Answer these Question ns based on the passages.

Passage V
For years, the contents of a child’s sandbox have confounded some of the nation’s top physicists. Sand and other granular materials, such as powders, seeds, nuts, soils, and detergent, behave in ways that seem to undermine natural laws and cost industries ranging from pharmaceuticals to agribusiness and mining, billions of dollars.

Just shaking a can of mixed nuts can show you how problematic granular material can be. The nuts do not ‘mix’; they ‘unmix’ and sort themselves out, with the larger brazil nuts on top and the smaller peanuts at the bottom. In this activity and others, granular matter’s behaviour apparently goes counter to the second law of thermodynamics, which states that entropy, or disorder, tends to increase in any natural system.

Mimicking the mixed-nut conundrum with a jar containing many small beads and one large bead, one group of physicists claimed that vibrations causing the beads to percolate open up small gaps rather than larger ones. Thus, when a brazil nut becomes slightly airborne, the peanuts rush in underneath and gradually nudge it to the top. Another group of physicists colour coded layers of beads to track their circulation in a container and achieved a different result. Vibrations, they found, drive the beads in circles up the centre and down the sides of the container. Yet downward currents, similar to convection currents in air or water, are too narrow to accommodate the larger bead, stranding it on top.

One industrial engineer who has studied the problem says that both the ‘percolation’ and ‘convection current’ theories can be right, depending upon the material, and that percolation is the major factor with nuts. Given the inability of scientists to come up with a single equation explaining unmixing, you can see why industrial engineers who must manage granular materials go a little, well, ‘nuts’! Take Pharmaceuticals, for instance. There may be six types of powders with differentsized grains in a single medicine tablet. Mixing them at some speeds might sort them, while mixing at other speeds will make them thoroughly amalgamated. One aspirin company still relies on an experienced employee wearing a latex glove who pinches some powder in the giant mixing drum to see if it ‘feels right’.

Granular material at rest can be equally frustrating to physicists and engineers. Take a tall cylinder of sand. Unlike a liquid, in which pressure exerted at the bottom increases in direct proportion to the liquid’s height, pressure at the base of the sand cylinder doesn’t increase indefinitely. Instead, it reaches a maximum value and stays there. This “quality allows sand to trickle at a nearly constant rate through the narrow opening separating the two glass bulbs of an hourglass, thus measuring the passage of time.

Physicists have also found that forces are not distributed evenly throughout granular material. It is this characteristic that may account for the frequent rupturing of silos in which grain is stored. In a silo, for instance, the column’s weight is carried from grain to grain along jagged chains. As a result the container’s walls carry more of the weight than its base, and the force is significantly larger at some points of contact than at others. Coming up with equations to explain, much loss, predict the distribution of these force chains” is extremely difficult.

Again, using beads, physicists developed a simple theoretical model in which they assume that a given bead transmits the load it bears unequally and randomly onto the three beads on which it rests. While the model agrees well with experimental results, it does not take into account all of the mechanisms of force transmission between grains of sand or wheat.

In the struggle to understand granular materials, sand-studying physicists have at least one thing in their favour. Unlike particle physicists who must secure billions of dollars in government funding for the building of super-colliders in which to accelerate and view infinitesimal particles, they can conduct experiments using such low-cost, low-tech materials as sand, beads, marbles, and seeds. It is hoped that more low-tech experiments and computer simulations will lead to equations that explain the unwieldy stuff and reduce some of the wastage, guesswork, and accidents that occur in the various industries that handle it.

Question 19

Which of the following appears to be the best solution for combating the ‘unmixing’ problem faced by pharmaceutical manufacturers that must prepare large quantities of powders?

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