant force acting on a liquid droplet, it will be pulled into a round shape as a result of the tendency to minimise surface tension by minimising surface area (Laplace’s Law).

Surface tension is also responsible for the fog that blinds all glasses-wearers when they enter a warm building on a cold winter’s day. When molecules of water vapour in the warm air hit the cold surface of glasses lenses, there is a net heat transfer of energy from the more energetic molecules inside the room to the cold glasses lenses, as the system tends toward thermodynamic equilibrium. Subsequently, the molecules have insufficient energy to exist in the gaseous state, condensing into liquid form since molecules in this state have less internal energy. The aforementioned polarity of liquidous H2O molecules causes them to be more attracted to each other than the lenses. As a result, they form tiny droplets on the surface of the glasses lenses that reduce the light transmission, fogging up the glasses.

Capillary Action

Adhesive forces refer to intermolecular attractive forces between dissimilar molecules; the relative strength of cohesive and adhesive forces is an important, determining factor in the macroscopic behaviour of liquids. Surface tension and the interplay of these two types of force are, together, responsible for capillary action, the tendency of a fluid to rise in a narrow tube, even in opposition to external forces such as gravity.

If the strength of the cohesive forces is greater than that of adhesive forces, the angle between the tangent to the liquid surface and the surface (contact angle) is larger, thus the liquid is more likely to form a droplet.

The contact angle between water and a wax surface (left) and a paint surface (right) differ due to the relative strengths of cohesive and adhesive forces.

Conversely, if the strength of adhesive forces dominates over cohesive forces, there will be a smaller contact angle and so water molecules flock to the glass surface, to form these preferable adhesive bonds. As depicted below, the water forms a concave shape, with the outer rim of the liquid turned up against the glass; this is called capillary attraction.

Adhesive forces between water and glass molecules are stronger than cohesive forces between molecules, causing a concave shaped meniscus to form.

The tendency to minimise the surface tension on the surface of the liquid by minimisation of surface area results in an inclination to reduce this curvature, pushing the centre of the liquid upwards. In a thinner tube, where there is more relative surface area inside the tube, liquid can be pulled up higher than in tubes with wider diameters.

Despite its ubiquitous presence in our daily lives, from plants to the human body, the potential of this effect for use in drug delivery has not attracted much attention until recently. It could soon become a vital choice for drug delivery processes to optimise drug efficacy and disease treatment as its potential becomes increasingly explored with the development of more drug delivery platforms.

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