Research Topics

Natural Quasicrystals

Quasicrystals are solids whose atomic arrangements have symmetries forbidden for periodic crystals, including configurations with five-fold symmetry. All examples identified to date have been synthesized in the laboratory under controlled conditions. Here we present evidence of a naturally-occurring icosahedral quasicrystal that includes six distinct five-fold symmetry axes. The mineral, an alloy of aluminum, copper and iron, occurs as micron-sized grains associated with crystalline khatyrkite and cupalite in samples from the Koryak mountains in Russia.

Drying of Complex Suspensions

Mixtures of immiscible fluids with colloids can be very complex but they are technologically important for industries such as paints and protective coatings, especially when such materials undergo drying. Emulsions containing colloidal particles are particularly interesting as controllable test cases of such systems, but they are difficult to image because these mixtures typically scatter light strongly. Lei Xu, Alexis Bergès, myself and others describe a full 3D picture of what happens when these emulsions dry out.

Decagonal and Quasicrystalline Tilings in Medieval Islamic Architecture

The conventional view holds that girih (geometric star-and-polygon) patterns in medieval Islamic architecture were conceived by their designers as a network of zigzagging lines, where the lines were drafted directly with a straightedge and a compass. We show that by 1200 C.E. a conceptual breakthrough occurred in which girih patterns were reconceived as tessellations of a special set of equilateral polygons (girih tiles) decorated with lines.

PLuTARC: Target-Locking Microscopy

In any typical data gathering process, objects are observed from a fixed viewpoint (think of a camera on a tripod). If the objects are moving, this limits the observation time, as the objects move out of the field of view. In a microscope, this is a particularly severe problem when studying moving objects like swimming cells, or freely-diffusing cluster of colloidal beads. What we've done with the PLuTARC (Peter Lu Target-Locking Acquisition in Real-time Confocal) system is to implement target-locking.

Colloidal phase transitions on the Int'l Space Station with GPGPU

Under normal conditions on earth, the separation between liquids and gases is usually a bit mundane: the level of water in a drinking glass simply falls as the liquid evaporates into a gas, since the liquid is denser. Two liquids of different densities, such as the oil and vinegar of salad dressing, are similarly unexciting; the oil just floats to the top after a period of time. However, when the effects of gravity are taken away, such as in the environment of the International Space Station (ISS), this separation creates far more interesting and complex patterns.

Physics of Attractive Colloids

Colloids are microscopic particles so small that they move diffusively when dispersed in a fluid, exhibiting Brownian motion, controlled by the temperature of the system, like atoms. However, unlike atoms, colloids are big enough to see with light, so they can be probed with microscopes and laser light scattering. The interactions between atoms are fixed, dictated by quantum mechanics, but those in colloids can be very finely tuned.

Incompleteness of the Fossil Record affects Biodiversity

The fossil record is clearly incomplete; the vast majority of organisms die without leaving a trace. But does this have any effect on the big picture of large-scale biodiversity, and how it changes through time? Motohiro Yogo, Charles R. Marshall and myself showed that indeed this incomplete preservation has significant effects. Earlier work suggested that, following a mass-extinction in which a lot of species were wiped out, it took a long time to recover back to a high level of biodiversity, due to an unexplained "speed limit" on recovery.

First Use of Diamond

It has been long thought that man first used diamond in India, around the time of Christ, based on the record of documents from India in the latter half of the first millennium, BC. There certainly did not seem to be any evidence from earlier historical periods, let alone prehistoric times. However, we have uncovered evidence that the neolithic Chinese were using diamonds to polish a special group of ceremonial stone burial axes as early as 2500 BC, placing the earliest known use of diamond two thousand years before the mineral is known to have been used elsewhere.

Early Precision Compound Machines

Distinctive spiral grooves carved on ritual jade rings buried in tombs dating from China’s Spring and Autumn period (771–475 BC) follow a precise mathematical form described by the spiral of Archimedes, 300 years before he lived. I show that the precise drafting would have required a precision compound machine in 550 BC, making it the first machine to precisely interconvert linear and rotational motion by half a millennium, and propose a basic mechanical design relying only upon technologies known to have existed at that time.

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Peter J. Lu  |  Harvard University  |  Cambridge, MA 02138 USA |  plu_at_fas.harvard.edu