Planar granular shear flow under external vibration.

We present results from a planar shear experiment in which a two-dimensional horizontal granular assembly of pentagonal particles sheared between two parallel walls is subjected to external vibration. Particle tracking and photoelastic measurements are used to quantify both grain scale motion and interparticle stresses with and without imposed vibrations. We characterize the particle motion in planar shear and find that flow of these strongly interlocking particles consists of transient vortex motion with a mean flow given by the sum of exponential profiles imposed by the shearing walls. Vibration is applied either through the shearing surface or as bulk vertical vibration of the entire shearing region with dimensionless accelerations Γ=A(2πf)^{2}/g≈0-2. In both cases, increasing amplitude of vibration A at fixed frequency f leads to failure of the force network, reduction in mean stress, and a corresponding reduction in imposed strain. Vibration of the shearing surface is shown to induce the preferential slipping of large-angle force chains. These effects are insensitive to changes in frequency in the range studied (f=30-120 Hz), as sufficiently large displacements are required to relieve the geometrical frustration of the jammed states.

Detection of deoxyribonucleic acid (DNA) targets using polymerase chain reaction (PCR) and paper surface-enhanced Raman spectroscopy (SERS) chromatography.

Surface-enhanced Raman spectroscopy (SERS) enables multiplex detection of analytes using simple, portable equipment consisting of a single excitation source and detector. Thus, in theory, SERS is ideally suited to replace fluorescence in assays that screen for numerous deoxyribonucleic acid (DNA) targets, but in practice, SERS-based assays have suffered from complexity and elaborate processing steps. Here, we report an assay in which a simple inkjet-fabricated plasmonic paper device enables SERS-based detection of multiple DNA targets within a single polymerase chain reaction (PCR). In prior work, we demonstrated the principles of chromatographic separation and SERS-based detection on inkjet-fabricated plasmonic paper. The present work extends that capability for post-PCR gene sequence detection. In this design, hydrolysis DNA probes with 5' Raman labels are utilized; if the target is present, the probe is hydrolyzed during PCR, freeing the reporter. After applying the PCR sample to a paper SERS device, an on-device chromatographic separation and concentration is conducted to discriminate between hydrolyzed and intact probes. SERS is then used to detect the reporter released by the hydrolyzed probes. This simple separation and detection on paper eliminates the need for complex sample processing steps. In this work, we simultaneously detect the methicillin-resistant Staphylococcus aureus genes mecA and femB to illustrate the concept. We envision that this approach could contribute to the development of multiplex DNA diagnostic tests enabling screening for several target sequences within a single reaction, which is necessary for cases in which sample volume and resources are limited.

Highly sensitive and flexible inkjet printed SERS sensors on paper.

Surface enhanced Raman spectroscopy (SERS) has the potential to be utilized for the detection of a broad range of chemicals in trace quantities. However, because of the cost and complexity of SERS devices, the technology has been unable to fill the needs of many practical applications, in particular the need for rapid, portable, on-site detection in the field. In this work, we review a new methodology for trace chemical detection using inkjet-printed SERS substrates on paper. The detection performance of the inkjet-printed SERS devices is demonstrated by detecting 1,2-Bis(4-pyridyl)ethylene (BPE) at a concentration as low as 1.8 ppb. We then illustrate the primary advantages of paper SERS substrates as compared to conventional SERS substrates. By leveraging lateral flow concentration, the detection limit of paper SERS substrates can be further improved. Two real-world applications are demonstrated. First, the inkjet-printed SERS substrates are used as "dipsticks" for detecting the fungicide malachite green in water. Then, the flexible paper-based SERS devices are used as swabs to collect and detect trace residues of the fungicide thiram from a surface. We predict that the combination of ultra-low-cost fabrication with the advantages of easy-to-use dipsticks and swabs and the option of lateral flow concentration position ink-jet printed SERS substrates as a technology which will enable the application of SERS in solving critical problems for chemical detection in the field.

Demonstration of the coupling of optofluidic ring resonator lasers with liquid waveguides.

Optofluidic lasers are of particular interest for lab-on-a-chip-type devices, with broad spectral tunability, convenient microfluidic integration, and a small footprint. Optofluidic ring resonator (OFRR) lasers are advantageous in terms of size but typically generate nondirectional emission that is of minimal practical use. We introduce two unique geometries for soft-lithography-based OFRR lasers--side-coupled rings and spiral rings--both of which can be produced in polydimethyl siloxane substrates with contact molding. These rings utilize evanescent and direct butt-coupling, respectively, to effectively couple the OFRR laser emission into microfluidic channels. A laser threshold of a few to tens of microJ/mm(2) is achieved.