Tuesday, November 10, 2009

droplets for laser-driven fusion

11.09.09. Nuclear fusion is promising as a potential source of sustainable energy. When two deuterium nuclei (a deuterium nucleus consists of a proton bound to a neutron) fuse, for example, they can produce a Helium isotope (two protons bound to one neutron) and a bare high-energy neutron. The energy of these bare neutrons can be converted to consumable energy (for example by using the neutrons to heat water, produce steam, and drive turbines). Deuterium exists naturally, albeit rarely, in the form of deuterized water in the oceans, so the problem consists of extracting the deuterized water from the natural water and imbuing the deuterium nuclei with enough initial energy to fuse. A promising way of achieving this initial energy is by irradiating small droplets of the deuterized water with a powerful, fast-pulsed laser; in this case the diameter of droplet that maximizes fusion yield is in the 0.01 to 1 micron range. As an undergraduate I (Mr. Mugler) worked with Professor Tom Donnelly and other students at Harvey Mudd College to develop a controllable source of droplets of this size, an experience which featured prominently in my "path" (see previous post) from middle school to the present day. We used a piezoelectric oscillator to vibrate (at Megahertz frequencies) a column of fluid at its base, which produced from its surface an aerosol of micron-scale droplets whose diameter we characterized by measuring the scattering pattern of a laser shone through the aerosol (the diagram shown is from Donnelly et al, Phys Fluids, 2004). More recently, current Harvey Mudd undergraduates have brought the droplet source to the University of Texas at Austin, which houses one of the most powerful lasers in the world, and have successfully achieved laser-driven fusion with deuterized water droplets. For more information see a recent article in the Harvey Mudd College Bulletin.

Monday, November 9, 2009

path to college and beyond

11.09.09. Great scientific discoveries are often made rather incidentally by people blindly following their most passionate interests. The same can be true for great careers, scientific and otherwise. We took some time to encourage the students to think critically and specifically about their own interests, and how these interests might influence their choice of high school (many New York City public high schools are highly specialized and students must apply), college, and beyond. Students made flow charts showing their own projected paths through middle school, high school, college, and (potentially) grad school. Mr. Mugler and Mr. Seymour then shared their own charts detailing their windy paths. This activity generated many interesting questions from students, on topics ranging from green energy to laser-driven fusion to graduate study in dance.

Wednesday, November 4, 2009

laser tweezers

11.04.09. An optical trap (also called "laser tweezers") is an experimental setup in which the motion of a small (on the order of a micron in diameter) plastic bead is controlled by a laser beam. The refraction of light through the bead exerts an attractive force (on the order of several piconewtons) on the bead directed toward the center of the beam. Optical traps are useful for a variety of biophysical applications in which one wishes to move or manipulate a single molecule. For example, when the bead is affixed to the cargo end of a motor protein, the laser can be used to control the force against which the protein pulls, which can affect the protein's step frequency or step size (see previous posts). As a visual aid to the lessons below, students were presented with this video, taken by Columbia physics majors Dan Amrhein and Alex Kaz, which shows a fixed laser beam (lower-left corner, directed into the screen) alternately "trapping" and losing a 2-micron bead as the microscope stage is moved from side to side.

motor protein syncrony and LCM

11.04.09. The step length of a motor protein (see previous posts) is affected by external factors both deterministic, such as the force exerted by its cargo, and random, such as collisions with other particles in the cell (see, e.g., Clemen et al, Biophys J, 2005). We imagined two motor proteins with different step lengths starting at the same point on a filament, and we asked at what distance would they coincide once more? Viewing each protein as stepping along a number line, it is clear that the answer is the least common multiple (LCM) of their step sizes. Students practiced finding LCMs in the context of stepping proteins, and this led to a comparison of two very different techniques for finding the LCM: (1) listing out the multiples of each number explicitly and (2) finding the prime factorization of each number and multiplying the greatest common factor (GCF) by the unshared factors.