Strange Paths

Liquid surfaces are pulled by the intermolecular forces, which are unbalanced on the boundary, producing surface tension. When liquid layers with different surface tension get in contact, these forces cause a flow, also known as Marangoni effect,¹ which is also the origin of the beautiful patterns found in the ancient japanese art of Suminagashi (”floating ink”). In this image, a film of oleic acid surfactant (with surface tension 32.5 mN/m) quickly spreads spontaneously about 2.5 mm over a layer of glycerol (with surface tension 63.4 mN/m). Both Marangoni and capillary stresses cause variations in the film thickness, leading to dendritic flow patterns. The contour lines are interference fringes.

Click image to zoom²

1. C. Marangoni, “Über die Ausbreitung der Tropfen einer Flüssigkeit auf der Oberfläche einer anderen”, Ann. Phys. Leipzig, 143:337-354 (1871). [↩]
2. © B. J. Fischer, A. A. Darhuber, S. M. Troian, Department of Chemical Engineering, Princeton University [↩]

Terrestrial clouds are the result of extraordinarily complex interactions between water and air, with several feedback mechanisms combining the effects of fluid dynamics and thermodynamics.¹

Click image to zoom²

The kind of convective clouds known as cumulus are produced by the vertical winds occurring in regions of warm moist air, according to Archimedes principle. The rapid lifting results in adiabatic expansion and cooling, and consequent accretion of water droplets. The irregular distribution of droplets scatters sunlight geometrically in all directions, producing a bright white appearance like in snow, decaying into gray shades as per their optical thickness. Each cloud is short-lived, lasting approximately 15 minutes in average.

1. H. R. Pruppacher, J. D. Klett, “Microphysics of clouds and precipitation“, Springer (1997); R. A. Houze, “Cloud Dynamics“, Academic Press (1994) [↩]
2. © 2004 Sarah Robinson, Flow Visualization Course, University of Colorado [↩]

Animation showing the interaction of four charges of equal mass¹, two positive and two negative, in the approximation of classical electromagnetism. The particles interact via the Coulomb force, mediated by the electric field represented in yellow. A repulsive “Pauli force” of quantum mechanical origin, which becomes very large at a critical distance of about the radius of the spheres shown in the animation, keeps the charges from collapsing into the same point. Additionally, the motion of the particles is damped by a term proportional to their velocity, allowing them to “settle down” into stable (or meta-stable) states.

When the charges are allowed to evolve from the initial state, the first thing that happens (very quickly, since the Coulomb attraction between unbalanced charges is very large) is that they pair off into dipoles. Thereafter, there is still a (much weaker) interaction between neighboring dipoles (van der Waals force). Although in principle it can be either repulsive or attractive, there is a torque that rotates the dipoles so that it is attractive, eventually bringing the two dipoles together in a bound state. This mechanism binds the molecules of some substances into a solid.

1. © 2004 MIT TEAL/Studio Physics Project, John Belcher [↩]

Using computer animation¹ based on molecular research² it is possible to see how DNA is actually copied in living cells. This animation shows the “assembly line” of biochemical machines which pull apart the DNA double helix and output a copy of each strand. The DNA to be copied enters the whirling blue molecular machine, called helicase, which spins it as fast as a jet engine as it unwinds the double helix into two strands. One strand is copied continuously, and can be seen spooling off on the other side. Things are not so simple for the other strand, because it must be copied backwards, so it is drawn out repeatedly in loops and copied one section at a time. The end result is two new DNA molecules.

1. Drew Berry, “DNA animation”, The Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia (courtesy of the author). © 2007 Howard Hughes Medical Institute [↩]
2. T. A. Baker, S. P. Bell, “Polymerases and the Replisome: Machines within Machines“, Cell, 92:295-305 (1998); K. P. Lemon, A. D. Grossman, “Movement of Replicating DNA through a Stationary Replisome“, Molecular Cell, 6, 6:1321-1330 (2000); M. R. Singleton, M. R. Sawaua, T. Ellenberger, D. B. Wigley, “Crystal structure of T7 gene 4 ring helicase indicates a mechanism for sequential hydrolysis of nucleotides“, Cell 101:589-600 (2000); D. S. Johnson, L. Bai, B. Y. Smith, S. S. Patel, M. D. Wang, “Single-Molecule Studies Reveal Dynamics of DNA Unwinding by the Ring-Shaped T7 Helicase“, Cell 129, 7:1299-1309 (2007). [↩]

In 1610 Galileo published the astonishing report of his first telescope observations,¹ containing detailed drawings of the moon surface and his discovery of four “planets” orbiting around Jupiter (now known as the “Galilean Moons”). About two years later, he wrote an even more precise observation² with more than a hundred drawings of their relative daily positions. This animation³ brings back life to Galileo’s observation, as made in Florence, March 1613.

1. G. Galilei, “Sidereus Nuncius” (”The Starry Messenger“) (1610) [↩]
2. G. Galilei, “Istoria e Dimostrazioni intorno alle Macchie Solari” (”The Sunspot Letters to Marc Welser”) (1613) [↩]
3. Massimo Mogi Vicentini, © Planetario di Milano, Italy [↩]

The genetic information of all living organisms is encoded into long sequences of four molecular symbols, structured like the steps of giant DNA ladders named chromosomes. Human cells contain two sets of 23 chromosomes, each having 50 to 250 millions symbols or base pairs for a total of 3 billions, like a book of one million pages written in a mostly unknown language. In this image, a short excerpt from human chromosome 20, which has 63 644 868 base pairs, is represented by using the letters A C G T and dots for apparently unused sections.

Click image to zoom¹

1. © Ben Fry, Computation Group MIT Media Lab [↩]

Timelapse photo of the total lunar eclipse of October 27, 2004.¹ Celestial bodies orbiting around a star cast shadows, which may partially or totally obscure other bodies aligned behind them, “eclipsing” the star from their viewpoint (from Greek ekleipein, “failing to appear”).

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Given their short duration, eclipses are amongst the phenomena where cosmic scale dynamics may be perceived most dramatically. In the picture, the moon’s curved path is primarily due to Earth’s rotation, and to a small extent to the lunar motion in its elliptical orbit around the Earth. During the totality stage the Moon appears red, because Earth’s atmosphere scatters sunlight and only red wavelengths are refracted into the shadow. An observer on the moon would see a bright ring of red light, coming from all simultaneous Earth’s sunrises and sunsets.²

A total lunar eclipse will happen Saturday, March 3, 2007, and will be visible from Europe, Africa, Western Asia and Eastern America.

1. Picture © Forrest J. Egan, Digital Astro [↩]
2. Eclipse seen from the moon, Surveyor 3 mission, 24 April 1967 (artificial color) © NASA [↩]

Caustics (from the Greek kaustikos, kaiein, ‘to burn’) are geometrical entities formed by the singular concentration of curves, which model approximately the behavior of light rays focused by lenses or curved mirrors, leading to very bright regions when they encounter a surface. The light patterns at the bottom of swimming pools are examples of caustics, produced by the refraction on the wavy surface of water. In this computer image are discovered light caustics produced by two consecutive wavy surfaces, as if light was entering a second sea under the sea.

Click image to zoom¹

1. Digital Artwork © Eric J. Heller, Resonance Fine Art [↩]