The new switchable surface essentially consists of a forest of molecules only a billionth of a meter tall, engineered to stand at a precise distance from each other. In this particular case, the team makes the top of each molecule negatively charged and hydrophilic (water-loving), and the trunk positively charged and hydrophobic (water-repelling).
When a positive electrical potential is applied, the induced attractive force causes the top to bend down. The resulting loop that is now exposed is hydrophobic. Reverse the electrical potential and the molecules will straighten to their full height, the hydrophilic tops once more attracting water. Look at your hand, Langer explained. Imagine that your fingertips have property A and your knuckles have property B. Weve created a reversible way to move those fingers up and down, exposing either the fingertips or the knuckles with their different properties.
One important challenge for the team was finding a way to create a molecular forest, or self-assembled monolayer (SAM), with enough space between molecules to allow each to bend down. Conventional SAMs are characterized by very dense assemblies of molecules so tightly packed together that they have no room to move.
The MIT engineers solved the problem by adding bulky hats to each molecule during the assembly of the SAM, creating the equivalent of a field of molecular mushrooms. By then removing the hats, we ended up with a low-density monola
Contact: Elizabeth Thomson
Massachusetts Institute of Technology