Meanwhile in the Whitesides lab

24 September 2011 - Part II

Whitesides science 2011.gifAfter our virtual visit to the Whitesides lab in 2010 here let's find out what is new from Harvard in 2011.
Take a piece of transparent and grated polyester film, apply a platinum coating on top and stick double-sided tape on the bottom. Then render the top layer hydrophilic by a 3-mercapto-1-propanesulfonic acid coat and the bottom hydrophobic with teflon. Apply iron powder to the bottom using the double-sided tape. Cut up the film into tiny squares (2x2 mm) and set them afloat in a petri dish filled with perfluoro-1-methyldecalin and water. What do you get? Tang et al. (DOI) used this recipe and in their petri dish the floating tiles behaving as reflective Diffraction grating elements, all lit up, each with a different color. In addition to that, due to the iron cargo, the tiles could be steered using an magnetic field. When water was replaced by an agarose solution, a specific tile configuration could be preserved by gel formation. PDMS was then pored over the surface and the agarose removed by melting. What do you then get? A fancy bracelet!.

And what about biomimetic hair? In deep reactive-ion etching the combination of sulfur hexafluoride and a plasma results in a directed beam of radicals that can eat away the surface of a silicon wafer not protected by a resist. In this way you can punch nanometer-size holes or trenches into the silicon. To prevent wall collapse etching is periodically replaced with passification: adding a teflon layer by introducing gaseous octafluorocyclobutane. This protects the walls more than the bottom of the hole resulting in deeper trenches. Kim et al. (DOI) used such a wafer or rather its mirror image made from a UV-epoxy resin cast. In SEM images the surface of this epoxy resin is covered with a forest of coin stacks or hairs. These hairs were then covered in a layer of platinum (sputtering) and then in a layer of polypyrrole by pyrrole electrodeposition: STEPS or structural transformation by electrodeposition on patterned substrates. And what to do with this kind of devices? In one demonstration a distance gradient was engineered between the pillars and then bacteria were set loose on the surface. The way they deal with available surface space will determine where they congregate.

The Whitesides group has a continuing interest in Magnetic levitation. Latest innovation: component assembly in 3D that does not require mechanical contact. ( Mirica et al. DOI). Various objects are suspended simultaneously between two magnets. The relative horizontal position can be manipulated by the density of each object, the strength of the magnetic field and the density of the liquid medium. One demonstration concerned a suspended optical table. Sounds like a solution of the age-old thread-in-needle problem is just around the corner.

And on an entirely different topic. Do thunderstorms trigger the nucleation of ice? only one way to find out. Stan et al. (DOI) built themselves a tabletop contraption allowing water droplets traverse a microchannel in a complex carrier gas while cooling and at the same time being exposed to an electric field. Visually observed (video) darkening of the droplets signalled the onset of freezing. A strong enough field the researchers say should be able to align the water molecules in a way that triggers freezing but after much experimenting the inevitable conclusion was a negative one.

By the way, back in January Whitesides (together with John Deutch) had thing to say about the future of chemistry in an open comment to Nature. The message: do away with the traditional scientific disciplines and get serious about integrated research. Here is a reminder of some interesting quotes: academic chemistry is overpopulated (...) and produced too few new ideas and too many average scientists (...) or jobs and Many subdisciplines of chemistry still use an apprenticeship model in which a professor conceives the problem and strategy, and graduate students execute the bench work. It is hard to imagine a worse way to prepare tomorrows chemists to work at the integration of many disciplines