Experimental Soft Condensed Matter Group
Harvard University, Prof. David A. Weitz

Pictures & Movies from our group
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Visit the various "cool pictures" that we have found:

[particles] LEFT:Click here to see an animated GIF movie showing two micron diameter particles diffusing in water, and in a concentrated DNA solution. The movie shows 4 s of data.

RIGHT: We can track colloidal particles diffusing in 3D, click the picture to see an animated GIF movie of this. This image has been processed, but is from experimental data.

[picture]
(click here to see animated GIF)

Click here to see a slower-motion version of this movie (123K).
Two small water droplets in a nematic liquid crystal viewed through crossed polarizers in an optical microscope. The orientational elasticity of the liquid crystal results in an attractive interaction between the droplets, and this movie shows a simple scheme devised by Philippe Poulin, a former post doc in our group, to measure the nature of the attractive interaction.

The water droplets are filled with a ferrofluid, which is superparamagnetic. When a magnetic field is applied normal to the plane of observation, magnetic dipoles are induced in the droplets which repel, and force the droplets apart. When the field is removed, the attractive interaction pulls the droplets together. Since the motion is viscously damped, the attractive force is exactly balanced by the viscous drag. By measuring the velocity as a function of separation, we are able to determine the force-distance behavior, and show that it is dipolar in nature, as predicted for these structures in a nematic host. Read more about it.

Polystyrene balls (diameter 0.8 microns) trapped between two bilayers.
[picture]
(click here for larger picture, 98K)
[picture] A large drop of nematic liquid crystal which contains a small water droplet in the center. The small water droplet behaves as a hedgehog defect, trapped in the center of the larger nematic drop. The sample is viewed through crossed polarizers, and the cross pattern results from the orientation of the nematic liquid crystal. The anchoring conditions at all the surfaces ensure that the nematic molecules are normal to the interface.
Drops of nematic liquid crystal with small water droplets inside. The boundary conditions for the interface force the nematic molecules to align parallel to the surface, resulting to two Boojum defects at either side of the drops. This causes to water droplet to migrate to these defects. [picture]
(click here for larger picture, 38K)
[picture]
(click here for larger picture, 152K)
 Thin water film on a patterned surface; the periodicity of the pattern is 10 microns.
Diffraction pattern from a colloidal crystal made from PMMA spheres that have been indexed matched to the solvent. This is a binary colloidal alloy crystal, formed from a mixture of two different sizes of particles. The only interparticle interaction is volume exclusion, and the crystallization is purely entropic in origin. The crystal structure is AB13. [picture]
(Click here for larger picture, 40K)
[picture]
(Click here for larger picture, 183K)
 Liquid crystal emulsion droplets stuck to the hydrophobic patches of a hydrophilic surface.
A collage of pictures of multiple emulsions consisting of large drops of nematic liquid crystal with small water drops inside them. The orientational order of the nematic liquid crystal causes the water droplets to attract one another and causes them to form chains, as can be seen in the largest drop.
[picture]
(Click here for larger picture, 172K)
[picture]
(Click here for larger picture, 164K)
 Another collage of pictures of multiple emulsions consisting of large drops of nematic liquid crystal with small water drops inside them. The interaction between the water droplets in the nematic liquid crystal can be described as a dipolar attraction, which results in the formation of the linear chains of droplets seen in the largest nematic drop.
Another collage of pictures of multiple emulsions consisting of large drops of nematic liquid crystal with small water drops inside them. This picture appeared on the cover of the PENN TECH news in 1997.
[picture]
(Click here for larger picture, 214K)

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