PSAT Reading Practice Test 11

Questions 1-10 refer to the following information.

These passages are adapted from KU Leuven, "Bacterium counteracts 'coffee ring effect.'" © 2013 and University of Pennsylvania, "Physicists undo the 'coffee ring effect.'" © 2011 Science Daily.

Passage 1
A team of University of Pennsylvania physicists
has shown how to disrupt the "coffee ring effect"—
the ring-shaped stain of particles leftover after coffee
drops evaporate—by changing the particle shape. The
05discovery provides new tools for engineers to deposit
uniform coatings.
The research was conducted by professor Arjun
Yodh, director of the Laboratory for Research on the
Structure of Matter; doctoral candidates Peter Yunker
10and Matthew Lohr; and postdoctoral fellow Tim Still.
"The coffee ring effect is very common in everyday
experience," Yunker said. "To avoid it, scientists have
gone to great lengths designing paints and inks that
produce an even coating upon evaporation. We found
15that the effect can be eliminated simply by changing
the shape of the particle."
University of Chicago physicists Sidney Nagel,
Thomas Witten and their colleagues wrote an
influential paper about this process in 1997, which
20focused mainly on suspended spherical particles, but it
was not until the Yodh team's recent experiments that
the surprising role played by suspended particle shape
was discovered.
Yodh's team used uniformly sized plastic particles
25in their experiments. These particles were initially
spherical but could be stretched into varying degrees of
eccentricity, to ensure the experiments only tested the
effect of the particle's shape on the drying pattern. The
researchers were surprised at how big an effect particle
30shape had on the drying phenomenon.
"Different particle geometries change the nature of
the membrane at the air-water interface," Yodh said.
"And that has big consequences." Spherical particles
easily detach from the interface, and they flow past one
35another easily because the spheres do not substantially
deform the air-water interface. Ellipsoid particles,
however, cause substantial undulation of the air-water
interface that in turn induces very strong attractions
between the ellipsoids. Thus the ellipsoids tend to get
40stuck on the surface, and, while the stuck particles
can continue to flow towards the drop's edges during
evaporation, they increasingly block each other,
creating a traffic jam of particles that eventually covers
the drop's surface.
45After experimenting with suspended particle
shape, the researchers added a surfactant, essentially
soap, into the drops to show that interactions on the
drop's surface were responsible for the effect. With the
surfactant lowering the drop's surface tension, ellipsoid
50particles did not get stuck at the interface and flowed
freely to the edge.
"We were thinking it would be useful if you could
just sprinkle in a few of these ellipsoid particles to
remove the coffee ring effect," Yodh said, "and we
55found that sometimes this idea works and sometimes
it doesn't."
Passage 2
Researchers from the Departments of Chemical
Engineering and Chemistry at KU Leuven have
now discovered how to counteract coffee rings with
60surfactants', i.e., soap. The key to the discovery was not
a kitchen towel, but a bacterium that counteracts the
coffee ring effect at the microscopic level.
When a coffee ring dries, its edges become
noticeably darker and thicker. This occurs because the
65coffee particles move toward the edge of the stain while
the water in the liquid evaporates. At a microscopic
level, this coffee ring effect can also be seen in liquids
with particles of other materials such as plastic and
wood.
70In various industrial applications—applying
an even coat of paint or varnish, for example—the
coffee ring effect can be particularly troublesome and
scientists have long been seeking ways to counteract
it. Raf De Dier and Wouter Sempels (Departments
75of Chemical Engineering and Chemistry) have now
described a solution based on examples found in
nature. De Dier and Sempels carried out experiments
and calculations on nanomaterials as well as on a
particularly promising bacterium, Pseudomonas
80aeruginosa.
Pseudomonas aeruginosa is a dangerous bacterium
that can cause infections in open wounds. "A
Pseudomonas aeruginosa bacteria colony wants to
find as large a breeding ground as possible. To avoid
85overconcentration on the edges of a wound when
spreading itself during the drying-out process, the
bacterium produces substances that counteract the
coffee ring effect."
These surface-tension-disrupting substances are
90called surfactants. Detergents such as soap are also
surfactants. "Add soap to a stain—a coffee stain or any
other stain—and you will still get a coffee ring effect.
But at the same time the soap causes a counterflow
from the edge back towards the centre of the stain
95in such a way that the small particles—material or
bacteria—end up in a kind of whirlwind. In this way,
you get a more uniform distribution of particles as
evaporation occurs."