We first report that, for planar nematic MBBA, the electroconvection threshold voltage has a nonmonotonic temperature dependence, with a well-defined minimum, and a slope of about -0.12 V/degree near room temperature. Motivated by this observation, we have designed an experiment in which a weak continuous-wave absorbed laser beam with a diameter comparable to the pattern wavelength generates a locally supercritical region, or pulse, in dyedoped MBBA. Working 10-20% below the laser-free threshold voltage, we observe a steadystate pulse shaped as an ellipse with the semimajor axis oriented parallel to the nematic director, with a typical size of several wavelengths. The pulse is robust, persisting even when spatially extended rolls develop in the surrounding region, and displays rolls that counterpropagate along the director at frequencies of tenths of Hz, with the rolls on the left (right) side of the ellipse moving to the right (left). Systematic measurements of the samplevoltage dependence of the pulse amplitude, spatial extent, and frequency show a saturation or decrease when the control parameter (evaluated at the center of the pulse) approaches ∼ 0.3.We propose that the model for these pulses should be based on the theory of control-parameter ramps, supplemented with new terms to account for the advection of heat away from the pulse when the surrounding state becomes linearly unstable. The advection creates a negative feedback between the pulse size and the efficiency of heat transport, which we argue is responsible for the attenuation of the pulse at larger control-parameter values.

G el electrophoresis, used by geneticists and forensic experts alike, is an immensely popular technique that utilizes an electric field to separate molecules and proteins by size and charge. At the microscopic level, a dye or complex protein like DNA is passed through agarose, a gelatinous three-dimensional matrix of pores and nano-sized tunnels. When forced through a maze of holes, the molecule unravels, forming a long chain, slithering through the field of pores in a process colloquially coined "reputation." ' As a result, the smaller molecules travel farther through the gel when compared to molecules of larger molecular weight. This highly effective "molecular sieve" provides consistent data and allows scientists to compare similar sequences of DNA base pairs in a routine fashion.^ When performed at the high school level, gel electrophoresis provides students the opportunity to learn about a contemporary lab technique of great scientific relevance. Doing real science certainly excites students and motivates them to learn more.In many cases, students learn how to use a micropipette, create an agarose gel, and (if all goes to plan) have the opportunity to actually see dyes move across the gel. Instructors typically provide a qualitative explanation (the electric field/ current moves the molecules; the smaller the molecule, the faster and farther it moves across the gel, for example) or an explanation that is beyond the scope ofthe class.A common misconception held by students (and instructors alike) is that the electric field provides a force that continuously accelerates a charged molecule across the gel; this explanation is correct to some extent, but fails to explain the role ofthe molecules drag force and subsequent terminal velocity.^ In gel electrophoresis, the drag force plays a dominant role in the early stages ofthe dynamics, and, as a consequence, the molecule has no net acceleration through the duration of the experiment. Such details are trivial when introducing this lab to students at a conceptual level. Excited as the students are, many are left with little to no opportunity to explore on their own, and ultimately follow a rigid sequence of procedures that does little more than verify that the process actually works.This paper proposes an inquiry-based method to investigate the robust dynamics of gel electrophoresis at the high school level. The goal is to motivate students to develop a working kinematic model and to test the model experimentally. Such an exercise works best in an AP Physics classroom, at a point in the course when concepts in electrostatics, specifically dealing with charges and electric forces and fields, are fresh in students' minds. Student misconceptionsPrior to performing the experiment, it is important to address three common misconceptions (dealing with electric fields and forces) held by students and how this exercise may address these problems. As outlined by Randall Knight:'* The three-dimensionality of electric fieldsStudents are introduced to the concept ofthe electric...

We report a liquid-crystal pattern-formation experiment in which we break the lateral (translational) symmetry of a nematic medium with a laser-induced thermal gradient. The work is motivated by an improved measurement (reported here) of the temperature dependence of the electroconvection threshold voltage in planar-nematic 4-methoxybenzylidene-4-butylaniline (MBBA). In contrast with other broken-symmetry-pattern studies that report a uniform drift, we observe a strip of counterpropagating rolls that collide at a sink point, and a strong temporally periodic amplitude modulation within a width of 3-4 rolls about the sink point. The time dependence of the amplitude at a fixed position is periodic but displays a nonsinusoidal relaxation-oscillation profile. After reporting experimental results based on spacetime contours and wavenumber profiles, along with a measurement of the change in the drift frequency with applied voltage at a fixed control parameter, we propose some potential guidelines for a theoretical model based on saddle-point solutions for Eckhaus-unstable states and coupled complex Ginzburg-Landau equations. Published in PRE 73, 036317 (2006).