Studying the effect of high powered lasers on material phase changes
Phase changes are an important part of chemistry and materials science. From the food we eat and the medicine that we use, to the renewable energy devices we wish to implement, phase changes play a big role. Usually, phase changes such as nucleation or crystallization can take days or weeks, uses a labor intensive process, and does not form pure or controlled new phase. That's why studying the formation of new phases within materials is so important. We do so with lasers! Using these lasers, in nanoseconds rather than days, we are able to form higher purity, 3-D controlled crystals.
Watch a presentation of my work below (8m:48s), or keep scrolling to read about it!
Non-Photochemical Pulsed-Laser-Induced Nucleation in a Continuous-Wave-Laser-Induced Phase-Separated Solution Droplet of Aqueous Glycine Formed by Optical Gradient Forces
If an optical tweezer, a tightly focused laser beam that acts as a non-mechanical tweezer for small particles, is applied to a droplet of a glycine and heavy water solution, then a new liquid phase, or a droplet within a droplet, is formed. This new droplet has some interesting properties: it sticks around for minutes after turning off the optical tweezer, its estimated supersaturation is well above the metastable zone but it doesn't crystallize, and it does not immediately form a crystal when the phase disappears.
By observing this new glycine liquid droplet phase after shooting with a single pulse of a high powered laser and inducing nucleation for 100 minutes (using a Non-Photochemical Laser Induced Nulceation technique), our research shows that this new liquid phase might consist of a concentration gradient of liquid like clusters sizes, with the largest clusters closer to the focal point.
Included: Video 1) 100 minute time-lapse of Laser Induced Phase Separated Droplet dissolving. 2) 100 minute time-lapse of Laser Induced Phase Separated Droplet after it was shot by high powered pulse laser. 3) The optical setup used. 4) Graphics of the experiments are also outlined.
Microfluidic Laser-Induced Nucleation of Supersaturated Aqueous KCl Solutions
The main bottleneck in Non-Photochemical Laser Induced Nucleation is the preparation of the solution. Seeing that each solution must have a minimum supersaturation in order to undergo NPLIN, one way to increase the saturation is to supercool the solution with a microreactor!
Diagram describing setup of microreactor setup, resulting KCl crystal, and the difference in crystal number yield vs saturation of solution!
Dendritic Growth of Glycine from Nonphotochemical Laser-Induced Nucleation of Supersaturated Aqueous Solutions in Agarose Gels
Control over the shape and location of a crystal in a solution is a difficult objective. In order to enhance the control of the crystals one can "hold" the forming crystal by forming it in a gel. We've shown that one can shoot a high powered, pulsed laser at a supersaturated solution in a gel and form a shaped crystal in a specific location (using NPLIN techniques).
Included: Video of supersaturated glycine in water solution (S=1.5) mixed with agarose gel being shot by a high powered pulse beam to induce NPLIN in the beam path. Check out the stellar dendrite (star shaped crystal)!
What is NPLIN?
What happens when you shoot a high powered laser at a supersaturated solution? Depending on the solution, crystals could form! The question is why? That's why we study Non-Photochemical Laser Induced Nucleation (NPLIN). We want to understand why do crystals form in the path of lasers shot at the solution (aka beampath) using different mechanisms.
In 1996, NPLIN was discovered by shooting an unfocused near infrared laser at an intensity roughly a trillion times more than a laser pointer (0.4 GW/cm vs 0.5 W/cm ) into a supersaturated solution of urea (an amino acid). Here is a video, in real time, that shows what happens when you do just that!
It was found that you can use this technique to form highly pure crystals in a fraction of the time it would take using other methods (seconds rather than weeks), and that you have more control over the shape, type, and purity of the crystal! Seeing that crystals are a critical part of the chemical industry (i.e. pharmaceuticals, cosmetics, dyes), this level of control is paradigm shifting! Through controlling the supersaturation levels of the solution, the intensity of the laser, the polarization of the beam, or even the solvent of the solution you can have reproducible, fast, and high control over your crystals.
A few NPLIN techniques were founded and several materials where found to be susceptible to the crystallization technique. 11 years later, it was discovered that you can also nucleate a glycine crystal in a thin
film of glycine/heavy water solution by applying laser tweezer, by tightly focusing a laser through the lens of a microscope, at the solution/air interface of the droplet.