Anomalous diffusion of E. coli under microfluidic confinement and chemical gradient.

We report a two-layer microfluidic device to study the combined effect of confinement and chemical gradient on the motility of wild-type E. coli. We track individual E. coli in 50 µm and 10 µm wide microchannels, with a channel height of 2 µm, to generate quasi-2D conditions. We find that contrary to expectations, bacterial trajectories are superdiffusive even in the absence of a chemical (glucose) gradient. The superdiffusive behaviour becomes more pronounced upon introducing a chemical gradient or strengthening the lateral confinement. Run length distributions for weak lateral confinement in the absence of chemical gradients follow an exponential distribution. Both confinement and chemoattraction induce deviations from this behaviour, with the run length distributions approaching a power-law form under these conditions. Both confinement and chemoattraction suppress large-angle tumbles as well. Our results suggest that wild-type E. coli modulates both its runs and tumbles in a similar manner under physical confinement and chemical gradient. Our findings have implications for understanding how bacteria modulate their motility behaviour in natural habitats.

Development of a PNA-DiSc2 based portable absorbance platform for the detection of pathogen nucleic acids.

We report the development of a portable absorption (PortAbs)-based pathogen nucleic acid detection system using peptide nucleic acid (PNA) and a cyanine dye, DiSc2(5). When the dye binds to the PNA-DNA hybrid, it results in a characteristic ∼110 nm shift in the dye absorbance, which we measure using PortAbs. The protocol involves amplification of the target DNA, PNA-DNA hybridization and dye complexing steps followed by absorption measurement. The system is built using a broad-spectrum photodiode whose output is amplified and then measured by a high resolution (24 or 32 bit) analog-to-digital converter. The excitation pulses of light are delivered by a color-changing LED. The sequence of excitation, measurement and display of results are all controlled by an embedded Raspberry-Pi board (or alternatively a laptop). At higher concentrations of the target amplicon (∼200 ng), the color change can be detected visually. At lower concentrations, PortAbs outperforms a plate reader and can detect target DNA as low as 30 ng or approximately 10 nM which is at least 10 fold better than previously reported studies. We validate the methodology using SARS-CoV-2 clinical samples containing about 1000 copies of the viral RNA and show that the entire workflow takes about 90 min. The cost of the complete standalone system is less than INR 40 000 (approx. 500 USD).

Drop impact on thin powder layers: pattern formation by air entrapment.

Impact of drops on thin powder layers displaces the powder particles radially outward producing shallow craters with thick rims, for example, as observed on dust layers on the floor. Here, we report that the patterns formed on thin powder layers by drop impact are not limited to such crater-like ones. Instead, depending upon the layer properties, disc or disc-plus-ring shaped patterns are formed at the impact point. We show that air entrapment and micro-bubble formation during the drop impact result in the formation of such patterns. Based on high-speed imaging, scaling analyses, and measurements with various liquids and powder layers, we propose a mechanism for the formation of such patterns. The phenomenon that we report can open further investigations on drop impact on the granular matter.