Fog Harvesting with Highly Wetting and Nonwetting Vertical Strips.

Highly wetting and nonwetting substrates have been widely used in fogwater collection systems for enhanced water harvesting. In this work, fog harvesting substrates comprising PVC strips of different wetting properties and widths ranging from 1-5 mm were vertically aligned and spaced apart at regular intervals to give the same solid area fraction of 0.8. Evaluation of the water collection efficiencies of the tested configurations revealed that 1 mm wide superhydrophilic strips was the most efficient, achieving double the amount of water harvested compared with 2.8 mm wide strips. This finding was attributed to the low Stokes numbers of the aerosol particle distribution of the fog which tended to result in them being brought by the flow streamlines toward the air gaps between the strips. Stagnant flow regions at the edges of each strip, revealed through potential flow calculations, then caused higher liquid imbibition and impaction there for water harvesting. It was also found that the Cassie nonwetting substrates that originally exhibited contact angles of 161° transformed to Wenzel wetting with zero contact angle within 60 min of fog interception. Optical profilometry revealed no obvious difference in surface roughness between the central region and edges of the strips, indicating that surface morphology was unlikely to be a contributing factor for enhanced water collection at the edges. The findings here indicated that highly wetting vertical strip architectures with narrow widths (1 mm) were favorable over wider strips for water harvesting provided that clogging and re-entrainment were not significant factors.

Low-cost Imaging of Fluorescent DNA in Agarose Gel Electrophoresis using Raspberry Pi cameras.

Low-cost analytical solutions built around microcomputers like the Raspberry Pi help to facilitate laboratory investigations in resource limited venues. Here, three camera modules (V1.3 with and without filter, as well as NoIR) that work with this microcomputer were assessed for their suitability in imaging fluorescent DNA following agarose gel electrophoresis. Evaluation of their utility was based on signal-to-noise (SNR) and noise variance metrics that were developed. Experiments conducted with samples were subjected to Polymerase Chain Reaction (PCR), and the amplified products were separated using gel electrophoresis and stained with Midori green. Image analysis revealed the NoIR camera performed the best with SNR and noise variance values of 21.7 and 0.222 respectively. In experiments conducted using UV LED lighting to simulate ethidium bromide (EtBr) excitation, the NoIR and V1.3 with filter removed cameras showed comparable SNR values.

Cryoprotectant-free preservation of bacteria using semi-spherical drops.

Cryopreservation is a widely used long-term preservation method to ensure the quality and vitality of microbes in laboratories and biological resource facilities. However, freeze-thaw damage and osmotic pressure changes during cryopreservation adversely impacts microbial survival. Significant expenditure of resources and expertise are required to select the right cryoprotectant and optimize its concentration for maximum survival of diverse microorganisms. This work describes a cryopreservation method that obviates the need for cryoprotectants by exploiting the unique thermal characteristics of semi-spherical drops. Here, a plurality of these drops, each 10 µl in volume, created on a highly non-wetting flat-sheet substrate with holes and frozen at -70 °C. Deriving an f (x) metric as a measure of relative viability, storage in drops in the absence of cryoprotectants was found to improve the survivability of Staphylococcus epidermidis by 1.91 times compared with the same sample stored in larger 50-µl volumes in standard 1.5-ml tubes. This also compares well with a value of 2.33 obtained with standard preservation with cryoprotectant. The drop method allows high throughput aliquoting of the bacterial culture into multiple discrete drops using multichannel pipettes or automated liquid handlers and the edges of the holes provides a pinning action that holds the drop stably against gravitational roll-offs. It also allows samples to be removed in discrete small volumes, thus, reducing the number of freeze thaw cycles and associated cell damage. The flat-sheet architecture of the substrate reduces the amount of plastic waste generated and augments green laboratory practices.

Inflight Polymerase Chain Reaction of samples with drones.

A system devised to conduct Polymerase Chain Reaction (PCR) in-flight on drones that uses the spatial displacement of capillary tubes on thermal blocks kept at 94 °C, 58 °C and 72 °C corresponding to cycling temperatures for denaturation, annealing and extension is demonstrated here. The use of acetal as the thermal block material reduced heat loss and the input power (within 18.5 W) needed to maintain the required temperatures. Tests showed that concentrations of samples down to 1.16 × 106 DNA copies/µL could be significantly and consistently detected above the background emission of the fluorescence signal intensity.

Liquid marble clearance and restoration via gas bubble insertion and bursting.

There is hitherto a lack of a simple way to disrupt the coating of particles from liquid marbles in order to introduce additional reagents. Here, a 40 µL liquid marble, created on a superhydrophobic substrate with a 2 mm hole, forms an overhead and overhanging liquid component from which a single gas bubble of up to 28 µL volume could be introduced via the latter. This caused a localized clearing of the particle shell at the apical region of the overhead component because the particles could not be energetically sustained at the thin film region of the bubble. The subsequent dispensation of 5 µL of an external liquid directly onto the shell-free apex of the liquid marble allowed the coalescence of the two liquid bodies, bubble rupture, and restoration of complete particle shell encapsulation. The addition of the liquid via the overhanging component was alternatively found incapable of increasing the size of the overhead drop component. The localized bubble-actuated transient shell clearance at the apex of the liquid marble to allow the addition of reagents shown here portends new vistas for liquid marbles to be used in biomedical applications.

Cryopreservation without dry ice-induced acidification during sample transport.

Dry ice (solid CO2) remains highly useful when temperature-sensitive biological samples need to be cryogenically transported. CO2 released during the sublimation of dry ice can diffuse through gas permeable receptacle material or any defective seals resulting in potential sample acidification and compromised integrity. In addition, the quality of cryopreservation can be undermined once the dry ice is exhausted. The dry ice carrier design described here has been demonstrated to prevent sublimated CO2 from reaching the samples while maintaining storage temperature below -60 °C for 19 h. It is also equipped with microcontroller-based temperature monitoring for traceability and CO2 gas monitoring for safety.

Drone inflight mixing of biochemical samples.

Autonomous systems for sample transport to the laboratory for analysis can be improved in terms of timeliness, cost and error mitigation in the pre-analytical testing phase. Drones have been reported for outdoor sample transport but incorporating devices on them to attain homogenous mixing of reagents during flight to enhance sample processing timeliness is limited by payload issues. It is shown here that flipping maneuvers conducted with quadcopters are able to facilitate complete and gentle mixing. This capability incorporated during automated sample transport serves to address an important factor contributing to pre-analytical variability which ultimately impacts on test result reliability.

Millimeter-Sized Hole Damming.

Valves used to control liquid filling and draining processes from storage typically need to be actuated. Here, we show that similar flow enabling and restricting operations can be achieved through millimeter scale holes that function according to the amount of hydrostatic pressure applied without any other intervention. This phenomena is exhibited using receptacles where the base is made of either a hydrophilic or superhydrophobic substrate with hole sizes ranging from 1.0-2.0 mm. The construction is such that the drainage flow velocities are of the same order in both substrates and follow Torricelli's law trends. Nevertheless, the primary mechanisms responsible for resisting the onset of flow in each substrate are different; nonbreaching of the advancing contact angle threshold in the former, and stable maintenance of an elastic-like deformation of the liquid-gas interface that is connected to the surrounding plastron in the latter. These differences are demonstrated using an upward jet of water delivered to the orifice, where a discharging flow from the hydrophilic base occurred before the threshold hydrostatic pressure condition was attained, while liquid from the jet is subsumed into the liquid body of the receptacle with the superhydrophobic base without any leakage. These findings portend advantages in simplicity and robustness for a myriad of liquid-related processes.

Stabilized dried blood spot collection.

During the collection phase of the dried blood spot method, practitioners need to ensure that there is no smearing of the blood sample on the filter paper or else readings from it will be invalid. This can be difficult to accomplish in the field if there is relative motion between the site of blood discharge on the finger and the filter paper. In this article, a gyroscope stabilization method is introduced and demonstrated to provide consistent and improved dried blood spot collection within a circular guide region notwithstanding the presence of rocking.

Linear Stepper Actuation Driving Drop Resonance and Modifying Hysteresis.

In this work, 2 µL water drops are placed on substrates that are created to have a circular hydrophilic region bounded by superhydrophobicity so that they exhibit high contact angles. When the substrate is translated by a linear stepper actuator, the random force components present in the actuator are shown to cause the drop to rock resonantly. When the substrate is translated downward at inclination angles of up to 6° with respect to the horizontal, the contact angle hysteresis increases progressively to a limiting condition. When the substrate is moved up at inclined angles, alternatively, the contact angle hysteresis increases initially to the limiting condition before it is progressively restored to its static state. These behaviors are accounted for by the reversible micro-Cassie to Wenzel wetting state transformations that are made possible by the hierarchical microscale and nanoscale structures present in the superhydrophobic regions.

Particles replaced axially in an optical trap.

Flow-based measures to automate optical trapping have significant limitations. A scheme is advanced here where a spherical bead is first located in a trap, and a second bead below the focus point is selectively drawn into the trap to replace the original particle. Experimentation conducted showed that it was possible to do so with little perturbation of other surrounding particles. Simulations done allowed for a clearer description of the exchange mechanism.

Plastron-Mediated Growth of Captive Bubbles on Superhydrophobic Surfaces.

Captive bubbles on a superhydrophobic (SH) surface have been shown to increase in volume via injection of air through the surrounding plastron. The experimental contact diameter against volume trends were found to follow that predicted by the Surface Evolver simulation generally but corresponded with the simulated data at contact angle (CA) = 158° when the volume was 20 µL but that at CA = 170° when the volume was increased to 180 µL. In this regime, there was a simultaneous outward movement of the contact line as well as a small reduction in the slope that the liquid-air interface makes with the horizontal as air was injected. At volumes higher than 180 µL, air injection caused the diameter to reduce progressively until detachment. The inward movement of the contact line in this regime allowed the bubble body to undergo shape deformations to stay attached onto the substrate with larger volumes (300 µL) than predicted (220 µL at CA = 170°) using simulation. In experiments to investigate the effect of translating the SH surface, movement of captive bubbles was possible with 280 µL volume but not with 80 µL volume. This pointed to the possibility of transporting gas-phase samples on SH surfaces using larger captive bubble volumes.

Controlled transport of captive bubbles on plastrons.

Captive bubbles that reside on superhydrophobic surfaces with plastrons move uncontrollably when tilted. A system based on creating moveable local apexes on flexible superhydrophobic foils is shown to allow controlled transport. Simulations done reveal that specific bubble transport speeds are needed to form concentration gradients suited for aerotaxis study and sensing.

Liquid-body resonance while contacting a rotating superhydrophobic surface.

We advance a scheme in which a liquid body on a stationary tip in contact with a rotating superhydrophobic surface is able to maintain resonance primarily from stick-slip events. With tip-to-surface spacing in the range 2.73 ≤ h < 2.45 mm for a volume of 10 µL, the liquid body was found to exhibit resonance independent of the speed of the drum. The mechanics were found to be due to a surface-tension-controlled vibration mode based on the natural frequency values determined. With spacing in the range 2.45 ≤ h < 2.15 mm imposed for a volume of 10 µL, the contact length of the liquid body was found to vary with rotation of the SH drum. This was due to the stick-slip events being able to generate higher energy fluctuations causing the liquid-solid contact areas to vary since the almost oblate spheroid shape of the liquid body had intrinsically higher surface energies. This resulted in the natural frequency perturbations being frequency- and amplitude-modulated over a lower frequency carrier. These findings have positive implications for microfluidic sensing.

Squeezed flow preconcentration for probe tip biosensors.

The preconcentration of analytes improves sensing using probe tips. In this work, we report a method based on creating a squeeze flow between a cylinder and circular coverslip to preconcentrate material at the liquid-gas interface while allowing a probe tip to be readily inserted there. In verification tests using enhanced green fluorescent protein, this capacity is proven. We estimated a 9.7 times increase in probability for fluorophores to be picked up at the tip using inference from fluorescence intensity distributions found. The method is expeditious, simple, and inexpensive, and it does not require any electrical energy source to operate.

Scribed transparency microplates mounted on a modified standard microplate.

The immense cost effectiveness of using transparencies as analyte handling implements in microplate instrumentation offers the possibility of application even in resource-limited laboratories. In this work, a standard microplate was adapted to serve as the permanent base for disposable scribed transparencies. The approach is shown to ameliorate evaporation, which can affect assay accuracy when analytes need to be incubated for some time. It also offers assurance against fluorescence measurement errors due to the cross-talk of samples from adjacent wells.

Liquid body formation from a semispherical superhydrophobic well on a small incline.

In this work, drop formation on a slightly inclined superhydrophobic substrate with liquid at various flow rates delivered through a semispherical well was investigated. Due to the initial dry well condition in the first drop produced, the inertial force from liquid filling allowed the well's edge hysteresis to be more readily breached, in which flow rates of 16 mL/min and above could create a jet that appeared to be able to "pierce" through the top of the semispherical drop without disrupting its form and growth very much. For subsequent drops, the well's edge hysteresis at flow rates of 14 mL/min and above helped to support an "egg" like form. In contrast, this form could not be developed on a similarly inclined superhydrophobic substrate without a well. The findings here assist to establish the flow rate ranges for consistent discrete volume delivery in biochemical analysis and serves as a means to conduct investigations to better reconcile the tendency of liquids to assume drops or develop jets.

Hydrostatic pressure effect on micro air bubbles deposited on surfaces with a retreating tip.

The effect of hydrostatic pressure on 6 µL air bubbles formed on micropillar structured PDMS and silicone surfaces using a 2 mm diameter stainless steel tip retreated at 1 mm/s was investigated. Dimensional analysis of the tip retraction process showed the experiments to be conducted in the condition where fluid inertial forces are comparable in magnitude with surface tension forces, while viscous forces were lower. Larger bubbles could be left behind on the structured PDMS surface. For hydrostatic pressures in excess of 20 mm H2O (196 Pa), the volume of bubble deposited was found to decrease progressively with pressure increase. The differences in width of the deposited bubbles (in contact with the substrate) were significant at any particular pressure but marginal in height. The attainable height before rupture reduced with pressure increase, thereby accounting for the reducing dispensed volume characteristic. On structured PDMS, the gaseous bridge width (in contact with the substrate) was invariant with tip retraction, while on silicone it was initially reducing before becoming invariant in the lead up to rupture. With silicone, hence, reductions in the contact width and height were both responsible for reduced volumes with pressure increase. Increased hydrostatic pressure was also found to restrict the growth in contact width on silicone during the stage when air was injected in through the tip. The ability to effect bubble size in such a simple manner may already be harnessed in nature and suggests possibilities in technological applications.

In situ probing the interior of single bacterial cells at nanometer scale.

We report a novel approach to probe the interior of single bacterial cells at nanometre resolution by combining focused ion beam (FIB) and atomic force microscopy (AFM). After removing layers of pre-defined thickness in the order of 100 nm on the target bacterial cells with FIB milling, AFM of different modes can be employed to probe the cellular interior under both ambient and aqueous environments. Our initial investigations focused on the surface topology induced by FIB milling and the hydration effects on AFM measurements, followed by assessment of the sample protocols. With fine-tuning of the process parameters, in situ AFM probing beneath the bacterial cell wall was achieved for the first time. We further demonstrate the proposed method by performing a spatial mapping of intracellular elasticity and chemistry of the multi-drug resistant strain Klebsiella pneumoniae cells prior to and after it was exposed to the 'last-line' antibiotic polymyxin B. Our results revealed increased stiffness occurring in both surface and interior regions of the treated cells, suggesting loss of integrity of the outer membrane from polymyxin treatments. In addition, the hydrophobicity measurement using a functionalized AFM tip was able to highlight the evident hydrophobic portion of the cell such as the regions containing cell membrane. We expect that the proposed FIB-AFM platform will help in gaining deeper insights of bacteria-drug interactions to develop potential strategies for combating multi-drug resistance.

Nanoscale focused ion beam tomography of single bacterial cells for assessment of antibiotic effects.

Antibiotic resistance is a major risk to human health, and to provide valuable insights into mechanisms of resistance, innovative methods are needed to examine the cellular responses to antibiotic treatment. Focused ion beam tomography is proposed to image and assess the detailed three-dimensional (3D) ultrastructure of single bacterial cells. By iteratively removing slices of thickness in the order of 10 nm, high magnification 2D images can be acquired by scanning electron microscopy at single-digit nanometer resolution. In this study, Klebsiella pneumoniae was treated with polymyxin B, and 3D models of both cell envelope and cytoplasm regions containing the nucleoid and ribosomes were reconstructed. The 3D volume containing the nucleoid and ribosomes was significantly smaller, and the cell length along the longitudinal axis was extended by 40% in the treated cells, implying stress responses to the drug treatment. More than a 200% increase in protrusions per unit surface area on the cell envelope was observed in the curvature analysis after treatment. Experiments by conventional transmission electron microscopy and atomic force microscopy were also performed, followed by comparison and discussions. In conclusion, the proposed 3D imaging method and associated analysis provide a unique tool for the assessment of antibiotic effects on multidrug-resistant bacteria at nanometer resolution.