question: Why does coke etc create foam when you pour it into a glass but fizzy/ carbonated water does not?
comment: Bubbles form in any carbonated beverage (sparkling water, coke, beer, etc.). In some beverages the bubbles survive on the surface and can form a foamy “head” because it contains something that makes the bubbles longer lasting (usually sugar or something similar). Diet soft drinks, like sparkling water, doesn’t have any sugar to make the foam, so they add extra carbonation to make it seem roughly as fizzy as non-diet soft drinks. That’s why the Mentos trick is usually done with diet soft drinks.
passages: For ideal cases, =0 and the created foam is dependent on the change in chemical potential of the solute. During foaming, the solute experiences a change in chemical potential as it goes from the bulk solution to the foam surface. In this case, the following equation can be applied where is the activity of the surfactant, is the gas constant, and is the absolute temperature:
  
  The Enrichment ratio demonstrates how effective the foaming is in extracting the protein from the solution into the foam, the higher the number the better the affinity the protein has for the foam state.
  
  formula_5
   The nucleation reaction can start with any heterogeneous surface, such as rock salt, but Mentos have been found to work better than most. Tonya Coffey, a physicist at Appalachian State University, found that the aspartame in diet drinks lowers the surface tension in the water and causes a bigger reaction, but that caffeine does not accelerate the process. It has also been shown that a wide variety of beverage additives such as sugars, citric acid, and natural flavors can also enhance fountain heights. In some cases, dissolved solids that increase the surface tension of water (such as sugars) also increase fountain heights. These results suggest that additives serve to enhance geyser heights not by decreasing surface tension, but rather by decreasing bubble coalescence. Decreased bubble coalescence leads to smaller bubble sizes and greater foaming ability in water. Thus, the geyser reaction will still work even using sugared drinks, but diet is commonly used both for the sake of a larger geyser as well as to avoid having to clean up a sugary soda mess.
   There are two types of foam that can form from this process. They are wet foam (or "kugelschaum") and dry foam (or "polyederschaum"). Wet foam tends to form at the lower portion of the foam column, while dry foam tends to form at the upper portion. The wet foam is more spherical and viscous, and the dry foam tends to be larger in diameter and less viscous. Wet foam forms closer to the originating liquid, while dry foam develops at the outer boundaries. As such, what most people usually understand as foam is actually only dry foam.
   Surface tension is visible in other common phenomena, especially when surfactants are used to decrease it:
  
  BULLET::::- Soap bubbles have very large surface areas with very little mass. Bubbles in pure water are unstable. The addition of surfactants, however, can have a stabilizing effect on the bubbles (see Marangoni effect). Note that surfactants actually reduce the surface tension of water by a factor of three or more.
   The physical characteristics of Mentos (surface roughness, easy dissolution into the liquid, etc.) have the effect of drastically reducing the activation energy for carbon dioxide bubble formation, so that the nucleation rate becomes exceedingly high. The activation energy for the release of carbon dioxide from Diet Coke by addition of Mentos has been found to be 25 kJ mol. This formation of gaseous carbon dioxide within the water, which due to hydrogen bonding and its high surface tension wants to maintain a connected matrix of molecules, causes the water to foam, ultimately generating the eruption-like nature of the effusion. The foaming is aided by the presence of food additives such as potassium benzoate, aspartame, sugars, and flavorings in Diet Coke, and gelatin and gum arabic in the Mentos candy, all which influence the degree to which water can foam.
   Normally, this process is relatively slow, because the activation energy for this process is high. The activation energy for a process like bubble nucleation depends on where the bubble forms. It is highest for bubbles that form in the liquid itself (homogeneous nucleation), and lower if the bubble forms on some other surface (heterogeneous nucleation). When the pressure is released from a soda bottle, the bubbles tend to form on the sides of the bottle. But because they are smooth and clean, the activation energy is still relatively high, and the process is slow. The addition of other nucleation sites provides an alternative pathway for the reaction to occur with lower activation energy, much like a catalyst. For instance dropping grains of salt or sand into the solution lowers the activation energy, and increases the rate of carbon dioxide precipitation. 
   This hypothesis gained further support when rock salt was used as a "jump start" to the reaction. A paper by Tonya Coffey, a physicist at Appalachian State University in Boone, North Carolina, confirmed that the rough surface of the Mentos candy helps speed the reaction. Coffey also found that the aspartame in diet soda lowers the surface tension and causes a bigger reaction, but that caffeine does not accelerate the reaction. On the other hand, it has been shown that a wide variety of beverage additives such as sugars, citric acid, and natural flavors also enhance fountain heights. In some cases, dissolved solids that increase the surface tension of water (such as sugars) also increase fountain heights. These results suggest that additives serve to enhance geyser heights not by decreasing surface tension, but rather by decreasing bubble coalescence. Decreased bubble coalescence leads to smaller bubble sizes and greater foaming ability in water.
   As in many chemical processes, there are competing considerations of recovery (i.e. the percentage of target surfactant that reports to the overhead foamate stream) and enrichment (i.e. the ratio of surfactant concentration in the foamate to the concentration in the feed). A crude method of moving upon the enrichment-recovery spectrum is to control the gas rate to the column. A higher gas rate will mean higher recovery but lower enrichment.
  
  Foam fractionation proceeds via two mechanisms:
  
  BULLET::::1. The target molecule adsorbs to a bubble surface, and
   Section::::Foam types.
  
  CAFS is able to deliver a range of useful foam consistencies, labeled from type 1 (very dry) to type 5 (wet), which are controlled by the air-to-solution ratio, and, to a lesser extent, by the concentrate-to-water percentage. Type 1 and 2 foams have long drain times, meaning the bubbles do not burst and give up their water quickly, and long duration. Wet foams, such as type 4 and 5, drain more quickly in the presence of heat. 
   Section::::Background.:Theory.
  
  Section::::Background.:Theory.:Young-Laplace equation.
  
  As vapor bubbles form in a liquid solvent, interfacial tension causes a pressure difference, Δ"p", across the surface given by the Young-Laplace equation. The pressure is greater on the concave side of the liquid lamellae (the inside of the bubble) with radius, R, dependent on the pressure differential. For spherical bubbles in a wet foam and standard surface tension , the equation for the change in pressure is as follows:
   The absence of oil (or the significantly lower oil content compared to traditionally brewed coffee) makes the system more stable and the bubbles do not collapse with the same ease as in cream. Soon after the foam is created, a process of thickening takes place where water molecules are constantly pushed out of the frothy mixture. The water is pushed out due to drainage occurring due to pressure differentials along the foam septum. Higher viscosity will retard the phenomenon, and that is the reason that the addition of sugar will create a better foam. The phenomenon continues until bubbles come very close together and the foam almost solidifies. This process can take between 2 minutes to 10 minutes and depends strongly on the agitation process during mixing. As the bubbles come closer together they will slowly start to coalesce and create bigger bubbles. According to the Laplace pressure equation, variation in bubble size will result in faster collapsing of the bubbles since the bigger bubbles will consume the smaller ones. Hand-mixers create smaller and more uniformly sized bubbles. The smaller bubble size reduces the bubble pressure gradient and forms a much longer lasting foam.
   The setup for continuous foam separation consists of securing a column at the top of the container of solution that is to be foamed. Air or a specific gas is dispersed in the solution through a sparger. A collecting column at the top collects the foam being produced. The foam is then collected and collapsed in another container.
  
  In the continuous foam separation process a continuous gas line is fed into the solution, therefore causing continuous foaming to occur. Continuous foam separation may not be as efficient in separating solutes as opposed to separating a fixed amount of solution.
  
  Section::::History.
   The basic requirements for formation of foam are an abundance of gas, water, a surfactant, and energy. The steam wand of an espresso machine supplies energy, in the form of heat, and gas, in the form of steam. The other two components, water and surfactants, are naturally occurring ingredients of milk. Varying the balance of these factors affects the size of bubbles, the foam dissipation rate, and the volume of foam.
   At equilibrium, the difference in capillary pressure must be balanced by the difference in hydrostatic pressure. Hence,
   First demonstrated by chemistry teacher Lee Marek on the "Late Show with David Letterman" on September 14, 1999, and later popularized in a June 2006 viral Internet video by Eepybird, a Mentos mint expedites a rapid release of carbon dioxide when dropped into a carbonated liquid, such as a soft drink. "MythBusters" concluded that the potassium benzoate, aspartame, and CO gas contained in the Diet Coke, in combination with the gelatin and gum arabic ingredients of the Mentos, all contribute to the formation of the foam.
   Closed-cell foams retain the fire resistance and recycling potential of other metal foams, but add the property of flotation in water.
  
  Section::::Closed-cell.:Manufacturing.
   For the Marangoni effect to occur, the foam must be indented as shown in the first picture. This indentation increases local surface area. Surfactants have a larger diffusion time than the bulk of the solution—so the surfactants are less concentrated in the indentation.
   Foam fractionation is closely related to the allied process of froth flotation in which hydrophobic particles attach to the surface of bubbles which rise to form a pneumatic (i.e. rising) foam. In this way, relatively hydrophobic particles can be separated from relatively hydrophilic particles. Froth flotation is typically used to separate coal particles from ash or particles of valuable minerals from gangue material. It was research into the froth phase of froth flotation conducted at the University of Newcastle, Australia, specifically into the prediction of liquid fraction and liquid flux in a pneumatic foam, that enabled a preliminary mechanistic description of foam fractionation. The synergies between foam fractionation and froth flotation have been explored in a 2009 special issue of the Asia Pacific Journal of Chemical Engineering.
   BULLET::::2. The bubbles form a foam which travels up a column and is discharged to the foamate stream of foam fractionation.
  
  The rate at which certain non-ionic molecules can adsorb to bubble surface can be estimated by solving the Ward-Tordai equation. The enrichment and recovery depend on the hydrodynamic condition of the rising foam, which is a complex system dependent upon bubble size distribution, stress state at the gas-liquid interface, rate of bubble coalescence, gas rate "inter alia". The hydrodynamic condition is described by the Hydrodynamic Theory of Rising Foam.
  
  Section::::Applications.
   The conversion of dissolved carbon dioxide to gaseous carbon dioxide forms rapidly expanding gas bubbles in the soda, which pushes the beverage contents out of the container. Gases, in general, are more soluble in liquids at elevated pressures. Carbonated sodas contain elevated levels of carbon dioxide under pressure. The solution becomes supersaturated with carbon dioxide when the bottle is opened, and the pressure is released. Under these conditions, carbon dioxide begins to precipitate from solution, forming gas bubbles. 
   Milo dissolved in water has a Glycemic Index (GI) of 55, the same as Coca-Cola. However, milk has a much lower GI of 30 - 33, so mixing a very small amount of Milo into a mug of milk yields an overall GI closer to 33, and mixing a large amount of Milo into a mug of milk will give a GI closer to 55.
   Section::::Manufacturing.
  
  Foamed concrete typically consists of a slurry of cement or fly ash and sand and water, although some suppliers recommend pure cement and water with the foaming agent for very lightweight mixes. This slurry is further mixed with a synthetic aerated foam in a concrete mixing plant. The foam is created using a foaming agent, mixed with water and air from a generator. The foaming agent used must be able to produce air bubbles with a high level of stability, resistant to the physical and chemical processes of mixing, placing and hardening.
   Home soda siphons can carbonate flat water through the use of a small disposable steel bulb containing carbon dioxide. The bulb is pressed into the valve assembly at the top of the siphon, the gas injected, then the bulb withdrawn. Soda water made in this way tends not to be as carbonated as commercial soda water because water from the refrigerator is not chilled as much as possible, and the pressure of carbon dioxide is limited to that available from the cartridge rather than the high-pressure pumps in a commercial carbonation plant.
  
  Section::::Products for carbonating water.:Home.:Gasogene.
   Several conditions are needed to produce foam: there must be mechanical work, surface active components (surfactants) that reduce the surface tension, and the formation of foam faster than its breakdown.
  
  To create foam, work (W) is needed to increase the surface area (ΔA):
  
  where γ is the surface tension.
   The process of dissolving carbon dioxide in water is called carbonation. Commercial soda water in siphons is made by chilling filtered plain water to or below, optionally adding a sodium or potassium based alkaline compound such as sodium bicarbonate to reduce acidity, and then pressurizing the water with carbon dioxide. The gas dissolves in the water, and a top-off fill of carbon dioxide is added to pressurize the siphon to approximately , some higher than is present in fermenting champagne bottles.
  
raw_presuppositions: Carbonated water does not create foam when it is poured into a glass.
raw_corrections: Carbonated water does create foam when it is poured into a glass, but since there is no sugar, the foam does not last long.