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Electronic cigarette elements that heat the fluid are usually termed "atomizers." They are usually just some nichrome wire or similar wrapped around a wick that becomes saturated with the nicotine fluid.

What is the process that "vaporizes" the fluid? Does the element just heat the fluid until it evaporates? Or something else, e.g., is the majority of fluid sort of spattered off the wick due to a smaller proportion of it violently evaporating? Or some other process/phenomenon? Is the term "atomizer" a scientific term in this context, or just something adopted for electronic cigarettes?

Chris Mueller
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Jodes
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    Atomise meaning to break into very small parts is a 17th century word, and specifically of liquids into droplets like a perfume atomiser or e-cigarette does mid-19th century. It is not directly related to Dalton's early-19th century atomic view of matter except etymologically, in which it is tecnically incorrect but so then, it turned out was Dalton (because atoms aren't atoms in the classical sense; "sub-atomic" is an oxymoron). – Jon Hanna Apr 17 '15 at 15:27

2 Answers2

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Yes, it evaporates the fluid as you describe, using a wicking substance (usually silica fibers or wire wool) and a heating coil. Note that it doesn't boil the fluid to achieve evaporation.

There are a number of factors that influence the rate of evaporation:

  • Surface area. Evaporation happens at the boundary between the liquid and air, so the greater surface area you achieve, the greater the possible rate of evaporation. The wick draws the liquid by capillary action and allows it to spread out along its fibers, creating a large surface area in a small space.
  • Temperature. The state-change from liquid to gas requires energy, this comes from heat in the surrounding environment. That's why when you blow on your damp hands they get cold. The heating coil ensures there's a plentiful supply of heat energy to help with this state-change.
  • Low Pressure. Air pressure pushes against the surface and makes it harder for evaporation to happen. Due to Bernoulli's principle, the moving air-flow from the drag lowers the air pressure and helps with evaporation.
  • Low Concentration. If the air is already saturated with vapor (from the liquid or other substances already in the air) then it will be harder for evaporation (new vapor going into the air). The flow of fresh air from outside going across the wick ensures maximum evaporation. You may notice that when it is humid, you get much less of the cigarette vapor per drag.

Regarding the term atomizer:

It usually means converting a liquid into very small particles (for example by means of vibration or a nozzle), small enough to be suspended in air. However, the goal of this is sometimes to cause evaporation (it creates a large surface area on all the tiny droplets, making evaporation easier), for example in an ultrasonic humidifier. A counter example (where the goal is not evaporation) would be a spray-paint nozzle. So that is the typical use of the term, I speculate that the reason e-cigarette manufacturers have adopted this term is the shared goal of vaporization along with the fact that atomizer sounds pretty cool.

jhabbott
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    The term atomizer is used because the vapor condenses after being vaporized, creating a tiny cloud (i.e. a suspension of liquid droplets in air). Vaporizers have been around for many years, and the good ones don't produce any visible traces in the air. Cigarette smokers like to blow something out when they exhale though. – Chris Mueller Apr 17 '15 at 12:32
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    Oh interesting! Makes sense. – jhabbott Apr 17 '15 at 14:18
  • @ChrisMueller "The term atomizer is used because the vapor condenses"? Makes little sense (outside of marketing) to me as neither is anything split down to the size of atoms, nor is there anything relating to the original meaning of atomos=unsplittable – Hagen von Eitzen Apr 17 '15 at 14:21
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    @HagenvonEitzen Atomization is a term which proceeds the use of the word as applied to the things which make up molecules (atoms). According to the linked Wikipedia article it means: "The making of an aerosol, which is a colloid suspension of fine solid particles or liquid droplets in a gas." The use I'm most familiar with is in old perfume bottles where the nozzle which turned the liquid into a fine mist was called the atomizer. – Chris Mueller Apr 17 '15 at 14:25
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Vapor is something that forms over any liquid and does not require the liquid to be above its boiling temperature. The atmosphere is a great example; it contains about 37 billion gallons of water in the form of vapor, but the temperature of (most of) the atmosphere is well below the boiling point of water. The goal of a vaporizer is simply to increase the rate of this process.

To understand how it does this you need to know a little bit about vapor pressure. A liquid at a given temperature will have some of its molecules which are moving fast enough to escape the surface of the liquid and end up in the vapor phase above the liquid. If you put the liquid in a sealed container, this process will continue until the partial pressure of the vapor reaches the vapor pressure of the liquid. What is important about this process is that it is only the pressure of the substance itself (i.e. partial pressure) which matters, even if the rest of the air above the substance is at a higher pressure than the vapor pressure, the liquid will still vaporize until its partial pressure reaches the vapor pressure. The vapor pressure increases with increasing temperature until it reaches the atmospheric pressure at the boiling point of the liquid.

Vapor pressure illustration and plots

With that background, we can now discuss how the vaporizer in an e-cigarette works. The amount of ambient vapor (in the surrounding air) of the nicotine solution in e-cigarettes is very low so turning it into vapor is simply a matter of letting it sit for long enough. The amount of time it would take is too long for a smoker to wait though so they use two tricks to speed up the process. 1) The fluid is heated so that the vapor pressure increases, and 2) a wick is used to drastically increase the surface area between which the air and fluid interact.

The heating of the fluid has the desirable side effect of making the cloud of vapor visible too. The best way to understand this is to think about a boiling pot of water. The water is converting to a gas at the surface and rising away from the pot, but you can't see water in its gaseous stage. You can, however, see steam because the water gas is mixing with the cool air above the pot and small droplets of water are condensing, much like a cloud. The same thing happens in an e-cigarette. Local heating in the vaporization area heats the air along with it and produces a vapor which you can't see. When this vapor mixes with the cool air as you inhale, it condenses to tiny droplets which give it the desirable appearance of smoke. So, when you blow out after puffing you are actually blowing out a little nicotine filled cloud.

As for the term atomizer, it does have a semi-scientific definition. An atomizer is something which creates a fine suspension of liquid droplets in a gas. This is exactly what I described to you above; the vaporizer turns the liquid to vapor which then becomes a suspension as the vapor saturated air cools.

Chris Mueller
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