Session E01: General Experimental Physics

Experimental and Numerical Studies on the Projective Dye Visualization Velocimetry in a Squared Vertical Tube

In fluid flow experiments, there have been numerous techniques developed over the years to measure velocity. Most popular techniques are non-intrusive such as particle image velocimetry (PIV), but these techniques are not suitable for all applications. For instance, PIV cannot be used in examining in-vivo measurements since the laser is not able to penetrate through the patient, which is why medical applications typically use X-rays. However, the images obtained from X-rays, in particular digital subtraction angiography, are projective images which compress 3D flow features onto a 2D image. Therefore, when intensity techniques, such as optical flow method (OFM), are applied to these images the accuracy of the velocity measurements suffer from the 3D effects. To understand the error introduced in using projective images, a vertical square tube chamber was constructed to achieve multiple water flow rates with various dye injection points to capture dye visualization images. Two experiments were conducted with the dye inlet either in the center or against the tube’s wall. These images were then evaluated with OFM and compared with PIV measurements to quantify the error associated with dye visualization velocimetry. Results from dye visualization were comparable with PIV, but a correction method, more specifically the Levenberg-Marquardt algorithm, was needed to adjust the OFM results. Corrections matched well with the baseline velocity values with relatively low valued least-squares. CFD simulations were compared with PIV measurements and dye visualization images to validate proper boundary conditions and meshing. CFD simulation results provided more insights of the flow characteristics of the dye diffusion in the laminar vertical tube flow.   

A 3D Printable Microscope With Dynamic Pattern Illumination

We explored the utility of a video projector as the source of dynamic pattern illumination for a 3D printable microscope in conjunction with a Raspberry Pi module and camera system. This setup provides a low-cost solution in achieving different contrast-enhancement modalities, including dark-field, Rheinberg illumination, oblique illumination, and polarization microscopy. In commercially available microscopes, these methods are possible through the use of specialized physical filter attachments that typically add to the cost of the microscope. With our microscope design, the physical filters are replaced by projecting an illumination mask/pattern on the back focal plane of the microscope’s condenser lens. Our design also provides for ease in switching from one contrast-enhancement method to another with no optical re-alignment, as this can be done with a click of the button on a PC. We will demonstrate that in using the different illumination patterns with varying colors it is possible to highlight and optically stain the details of an unstained, transparent specimen. Furthermore, we will discuss how our system produces color-coded information regarding the birefringence of our samples within a single image.   

Manipulating random lasing correlations in liquid crystals doped with laser dyes and plasmonic nanoparticles

Understanding the correlations between light modes scattered from disordered media is intimately tied to developments in imaging capabilities and information science. In this study, liquid crystals, infiltrated with gold nanoparticles and laser dye (pyrromethene 597), were utilized to generate random lasing action. By varying the incident pump energy, we show that it is possible to exert control over the correlations of the random lasing modes. We use the Lévy tail of the emission spectra's survival function to phenomenologically characterize the extent of correlations in the system. The results from this experimental work demonstrate the possibility to control a key physical feature of a random laser. We will discuss the existence of potential interplay between persistence and correlations in random lasing and how these complex photonic media may provide insights into novel information transfer protocols.   

Experimental estimate of atomic recoil in a warm vapor

We compare the centers of the doppler-broadened absorption peaks in the 85Rb F=2 line in a (near) room temperature vacuum cell at different optical powers.

This indicate a systematic intensity-dependent shift associated with the light accelerating the atoms away from the light source. We describe the underlying physics, the experiment, the non-saturated and saturated limits of this process and a simple analytical approach we use to extract an estimate of the atomic recoil velocity.   

Colored Percolation Theory

Colored Percolation is a generalization to conventional percolation schemes where occupied sites are colored with distinctive colors. A bond between two sites is open if the incident vertices are occupied and are of the same color. . Our goal for this project was to investigate the phenomena in three-dimensional lattices. We started with finding unique diagram configurations for every order and their respective perimeters. For the first time, we applied analytic diagram expansion for a multicomponent scenario. We employed Dlog-Pade approximants to analyze the perturbative analytical serious and calculate the transition temperatures as well as critical site occupancy probabilities.   

Towards Scanning Tunneling Microscopy studies of ultrafast laser damage

 Nowadays, femtosecond lasers are widely used, because of their short pulse duration which can be used to study reactions at the atomic level. Based on former thermal ablation research using femtosecond lasers, we start to explore surface morphology caused by femtosecond laser damage at the atomic scale using Scanning Tunneling Microscopy (STM). We perform atomic-resolution STM in Ultrahigh vacuum (UHV) which means there is a protective environment for the surface, especially for semiconductor surfaces which are more easily contaminated by air. 

 Challenges for combining a femtosecond laser with STM are the timescale resolution for material dynamics and locating the laser damage spot due to the STM’s sample rate and limited scan area. 

  In our recent work, we designed a custom UHV chamber, which can realize sputtering and annealing with our self-built electron-beam-heater (eheater) at one spot. Additionally, we designed a mount for an in-situ objective lens to tightly focus light onto the sample. To test the STM capability for locating laser damage areas, we have performed initial studies on Si(100) substrates, cleaned by immersion in HF. STM images of the control Si(100) sample show an atomically flat surface with a low coverage of surface contamination. These data will be compared with STM images of a laser-irradiated sample to identify laser damage areas.   

Waves on the tongue

Geographic tongue (GT) is a benign condition of unknown cause, characterized by spatio-temporal chronic lesions on the surface of the human tongue. The condition's appearance with propagating wave fronts of various forms can be modeled with two coupled, nonlinear reaction-diffusion (RD) equations. Here we present a realistic model, using the curvature dependent speed of RD waves, to explain the temporal evolution of GT patterns. We also compare our simulations with experimental time-series.