Phenomenal Surfaces

A pioneering effort to illuminate the nature of things

Felice Frankel, who was at the time an accomplished photographer of landscape architecture, came to Harvard in 1991 as a Loeb Fellow. These open-sesame fellowships let professionals concerned with the built environment spend a year in mid career studying anything they choose anywhere in the University. Frankel straightaway took herself to the Science Center, trailing memories of an undergraduate degree in biology, and enrolled in courses taught by Stephen Jay Gould, E.O. Wilson, and Mallinckrodt professor of chemistry George M. Whitesides. She invited herself to Whitesides's laboratory, and thus began an improbable collaboration, just the sort of coming together in unanticipated ways of people from disparate disciplines that is supposed to happen at Harvard.

Frankel "became enthralled by the possibilities of powerful imaging in science, and by the general ineptitude of scientists in exploiting those possibilities," Whitesides explains. He and colleagues were about to submit an article to Science. Frankel looked at the photographs meant to go with it and didn't think much of them. She made substitutes, including an image of square drops of water (see page 45), and, for the first time in a productive career, an article by Whitesides was featured on a magazine cover.

Whitesides and Frankel, now artist-in-residence and science photographer at MIT, went on to collaborate on an eye-opening book. On the Surface of Things: Images of the Extraordinary in Science will be published by Chronicle Books in October. On the following pages, Harvard Magazine is pleased to offer a preview.

 

Ferrofluid

Pity the gryphon, the mermaid, the silkie, the chimera: creatures assembled of incompatible parts, with uncertain allegiances and troubled identities. When nature calls, which nature is it? When instinct beckons, approach or flee?

A ferrofluid is a gryphon in the world of materials: part liquid, part magnet. It is prepared by grinding magnetite--the magnetic lodestone--in an oil. The grinding must be "just enough." If the particles of magnetite are too large, they remember who and what they were and behave like a fine magnetic powder, clumping and settling rapidly from the oil. If they are too small, they no longer show any of the wonderful cooperation between groups of atoms that is required for magnetism. If they are just the right size--if they are small enough that they are not so different in size and character from molecules of liquid, small enough that they have begun to lose their magnetic heritage, but still large enough that they again become fully magnetic when placed in a magnetic field--they develop a useful schizophrenia. Outside a magnetic field, they are non-magnetic liquids; in a magnetic field, they become magnetic. Grinding is carried out in the presence of soap, which coats the small particles with an oil-like surface film and makes it even more difficult to distinguish them from the oil. Properly reduced in size, and correctly coated, the particles remain dispersed and do not settle.

When placed in a magnetic field, the conflicting attractions of gravity, magnetism, and surface tension shape the ferrofluid. This drop of ferrofluid was placed on a glass sheet with yellow paper underneath for photographic contrast. Six small magnets were placed below the paper. In regions of high magnetic field, the fluid broke into spikes, trying to imitate the way iron filings line up in columns in a magnetic field. In regions of lower magnetic field it remained a liquid, forming flat drops in a compromise between the siren call of gravity and its own cautious cohesion. The result is shapes seen nowhere else in nature.

The unique properties of a ferrofluid--a stable liquid that responds to magnetic attraction--make it useful in devices where fluid properties and resistance to gravity are needed, such as rotary seals in disk drives for computers, and dampers for high-performance audio speakers.

 

Migrating Bacteria 

This dish records the history of a civilization of bacteria--the few days that it lasted. The civilization started at the center, when a fresh plate of culture medium--a gel containing nutrients--was inoculated with a few bacteria. They were the first colonists. The population of the initial colony grew, and it depleted the food that was available locally.

When the population became large enough, new colonists were sent out. These bacteria flowed radially outward toward more abundant food and uncontaminated space. Following their wave of migration, the colony consolidated its growth in the new region until it reached the critical density in population required to send out colonists again. These cycles of expansion and stationary growth created the patterns here.

The sectors follow the history of particular subgroups of these bacteria. Some families grew to the edge of the plate; less successful families were overwhelmed by the more successful, and they faded. The bacteria could not talk with one another, but they interacted by the chemicals--the "smells"--that they left, and by their influence on the local food supply and the level of pollution. When the food on their continent was exhausted, they died.

From these patterns, we infer how bacteria sense their environment. We sample our environment with different and more highly evolved senses than they do, but there are remarkable similarities between their sensors and ours. Understanding perception in all its complexity is one of the great challenges now facing biology. Evolution connects our eyes and nose to the senses of bacteria; their present is our history.

 

Square Drops of Water

To keep sheep, you need fences; unfenced sheep spread, wandering after grass.

These colored squares are drops of water. They rest on a surface across which they would normally form circular drops and spread until they coalesced. Instead, they are performing unnatural acts: forming squares, staying in place, not mixing. What fences pen them? What grass attracts them?

The surface beneath the drops has been coated with a single layer of molecules of a type much loved by water (a hydrophilic surface). The water molecules cover it as completely as they can. But the surface has also been broken into square fields by painted stripes a single molecule thick of a different type of molecule, one the water avoids (a hydrophobic surface). The molecules of water crowd to the edges of these stripes, but they do not jump them.

The backs of sheep are often spray-painted to identify them; the colors show when part of a flock has found a gap in a fence and drifted into another field. Dyes added to these drops do the same for water.

The square drops are approximately 0.4 centimeters across; the stripes dividing them are 1 micron across and one-thousandth of a micron high. If the water molecules were sheep, the fence would be 3 sheep tall and 1,500 sheep across, and the sheep would be piled 1 million deep in the center of the field. It would be a peculiar form of agriculture.

Controlling the spreading of liquids on surfaces--the wetting of the surface by the liquid--is surprisingly important, not only in painting, printing, and gluing, but also in growing mammalian cells; in fabricating the myriad micro-electronic devices that swarm around us in computers, cars, airplanes, and air conditioners; and in condensing steam in boilers.

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