| From household products to life-saving
medical applications, emulsions improve our daily activities in countless
ways. We coat our hardwood floors with emulsions that add shine and
prevent scratches. We keep cream cheese and margarine sealed against
bacteria in plastic tubs made from emulsions. We mold emulsions to
form intravenous tubes with glasslike clarity through which doctors
can spot fluid blockages. Emulsions, stable suspensions of one liquid
in another unmixable liquid, are some of the most versatile resources
available.
"The experimental study of emulsions dates back
to the ancient Egyptians and Babylonians, who mixed oil and vinegar
to season their salads. However, the numerical simulation of that
process is a more recent event," explains Yuriko Renardy of
the Virginia Tech.
For instance, the mayonnaise in your refrigerator
is an emulsion of vegetable oil in lemon juice, stabilized by a
molecule found in egg yolks. When you put mayonnaise on bread with
a knife, the spreading motion causes a process that scientists call
shearing. During shearing, lipid globules in the mayonnaise deform,
break up, and then coalesce. This drop deformation process resembles
a simple version of emulsions mixed on a large scale for industrial
use. Industries that use and produce emulsions want to know, if
they feed a mixture of liquids with different sized drops into a
mixer, how will it look when it comes out? The outcome is what scientists
call drop-size distribution.
Imagine you want to make an IV tube. To do so,
you must blend two unmixable liquids, each of which has a valuable
characteristic, such as transparency and flexibility. After the
liquids are blended, poured in molds, and cooled to a solid, the
tube formed must espouse both characteristics. In order to accomplish
this, you want the drop sizes of each liquid to be small and evenly
dispersed. Smaller drops approximate mixing better than large drops
because they provide more surface area in contact between the two
liquids. They will still be unmixable liquids, but the separate
drops will hold together much more securely. A secure emulsion will
make the original liquids in the IV tube less likely to separate
over time.
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