Emulsions and foams

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Emulsions and foams from Lectures for AP225.

Topics

Emulsions
W. Clayton, The theory of emulsions and emulsification; J.&A. Churchill:London, 1923, p. 89.
Foams
Jean Simeon Chardin 1699 - 1779
IceCream.jpg
Ice cream is a:

Foam of air bubbles,

Stabilized with small (yellow) oil drops,

In a matrix that is,

An emulsion of more oil drops

And a suspension of (blue) ice crystals,

In a continuous (grey) phase of surfactants, micelles, and solutes

in water

In a sugar cone.

Phase 1 Phase 2
Droplet Serum
Dispersed Medium
Discontinuous Continuous
Internal External

Cake batter is an emulsion and foam

A cake mixture is a complex, multi-component system, being simultaneously a foam, an emulsion and a complex colloidal dispersion. Of the main components of a cake batter - egg, flour, fats and sugar - only the sugar is non-colloidal. The process of transformation of these ingredients into cake (i.e. solid foam) is not completely understood, although it is known that for the success of the recipe it is vital to retain the air bubbles within the cooked batter.

Batter.jpg Cake.jpg

To better understand the 'foamy' nature of cakes, let us break up the cake batter into its most essential components:

1) Flour, which is mainly composed of gluten and starch. Gluten was named by the Chinese 'the muscle of flour' due to its elastic nature. Gluten is composed of long protein molecules ( gliadins and glutenins) which are responsible for the elastic behavior of dough. Upon kneading and stretching of dough, gluten proteins unfold and align. The coiled, spring-like structure of proteins can unfold and store some of the energy of stretching, but when the stretching is stopped, the molecules spring back to their compact coiled form. This is macroscopically manifested when stretched dough creeps up to its original shape. Although bread preparation benefits from a strong, elastic gluten, excessive amounts of the protein are not desirable in puffy pastries and raised cakes. Ways to limit gluten presence in batter are the use of low-protein flours as well as adding water in the dough, which dilutes the gluten proteins and limits their bonds.

Starch makes up 70% of the flour weight. Therefore it is a structural component of doughs, especially the low-gluten cake batters. Along with water, starch interpenetrates the gluten network and break it up tenderizing the dough. During the baking of bread and cake, starch granules absorb water and set to form the rigid bulk of the walls that surround the bubbles of carbon dioxide. At the same time, their swollen rigidity stops the expansion of bubbles and forces the water vapor inside to pop the bubbles and escape, turning the foam of separate bubbles into a continuous network of connected holes. If this didn't happen, then at the end of baking the cooling water vapor would contract and cause the cake to collapse.

2) Eggs, which contain proteins, fats and emulsifiers. The proteins in eggs coagulate during cooking and supplement the gluten structure. The fats and emulsifiers in eggs work like starch, weakening the gluten network and stabilizing the bubbles in the dough.

3) Fats, contained in oils and shortening. Fats are an important component in pie crusts and puff pastries, where layers of solid fats separate thin layers of dough from each other so that they cook into separate layers of pastry. In cakes, fat and oil molecules bond to parts of the gluten protein coils and prevent the proteins from forming a strong gluten. This is the reason why in making bread, which requires a strong gluten, flour and water are mixed alone.

4) Gas bubbles make up as much as 80% of a cake's volume. These weaken the gluten and starch network and divide it into millions of delicate sheets that form the bubble walls. Baking yeasts and chemical leavenings are routinely used to fill baked goods with gas bubbles. A common misconception is that these products create new bubbles: in fact, the carbon dioxide in yeast is released into the water phase of the batter, diffuses to the pre-existing air bubbles and enlarges them. This is why the initial aeration of dough and batter through kneading, strongly influences the final texture of baked goods.

Information adapted from:

  • Ian W. Hamley, 'Introduction to soft matter', John Wiley & Sons (2007)
  • Harrold McGee, 'On food and cooking: the science and lore of the kitchen', Scribner NY (2004)


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Bibliography

N.H Hyam, "Quantitative Evaluation of Factors Affecting the Sensitivity of Penetrant Systems". Materials Evaluation, pp. 31-38, February 1972.

A. V. Rode, "Electronic and magnetic properties of carbon nanofoam produced by high-repetition-rate laser ablation". Applied Surface Science 197–198: 644–649. 2002.


Ice Cream Floats

One interesting point building on the look into ice cream is the role it plays when you make an ice cream float. If you've ever made one, you'll remember that when the ice cream is added to the soda that it begins to rapidly foam up. You can see this in the image below:

Float.jpg

Looking at this, it begs the question of why the ice cream causes an increase in foam in the soda. The reason is simple. As mentioned above, ice cream is an emulsion of various things like fat and sugar. As it melts, these tiny particles are released into the soda. As you know, soft drinks are liquids that have been supersaturated with carbon dioxide. Soda fizz comes from the nucleation of the carbon dioxide. The particles from the ice cream alter the solubility of the carbon dioxide and enable the creation of foam nucleated on the small particles. The amount of foam created is directly proportional to how much CO2 is diluted in the liquid. Furthermore its the fat serving as the instigators of nucleation that further the stability of the foam produced.

References: http://www.newton.dep.anl.gov/askasci/chem00/chem00313.htm



The Mentos-Coke experiment

We have all surely heard, by now, of the famous Mentos in Diet Coke experiment (http://en.wikipedia.org/wiki/Diet_Coke_and_Mentos). I thought it worthy to mention here, because it seems to fit in the category of soft matter in numerous ways:

1) First of all, and specifically related to the topic of emulsions and foams, when the Mentos are dropped into the soda bottle, CO2 bubbles rapidly nucleate and grow on the rough surface of the candy, thus causing rapid expansion of an emulsion (CO2 gas in liquid soda) and allowing for very interesting science experiments, such as soda fountains, soda jets or soda rockets!

2) Interesting to note that mint Mentos work better than fruit Mentos, which have a soft surface coating that slows down nucleation of CO2 bubbles.

3) Interesting as well to note that Diet Coke is used preferably to Coke. The soda, in ths science experiment only provides highly dissolved carbon dioxide at a first glance. But from what has been reported in the TV series "Mythbusters" is that the replacement sugar used in Diet Coke, aspartame, is also responsible for the nice jetting regime that we see in this fun experiment. So although I haven't tried it myself, it would seem that regular Coke would not do so well.

4) One website (http://www.newscientist.com/article/dn14114-science-of-mentosdiet-coke-explosions-explained.html) claims that surface tension of soda with the sugar aspartame is lower than that of soda with regular sugar. And because lower surface tension would help bubbles grow faster, Diet Coke works better.

5) Moreover, the presence of other surfactants in Mentos may dissolve with the rest of the candy and help reduce the surface tension of the Diet Coke even further.


So we've got here a combination of surface geometry, surface tension, rapid foam formation, phase transition (from dissolved CO2 to foam), surface coating and others that really fit into the themes of soft condensed matter! That is perhaps why one of Harvard's former soft condensed matter post-docs Dan Blair, now assistant professor at Georgetown, was asked to comment on this phenomenon on NECN a while back (http://www.seas.harvard.edu/newsandevents/newsarchives/index.html?d=2006.06).


Aluminum Foam

Aluminum foam has applications to a variety of industries, including automotive, aviation, railway and engine building industries. It is an extremely stiff material but also low weight. It also has high energy absorption. Aluminum foam has specifically be used in the following common applications: fluid flow control devices, energy/impact absorption, compact heat exchangers, lightweight composite panels, and trabecular bone simulation (aluminum foam actually micmics bone tissue!) Some specific physical characteristics of aluminum foam are,

Compression Stretching 2.53 MPa

Elasticity Modulus (of compression) 93.08 MPa
Specific Heat .895 J/gC

Melting Point 660 C (quite high)

To produce aluminum foam, powdered aluminum is mixed with a product that releases gas at a high temperature. It is then compacted and put into a mold form. It is heated until the aluminum starts to foam. Then it is taken out of the heated environment so it stays in its desired shape.

Aluminum foam is preferably used over other polymeric foams (such as honeycomb and polymer foams), because it is recyclable, it has material stability over time, environmental and temperature conditions. It is also very cheap to make as compared to other commonly used foams. Aluminum foam also do not give off noxious fumes when they decompose, and they are stronger than other polymer foams.

Aluminum foam will be used more and more in the future!

Aluminium-foam.jpg

Extended Reading

Foams

  • Exerowa
    • Chapter 1. Formation and structure of foams;
      • It is not possible to obtain stable (long-living) foams from pure liquid. Stable polyhedral foams are only formed in the presence of an appropriate surfactant (or surfactant mixture)." p. 1
      • Foams can be formed by condensation (decreasing pressure or increasing temperature, or using a chemical reaction) or dispersion (injecting bubbles into a liquid by various methods). p. 4
    • Chapter 3. Physical chemistry of foam films;
    • Chapter 11. Black foam films: Application in medicine.
  • Mysels
    • Chapters 1 – 7
  • Prud’homme
    • Chapter 1. Thin liquid film physics
    • Chapter 2. Structure, drainage, and coalescence of foams and concentrated emulsions
    • Chapter 3. Foam rheology
      • "Liquid foams may be viewed as structured fluids possessing a complex rheological behavior strongly dependent upon local structure and physiochemical constitution." p. 189
    • Chapter 4. Experimental results on foam rheology
  • Norde
    • Chapter 18. Emulsions and foams
      • Emulsions and foams are thermodynamically unstable dispersions. p. 366
      • There dispersed phase is liquid, which means that their particles are deformable, the interface between the particles and the continuous phase is deformable, and the particles may coalesce. p. 366
  • Weaire
    • Chapter 1. Introduction
    • Chapter 2. Local equilibrium rules
    • Chapter 3. Quantitative description of foam structures
    • Chapter 8. Rheology
    • Chapter 12 Foam collapse

Emulsions

  • Becher
    • Chapter 1. Introduction and definitions
    • Chapter 3. Physical properties of emulsions
      • "In all probability, in the majority of cases emulsion droplets less than about 0.2 microns are not common; the largest droplets in an emulsion may well be of the order of a hundred times this value." p. 65
    • Chapter 4 - 6. Emulsion stability
    • Chapter 7 – 8. Emulsion instability
  • McClements
    • Chapter 6. Emulsion formation
    • Chapter 7. Emulsion stability
    • Chapter 8. Emulsion rheology
  • Sjöblom
    • Chapter 1. An experimental and theoretical approach to the dynamic behavior of emulsions;
      • "Practically all colloidal interactions are of a short range, almost never extending over distances greater than the size of the particle. Hence they have little influence over the transport of the particles, although they are crucial in determining the collision efficiency." p. 6
      • "The universal attractive forces between atoms and molecules, known as van der Waals forces, also operate between macroscopic objects and play a very important role in the interaction of colloidal particles. Indeed, without these forces, aggregations of particles would usually be prevented by the hydrodynamic interaction." p. 8
    • Chapter 2. Spontaneous emulsification
    • Chapter 3 Symmetric thin liquid films with fluid interfaces

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