Siddharth Maheshwari

Rapid bioparticle concentration and detection by combining a discharge driven vortex with surface enhanced Raman scattering

Rapid concentration and detection of bacteria in integrated chips and microfluidic devices is needed for the advancement of lab-on-a-chip devices because current detection methods require high concentrations of bacteria which render them impractical. We present a new chip-scale rapid bacteria concentration technique combined with surface-enhanced Raman scattering (SERS) to enhance the detection of low bacteria count samples. This concentration technique relies on convection by a long-range converging vortex to concentrate the bacteria into a packed mound of 200 mum in diameter within 15 min. Concentration of bioparticle samples as low as 10(4) colony forming units (CFU)ml are presented using batch volumes as large as 150 mul. Mixtures of silver nanoparticles with Saccharomyces cerevisiae, Escherichia coli F-amp, and Bacillus subtilis produce distinct and noticeably different Raman spectra, illustrating that this technique can be used as a detection and identification tool.

Evaporative Self-Assembly from Complex DNA : Colloid Suspensions

Evaporation-induced pattern formation has attracted considerable attention as a simple yet versatile method for generating self-assembled structures that have broad applications from photonic devices to biomacromolecular recognition. Previous study of evaporative self-assembly has mainly focused on single nonvolatile component systems, and the driving mechanisms have been extensively investigated. In contrast, pattern formation from evaporating multicomponent systems, despite its wide existence in nature and numerous engineering applications, has been rarely explored. In this work, we examine a DNA-colloid binary suspension as a model system to understand the evaporation-induced interfacial hydrodynamics and self-assembled morphology in multicomponent systems involving complex competing intermolecular and interfacial interactions. Direct microscopic observations show that the composition of the binary system plays a critical role in the multiple-ring formation upon evaporation: (1) suspensions with high DNA concentrations and low colloidal concentrations favor the formation of the multiple-ring pattern; (2) the size of colloidal particles added into DNA aqueous droplets can significantly disrupt smooth multiple rings to form rippled rings and curtain-like periodic patterns with a curious spoke-like structure as the size of colloidal particles increases; and (3) the enhancement of DNA-colloid interaction by oppositely charged colloidal particles results in considerably high irregularity of DNA stain ring spacing. We examine the disruption of the multiring morphology under varied conditions and attribute it to local hydrodynamics governed by colloid aggregation and sedimentation. Our results demonstrate the feasibility of fabricating periodic self-assembled hybrid structures via one-step evaporation of droplets consisting of multiple components.

Anomalous conical menisci under an ac field-departure from the dc Taylor cone

Observation of steady conical menisci with longitudinal growth is reported for electrospraying under high frequency alternating current (ac) electric fields. The authors report conical shapes that are analogous to the well-known direct-current (dc) cone-jet mode in the occurrence of conical menisci, but have some very distinctively ac features. The ac cones show a unique longitudinal growth that result in very high aspect ratio cones. The cone half angle for ethanol is approximately 9° for the ac case as compared to 47° for the dc case. These dissimilarities point to different mechanisms for dc and ac sprayings.

Effects of bulk charge and momentum relaxation time scales on ac electrospraying

The behavior at the liquid meniscus under the application of a high voltage (>4×103Vp-p), high frequency (>10kHz) alternating current (ac) signal is shown to depend very sensitively on the liquid conductivity and the applied frequency. For pure ethanol, after resonance effects become unimportant, the meniscus oscillates and then changes to a growing conical mode with increasing frequency. For ionic-liquid doped samples with higher conductivities, however, this transition to an oscillating mode and a conical mode is delayed and additional tip-streaming and elongated-fast dripping modes are seen. A linear proportionality between the transition frequency and the sample conductivity indicates that the critical period is approximately 20 times the bulk charge relaxation time scale; the mode transition takes place only when a specific and universal fraction of charge in the liquid bulk is unrelaxed. This observation for ac cones is in contrast with direct current cones with complete charge relaxation in the bul...