Mechanoluminescence Induced by Acoustic Cavitation
When a liquid is irradiated with high intensity ultrasound, micron-sized bubbles can be driven into violent collapse with a subsequent shockwave formed in the liquid from the bubble rebound. Small particles suspended in ultrasonically irradiated liquids can be accelerated to high velocities (several hundred m/s) by these shockwaves and undergo violent interparticle collisions. Under these conditions we have observed intense mechanoluminescence from a variety of organic microcrystalline solids (up to 1000 times more intense than from grinding). The light observed is a microdischarge that is a result of the local separation of charged surfaces due to the fracture of noncentric crystals. We have observed extensive atomic and molecular emission lines, many of which have not been previously reported for mechanoluminescence (e.g. He+, C2, CH, CO, CO+, and CO2+). In addition, we used optical emission spectroscopy of the atomic and molecular species to characterize the electron density, electron temperature, and temperature of the resulting discharge.
Remembering Words Not Presented in Sentences: How Study Context Changes Encoding Strategies
People falsely endorse semantic associates (Roediger & McDermott, 1995) and morpheme rearrangements (Jones & Jacoby, 2001) of studied words at high rates in recognition testing. The co-existence of these results is paradoxical: Models of reading that presume automatic extraction of meaning can not account for elevated false memory for lures that are related to studied stimuli only by their visual form, and models without such a process can not account for false memory to semantic lures. I will present evidence that the different study contexts that have been used in previous experiments encourage different encoding modes and consequently elicit different types of false remembering. In my study, participants studied compound words presented in a list or embedded in sentences. Sentence contexts, which promote a strategy of greater extraction of meaning and rapid discarding of surface information, led subjects to be better able to discriminate conjunction lures ("tailgate") from old words than did list contexts. However, list contexts led to superior discrimination of semantic lures ("nosedive") from old words than did sentence contexts. Taken together, these results show how encoding strategies are influenced by study contexts, and how those strategies influence remembering and false remembering.
Spiropyrans as Color-Generating Mechanophores
Mechanically activated reactions are emerging as a complement to traditional activation methods that use light, electrical, or thermal energy. Previous efforts by our group have shown that mechanical force, generated by ultrasound, can be transmitted through polymer chains to a centrally-located, bis-polymer functionalized mechanophore in order to break chemical bonds in the mechanophore. In an effort to expand this work, we designed and synthesized a symmetrically polymerized spiropyran that was expected to undergo a 6π electrocyclic ring opening and consequently change color upon the application of stress. It was found that the spiropyran-linked polymer exhibited reversible color changes both in solution and in the solid-state when mechanically stressed. The solid-state color change occurred in two different polymers with glass transition temperatures both above and below room temperature. A mechanically inactive control spiropyran, functionalized with polymer on only one side, did not exhibit any mechanochromic properties. These results have interesting applications in both the chemical and engineering fields as stress-responsive, self-assessing materials that could be used for damage detection.