Verdampfer & Coils

Capillary effect - how cotton wool transports the liquid to the coil

Capillary effect in the vaporiser: how cotton wool transports the liquid to the coil. Discover everything about the wick principle, absorbency and tips against dry hits now.

The capillary effect is the fundamental physical principle that enables any vape device to function in the first place: cotton or another type of wick absorbs e-liquid from the tank and feeds it to the hot coil. If this flow is too slow, you’ll get a dry, scratchy hit – a classic dry hit. If it’s too fast, your vape will leak. If you understand the interplay between cotton, coil and e-liquid, you can build more reliable rebuildables, choose more suitable pre-built coils and avoid typical priming mistakes. In this guide, you’ll learn how the capillary effect works, what role the wicking principle plays in vaping and what you should look out for.

The capillary effect describes the property of liquids to rise against gravity in narrow cavities. Two forces are key: the adhesion between the liquid and the surface, and the cohesion of the liquid itself. In very fine channels – for example, between the fibres of a cotton wick – adhesion ‘pulls’ the liquid upwards, whilst cohesion holds the liquid thread together.

In vaping, this phenomenon is also known as the capillary effect or, in everyday language, the wick principle. Exactly the same thing happens in a petrol lamp or in a plant stem: a porous material draws liquid from a reservoir and releases it evenly into an evaporation or combustion zone.

The capillary effect must be distinguished from pure capillary pressure in closed tubes and from simple absorption by volume: what matters is not how much e-liquid the cotton can store, but how quickly it transfers it through its fibre network to the heating coil.

In a typical coil, the heating coil sits in the centre, surrounded by wadding. This protrudes through the liquid slots (juice flow) into the tank. As soon as you inhale, the liquid evaporates on the hot coil. This immediately creates a vacuum in the fibres – and the capillary effect draws fresh liquid from the tank into the empty space. This supply must be exactly as fast as you produce vapour.

MTL atomisers: low power, fine fibres

For mouth-to-lung coils with 10–20 watts and resistances around 1.0 ohms, a narrower wicking path is sufficient. The low vapour production rate requires only moderate e-liquid flow.

Sub-ohm and DTL: plenty of vapour, high absorbency required

Direct-to-lung coils at 50–80 watts vaporise significantly more e-liquid per second. Here you need cotton with high absorbency and generous e-liquid channels – otherwise the wick will burn.

Capillary action when vaping depends on several factors. The following points determine whether your setup works smoothly or causes you problems:

Cotton quality: Unbleached organic cotton has long, even fibres and excellent capillary action. Cheap cotton delivers e-liquid unevenly.
Amount of cotton: Too much cotton blocks the e-liquid slots and slows the flow. Too little cotton causes leaks. The cotton should be able to be pulled through the coil without much force, but with noticeable resistance.
VG/PG ratio: High-VG e-liquids (70/30 or 80/20) are more viscous and flow more slowly. For such e-liquids, you need coils with large juice flow openings.
Priming: Always moisten new coils with 5–10 drops of liquid at all wadding openings and on top of the wick, then leave to soak for 5 minutes. This allows the capillary effect to fully saturate the wicking material.
Power: Stay within the wattage range printed on the coil. Too high a power setting will vaporise the e-liquid faster than the cotton can absorb it – resulting in a burnt wick.
Pauses between puffs: After several strong puffs in a row, the wick needs a few seconds to reabsorb the e-liquid. Short pauses prevent dry hits.

Why do I get a dry hit even though the tank is full?

This is usually because the capillary effect cannot keep up with the vapourisation rate. Causes: wattage setting too high, too much or incorrectly placed cotton, blocked juice flow slots, or very viscous high-VG e-liquid. Reduce the power or check the coil build.

Which cotton has the best absorbency?

Unbleached, organic cotton in pad or rolled form is considered the standard. It offers long, parallel fibres, absorbs e-liquid without affecting the flavour and provides consistent capillary action. Synthetic wicks are only used in specialised applications, not in standard sub-ohm vapes.

Does the temperature of the e-liquid matter?

Yes. Cold e-liquid is thicker and flows noticeably more slowly. If you take your battery out of your jacket pocket or use e-liquid straight from the fridge, give the setup a few minutes to reach room temperature – the e-liquid flow will then be much more consistent.

Can I test the capillary effect myself?

Dip a small piece of cotton wool into a few drops of e-liquid from the side. If it soaks up completely within a few seconds and the liquid visibly spreads through the fibre bundle, the cotton wool is suitable for rebuildables.

The capillary effect determines the flavour, vapour production and lifespan of your heating coil. If you understand the wicking principle, you can select cotton wool, coil and e-liquid that work well together and avoid classic mistakes such as dry hits or leaks. If you want to optimise your next vape setup, it’s worth taking a look at suitable coils as well as e-liquids with the right VG/PG ratio. Try out different setups and find out which capillary action works best for you when vaping.