We developed a drop-on-demand microdroplet generator for the discrete dispensing of biosamples into a bio-analytical unit. This disposable PDMS microfluidic device can generate monodisperse droplets of pico-liter volume directly out of a plane sidewall of the microfluidic chip by an electrohydrodynamic mechanism. The droplet generation was accomplished without using either an inserted capillary or monolithically built-in tip. The minimum droplet volume was around 4 pico-liters, and the droplet generation was repeatable and stable for at least 30 minutes, with a typical variation of less than 2.0% of drop size. The Taylor cone, which is usually observed in electrospray, was suppressed by controlling the surface wetting property of the PDMS device as well as the surface tension of the sample liquids. A modification of the channel geometry right before the opening of the microchannel also enhanced the continuous droplet generation without applying any external pumping. A simple numerical simulation of the droplet generation verified the importance of controlling the surface wetting conditions for the droplet formation. Our microdroplet generator can be effectively applied to a direct interface of a microfluidic chip to a bio-sensing unit, such as AMS, MALDI-MS or protein microarray-type biochips.
Motion in micro-channels of passive flow micro-fluidic systems can be controlled by proper design and estimated by careful modeling. We report on methods to describe the flow rate as function of time in a passive pump driven micro-fluidic systems. The model considers the surface energy present in small droplets, which prompts their shrinkage and induces flow. The droplet geometries are controlled by the micro-fluidic system geometry and hydrophilicity of the droplet channel contact area so that the chord of the droplet’s cross section is restrained as the fluid is pumped. The model uses interfacial thermodynamics and the Hagen-Poiseuille equation for calculating the flow rate in micro-channels. Existing analyses consider the theoretical relationships among sample volume and induced flow rate, surface energy of the drops at the entrance and exit ports, and the resistance to flow. This model provides more specific information on the influence of the experimental conditions in computations of the flow rate. The model was validated in four sets of experiments. Passive pumps with 1.8 mm diameter, hydrophobic or hydrophilic entry ports, 5.0 or 10.0 mm channel length, and 2.5 or 3.3 mm diameter reservoir ports provided initial flow rates between 85 nL/sec and 196 nL/sec.
Hierarchically structured flower leaves (petals) of many plants are superhydrophobic, but water droplets do not roll-off when the surfaces are tilted. On such surfaces water droplets are in the “Cassie impregnating wetting state”, which is also known as the “petal effect”. By analyzing the petal surfaces of different species, we discovered interesting new wetting characteristics of the surface of the flower of the wild pansy (Viola tricolor). This surface is superhydrophobic with a static contact angle of 169° and very low hysteresis, i.e., the petal effect does not exist and water droplets roll-off as from a lotus (Nelumbo nucifera) leaf. However, the surface of the wild pansy petal does not possess the wax crystals of the lotus leaf. Its petals exhibit high cone-shaped cells (average size 40 µm) with a high aspect ratio (2.1) and a very fine cuticular folding (width 260 nm) on top. The applied water droplets are in the Cassie–Baxter wetting state and roll-off at inclination angles below 5°. Fabricated hydrophobic polymer replicas of the wild pansy were prepared in an easy two-step moulding process and possess the same wetting characteristics as the original flowers. In this work we present a technical surface with a new superhydrophobic, low adhesive surface design, which combines the hierarchical structuring of petals with a wetting behavior similar to that of the lotus leaf.
anti-adhesive; petal effect; petal structures; polymer replication; superhydrophobic
A computer numerical control (CNC) apparatus was used to perform droplet centrifugation, droplet DNA extraction, and rapid droplet thermocycling on a single superhydrophobic surface and a multi-chambered PCB heater. Droplets were manipulated using “wire-guided” method (a pipette tip was used in this study). This methodology can be easily adapted to existing commercial robotic pipetting system, while demonstrated added capabilities such as vibrational mixing, high-speed centrifuging of droplets, simple DNA extraction utilizing the hydrophobicity difference between the tip and the superhydrophobic surface, and rapid thermocycling with a moving droplet, all with wire-guided droplet manipulations on a superhydrophobic surface and a multi-chambered PCB heater (i.e., not on a 96-well plate). Serial dilutions were demonstrated for diluting sample matrix. Centrifuging was demonstrated by rotating a 10 μL droplet at 2300 round per minute, concentrating E. coli by more than 3-fold within 3 min. DNA extraction was demonstrated from E. coli sample utilizing the disposable pipette tip to cleverly attract the extracted DNA from the droplet residing on a superhydrophobic surface, which took less than 10 min. Following extraction, the 1500 bp sequence of Peptidase D from E. coli was amplified using rapid droplet thermocycling, which took 10 min for 30 cycles. The total assay time was 23 min, including droplet centrifugation, droplet DNA extraction and rapid droplet thermocycling. Evaporation from of 10 μL droplets was not significant during these procedures, since the longest time exposure to air and the vibrations was less than 5 min (during DNA extraction). The results of these sequentially executed processes were analyzed using gel electrophoresis. Thus, this work demonstrates the adaptability of the system to replace many common laboratory tasks on a single platform (through re-programmability), in rapid succession (using droplets), and with a high level of accuracy and automation.
Droplet manipulations; Escherichia coli; Peptidase D; Droplet PCR; Rapid PCR
This paper demonstrates the ability to use capillary-electrophoresis (CE) separation coupled with laser-induced fluorescence for analyzing the contents of single femtoliter-volume aqueous droplets. A single droplet was formed using a T-channel (3 μm wide by 3 μm tall) connected to microinjectors, then the droplet was fluidically moved to an immiscible boundary that isolates the CE channel (50 μm wide by 50 μm tall) from the droplet generation region. Fusion of the aqueous droplet with the immiscible boundary effectively injects the droplet content into the separation channel. In addition to injecting the contents of droplets, we found aqueous samples can be introduced directly into the separation channel by reversibly penetrating and re-sealing the immiscible partition. Because droplet generation in channels requires hydrophobic surfaces, we have also investigated the advantages to using all hydrophobic channels versus channel systems with patterned hydrophobic and hydrophilic regions. To fabricate devices with patterned surface chemistry, we have developed a simple strategy based on differential wetting to deposit selectively a hydrophilic polymer (polystyrene sulfonate) onto desired regions of the microfluidic chip. Finally, we applied our device to the separation of a simple mixture of fluorescein-labeled amino acids contained within a ∼ 10 fL droplet.
A droplet of water on a heated surface can levitate over a film of gas produced by its own evaporation in the Leidenfrost effect. When the surface is prepared with ratchet-like saw-teeth topography, these droplets can self-propel and can even climb uphill. However, the extent to which the droplets can be controlled is limited by the physics of the Leidenfrost effect. Here, we show that transition boiling can be induced even at very high surface temperatures and provide additional control over the droplets. Ratchets with acute protrusions enable droplets to climb steeper inclines while ratchets with sub-structures enable their direction of motion to be controlled by varying the temperature of the surface. The droplets' departure from the Leidenfrost regime is assessed by analysing the sound produced by their boiling. We anticipate these techniques will enable the development of more sophisticated methods for controlling small droplets and heat transfer.
We apply optical manipulation to prepare lipid bilayers between pairs of water droplets immersed in an oil matrix. These droplet pairs have a well-defined geometry allowing use of droplet shape analysis to perform quantitative studies of the dynamics during the bilayer formation and to determine time-dependent values for the droplet volumes, bilayer radius, bilayer contact angle, and droplet centerline approach velocity. During bilayer formation, the contact angle rises steadily to an equilibrium value determined by the bilayer adhesion energy. When there is a salt concentration imbalance between the droplets, there is a measurable change in droplet volume. We present an analytical expression for this volume change and use this expression to calculate the bilayer permeability to water.
In the wetting of a solid by a liquid it is often assumed that the substrate is rigid. However, for an elastic substrate the rigidity depends on the cube of its thickness and so reduces rapidly as the substrate becomes thinner as it approaches becoming a thin sheet. In such circumstances, it has been shown that the capillary forces caused by a contacting droplet of a liquid can shape the solid rather than the solid shaping the liquid. A substrate can be bent and folded as a (pinned) droplet evaporates or even instantaneously and spontaneously wrapped on contact with a droplet. When this effect is used to create three dimensional shapes from initially flat sheets, the effect is called capillary origami or droplet wrapping.
In this work, we consider how the conditions for the spontaneous, capillary induced, folding of a thin ribbon substrate might be altered by a rigid surface structure that, for a rigid substrate, would be expected to create Cassie–Baxter and Wenzel effects. For smooth thin substrates, droplet wrapping can occur for all liquids, including those for which the Young’s law contact angle (defined by the interfacial tensions) is greater than 90° and which would therefore normally be considered relatively hydrophobic. However, consideration of the balance between bending and interfacial energies suggests that the tendency for droplet wrapping can be suppressed for some liquids by providing the flexible solid surface with a rigid topographic structure. In general, it is known that when a liquid interacts with such a structure it can either fully penetrate the structure (the Wenzel case) or it can bridge between the asperities of the structure (the Cassie–Baxter case).
In this report, we show theoretically that droplet wrapping should occur with both types of solid–liquid contact. We also derive a condition for the transition between the Cassie–Baxter and Wenzel type droplet wrapping and relate it to the same transition condition known to apply to superhydrophobic surfaces. The results are given for both droplets being wrapped by thin ribbons and for solid grains encapsulating droplets to form liquid marbles.
capillary origami; Cassie; contact angle; superhydrophobic; Wenzel
A macroscopic evaporating water droplet with suspended particles on a solid surface will form a ring-like structure at the pinned contact line due to induced capillary flow. As the droplet size shrinks, the competition between the time scales of the liquid evaporation and the particle movement may influence the resulting ring formation. When the liquid evaporates much faster than the particle movement, coffee ring formation may cease. Here, we experimentally show that there exists a lower limit of droplet size, Dc, for the successful formation of a coffee ring structure. When the particle concentration is above a threshold value, Dc can be estimated by considering the collective effects of the liquid evaporation and the particle diffusive motion within the droplet. For suspended particles of size ~100 nm, the minimum diameter of the coffee ring structure is found to be ~10 µm.
We report that a droplet dispensed from a micropipette almost always has a considerable electrical charge of a magnitude dependent on the constituents of the droplet, on atmospheric humidity and on the coating material of pipette tip. We show that this natural electrification of a droplet originates from the charge separation between a droplet and pipette tip surface by contact with water due to the ionization of surface chemical groups. Charge on a droplet can make it difficult to detach the droplet from the pipette tip, can decrease its surface tension, can affect the chemical characteristics of solutions due to interactions with charged molecules, and can influence the combination and localization of charged bio-molecules; in all cases, the charge may affect results of experiments in which any of these factors is important. Thus, these findings reveal experimental parameters that should be controlled in experiments that use micropipettes.
A simpler way for manipulating droplets on a flat surface was demonstrated, eliminating the complications in the existing methods of open-surface digital microfluidics. Programmed and motorized movements of 10 μL droplets were demonstrated using stepper motors and microcontrollers, including merging, complicated movement along the programmed path, and rapid mixing. Latex immunoagglutination assays for mouse immunoglobulin G, bovine viral diarrhea virus and Escherichia coli were demonstrated by merging two droplets on a superhydrophobic surface (contact angle = 155 ± 2°) and using subsequent back light scattering detection, with detection limits of 50 pg mL-1, 2.5 TCID50 mL-1 and 85 CFU mL-1, respectively, all significantly lower than the other immunoassay demonstrations in conventional microfluidics (~1 ng mL-1 for proteins, ~100 TCID50 mL-1 for viruses and ~100 CFU mL-1 for bacteria). Advantages of this system over conventional microfluidics or microwell plate assays include: (1) minimized biofouling and repeated use (>100 times) of a platform; (2) possibility of nanoliter droplet manipulation; (3) reprogrammability with a computer or a game pad interface.
A modified DNA combing method, which can precisely locate straightened DNA fibers on a substrate, has been developed. Precise motion control of a DNA solution droplet on hydrophobic surfaces has allowed detailed analyses of DNA straightening behavior. Our method provides a technique for consistently straightening λ phage DNA on a trace of droplet motion, though the straightened DNAs had several variations in their alignments. The dependence of the straightened DNA frequency upon motion rate, fluidity in the droplet and environmental humidity was investigated. Visualization of the solution flow in the moving droplet indicated that flows circulating parallel to the contour of the droplet markedly bias the direction of straightening in relation to the site in the droplet. As a result, the alignment variations caused by the site specificity of the bias direction revealed that environmental humidity significantly alters the straightening behavior.
Phase change accompanying conversion of a saturated or superheated vapor in presence of subcooled surfaces is one of the most common occurring phenomena in nature. The mode of phase change which follows such a transformation is dependent upon surface properties like as of contact angle and thermodynamic conditions of the system. In present studies, an experimental approach is used to study the physics behind droplet growth on a partially wetting surface. Superheated vapor at low pressures of 4–5 torr was condensed on subcooled silicon surface with static contact angle as of 60° in absence of non-condensable gases, and the condensation process monitored using Environmental Scanning Electron Microscope (ESEM) with submicroscopic spatial resolution. The condensation process was analyzed in the form of size growth of isolated droplets for before a coalescence event ended the regime of single droplet growth. Droplet growth obtained as a function of time reveals that the rate of growth decreases as the droplet increases in size. This behavior is indicative of an overall droplet growth law existing over larger time scales of which the current observations in their brief time intervals could be fitted in. A theoretical model based on kinetic theory further support the experimental observations indicating a mechanism where growth occurs by interfacial mass transport directly on condensing droplet surface. Evidence was also found which establishes the presence of submicroscopic droplets nucleating and growing in between microscopic droplets for partially wetting case.
A droplet-based bioreaction microsystem has been developed with automated droplet generation and confinement. On-chip electronic sensing is employed to track the position of the droplets by sensing the oil/aqueous interface in real time. The sensing signal is also used to control the pneumatic supply for moving as well as automatically generating four different nanoliter-sized droplets. The actual size of droplets is very close to the designed droplet size with a standard deviation less than 3% of the droplet size. The automated droplet generation can be completed in less than 2 sec, which is 5 times faster than using manual operation that takes at least 10 sec. Droplets can also be automatically confined in the reaction region with feedback pneumatic control and digital or analog sensing. As an example bioreaction, PCR has been successfully performed in the automated generated droplets. Although the amplification yield was slightly reduced with the droplet confinement, especially while using the analog sensing method, adding additional reagents effectively alleviated this inhibition.
In this work, we study metal droplets on a semiconductor surface that are the initial stage for both droplet epitaxy and local droplet etching. The distributions of droplet geometrical parameters such as height, radius and volume help to understand the droplet formation that strongly influences subsequent nanohole etching. To investigate the etching and intermixing processes, we offer a new method of wetting angle analysis. The aspect ratio that is defined as the ratio of the height to radius was used as an estimation of wetting angle which depends on the droplet material. The investigation of the wetting angle and the estimation of indium content revealed significant materials intermixing during the deposition time. AFM measurements reveal the presence of two droplet groups that is in agreement with nanohole investigations. To explain this observation, we consider arsenic evaporation and consequent change in the initial substrate. On the basis of our analysis, we suggest the model of droplet evolution and the formation of two droplet groups.
Droplet epitaxy; Local droplet etching; Quantum dots; Atomic force microscopy; Molecular beam epitaxy
Droplets of various liquids may float on the respective surfaces for extended periods of time prior to coalescence. We explored the features of delayed coalescence in highly purified water. Droplets several millimeters in diameter were released from a nozzle onto a water surface. Results showed that droplets had float times up to hundreds of milliseconds. When the droplets did coalesce, they did so in stepwise fashion, with periods of quiescence interspersed between periods of coalescence. Up to six steps were noted before the droplet finally vanished. Droplets were released in a series, which allowed the detection of unexpected abrupt float-time changes throughout the duration of the series. Factors such as electrostatic charge, droplet size, and sideways motion had considerable effect on droplet lifetime, as did reduction of pressure, which also diminished the number of steps needed for coalescence. On the basis of present observations and recent reports, a possible mechanism for noncoalescence is considered.
We carried out molecular dynamics simulations of water droplets on self-assembled monolayers of perfluorocarbon molecules. The interactions between the water droplet and the hydrophobic fluorocarbon surface were studied by systematically changing the molecular surface coverage and the mobility of the tethered head groups of the surface chain molecules. The microscopic contact angles were determined for different fluorocarbon surface densities. The contact angle at a nanometer length scale does not show a large change with the surface density. The structure of the droplets was studied by looking at the water density profiles and water penetration near the hydrophobic surface. At surface densities near close packed coverage of fluorocarbons, the water density shows an oscillating pattern near the boundary with a robust layered structure. As the surface density decreased and more water molecules penetrated into the fluorocarbon surface, the ordering of the water molecules at the boundary became less pronounced and the layered density structure became diffuse. The water droplet is found to induce the interfacial surface molecules to rearrange and form unique topological structures that minimize the unfavorable water-surface contacts. The local density of the fluorocarbon molecules right below the water droplet is measured to be higher than the density outside the droplet. The density difference increases as the overall surface density decreases. Two different surface morphologies emerge from the water-induced surface reorganization over the range of surface coverage explored in the study. For surface densities near closed packed monolayer coverage, the height of the fluorocarbons is maximum at the center of the droplet and minimum at the water-vapor-surface triple junction, generating a convex surface morphology under the droplet. For lower surface densities, on the other hand, the height of the fluorocarbon surface becomes maximal at and right outside the water-vapor-surface contact line and decreases quickly towards the center of the droplet, forming a concave shape of the surface. The interplay between the fluorocarbon packing and the water molecules is found to have profound consequences in many aspects of surface-water interactions, including water depletion and penetration, hydrogen bonding, and surface morphologies.
Contact angle analysis of cell surface hydrophobicity (CSH) describes the tendency of a water droplet to spread across a lawn of filtered bacterial cells. Colistin-induced disruption of the Gram-negative outer membrane necessitates hydrophobic contacts with lipopolysaccharide (LPS). We aimed to characterize the CSH of Acinetobacter baumannii using contact angles, to provide insight into the mechanism of colistin resistance.
METHODS AND RESULTS
Contact angles were analysed for five paired colistin-susceptible and -resistant A. baumannii strains. Drainage of the water droplet through bacterial layers was demonstrated to influence results. Consequently, measurements were performed 0.66-sec after droplet deposition. Colistin-resistant cells exhibited lower contact angles (38.8±2.8° to 46.8±1.3°) compared to their paired-susceptible strains (40.7±3.0° to 48.0±1.4°; ANOVA; p<0.05). Contact angles increased at stationary phase (50.3±2.9° to 61.5±2.5° and 47.4±2.0° to 50.8±3.2°, susceptible and resistant, respectively, ANOVA; p<0.05), and in response to colistin 32-mgL−1 exposure (44.5±1.5° to 50.6±2.8° and 43.5±2.2° to 48.0±2.2°, susceptible and resistant, respectively; ANOVA; p<0.05). Analysis of complemented strains constructed with an intact lpxA gene, or empty vector, highlighted the contribution of LPS to CSH.
Compositional outer-membrane variations likely account for CSH differences between A. baumannii phenotypes, which influence the hydrophobic colistin-bacterium interaction.
SIGNIFICANCE AND IMPACT OF STUDY
Important insight into the mechanism of colistin resistance has been provided. Greater consideration of contact angle mehodology is nescessary to ensure accurate analyses are performed.
Antimicrobials; Lipopolysaccharide; Mechanism of Action
Different strains of Pseudomonas putida and P. fluorescens isolated from the rhizosphere and phyllosphere were tested for surface activity in droplet cultures on polystyrene. Droplets of 6 of the 12 wild types tested spread over the surface during incubation, and these strains were considered surface active; strains not showing this reaction were considered non-surface active. Similar reactions were observed on pieces of wheat leaves. Supernatants from centrifuged broth cultures behaved like droplets of suspensions in broth; exposure to 100°C destroyed the activity. Average contact angles of the supernatants of surface-active and non-surface-active strains on polystyrene were 24° and 72°, respectively. The minimal surface tension of supernatants of the surface-active strains was about 46 mN/m, whereas that of the non-surface-active strains was 64 mN/m (estimations from Zisman plots). After 6 days of incubation, wheat flag leaves sprayed with a dilute suspension of a surface-active strain of P. putida (WCS 358RR) showed a significant increase in leaf wettability, which was determined by contact angle measurements. Increasing the initial concentration of bacteria and the amount of nutrients in the inoculum sprayed on leaves reduced the contact angles from 138° on leaves treated with antibiotics (control) to 43° on leaves treated with surface-active bacteria. A closely related strain with no surface activity on polystyrene did not affect leaf wettability, although it was present in densities similar to those of the surface-active strain. Nutrients alone could occasionally also increase leaf wettability, apparently by stimulating naturally occurring surface-active bacteria. When estimating densities of Pseudomonas spp. underneath droplets with low contact angles, it appeared that populations on leaves treated with a surface-active strain could vary from about 104 to 106 CFU cm−2, suggesting that the surface effect may be prolonged after a decline of the population. The possible ecological implications are discussed.
Caveolins form plasmalemnal invaginated caveolae. They also locate around intracellular lipid droplets but their role in this location remains unclear. By studying primary adipocytes that highly express caveolin-1, we characterized the impact of caveolin-1 deficiency on lipid droplet proteome and lipidome. We identified several missing proteins on the lipid droplet surface of caveolin-deficient adipocytes and showed that the caveolin-1 lipid droplet pool is organized as multi-protein complexes containing cavin-1, with similar dynamics as those found in caveolae. On the lipid side, caveolin deficiency did not qualitatively alter neutral lipids in lipid droplet, but significantly reduced the relative abundance of surface phospholipid species: phosphatidylserine and lysophospholipids. Caveolin-deficient adipocytes can form only small lipid droplets, suggesting that the caveolin-lipid droplet pool might be involved in lipid droplet size regulation. Accordingly, we show that caveolin-1 concentration on adipocyte lipid droplets positively correlated with lipid droplet size in obese rodent models and human adipocytes. Moreover, rescue experiments by caveolin- green fluorescent protein in caveolin-deficient cells exposed to fatty acid overload demonstrated that caveolin-coated lipid droplets were able to grow larger than caveolin-devoid lipid droplets. Altogether, these data demonstrate that the lipid droplet-caveolin pool impacts on phospholipid and protein surface composition of lipid droplets and suggest a functional role on lipid droplet expandability.
obesity; phospholipids; triacylglycerol; lipid storage
The eyes of most diurnal reptiles and birds contain coloured retinal filters-oil droplets. Although these filters are widespread, their adaptive advantage remains uncertain. To understand why coloured oil droplets appeared and were retained during evolution, I consider both the benefits and the costs of light filtering in the retina. Oil droplets decrease cone quantum catch and reduce the overlap in sensitivity between spectrally adjacent cones. The reduction of spectral overlap increases the volume occupied by object colours in a cone space, whereas the decrease in quantum catch increases noise, and thus reduces the discriminability of similar colours. The trade-off between these two effects determines the total benefit of oil droplets. Calculations show that coloured oil droplets increase the number of object colours that can be discriminated, and thus are beneficial for colour vision.
Many studies involving interacting microorganisms would benefit from simple devices able to deposit cells in precisely defined patterns. We describe an inexpensive bacterial piezoelectric inkjet printer (adapted from the design of the POSaM oligonucleotide microarrayer) that can be used to “print out” different strains of bacteria or chemicals in small droplets onto a flat surface at high resolution. The capabilities of this device are demonstrated by printing ordered arrays comprising two bacterial strains labeled with different fluorescent proteins. We also characterized several properties of this piezoelectric printer, such as the droplet volume (of the order of tens of pl), the distribution of number of cells in each droplet, and the dependence of droplet volume on printing frequency. We established the limits of the printing resolution, and determined that the printed viability of Escherichia coli exceeded 98.5%.
Acoustic droplet vaporization (ADV) is an ultrasound method for converting biocompatible microdroplets into microbubbles. The objective is to demonstrate that ADV bubbles can enhance high intensity focused ultrasound (HIFU) therapy by controlling and increasing energy absorption at the focus. Thermal phantoms were made with or without droplets. Compound lesions were formed in the phantoms by 5-second exposures with 5-second delays. Center to center spacing of individual lesions was 5.5 mm in either a linear pattern or a spiral pattern. Prior to the HIFU, 10 cycle tone bursts with 0.25% duty cycle were used to vaporize the droplets, forming an “acoustic trench” within 30 seconds. The transducer was then focused in the middle of the back bubble wall to form thermal lesions in the trench. All lesions were imaged optically and with 2T MRI. With the use of ADV and the acoustic trench, a uniform thermal ablation volume of 15 cm3 was achieved in 4 minutes; without ADV only less than 15% of this volume was filled. The commonly seen tadpole shape characteristic of bubble-enhanced HIFU lesions was not evident with the acoustic trench. In conclusion, ADV shows promise for the spatial control and dramatic acceleration of thermal lesion production by HIFU.
thermal therapy; ultrasound; microbubble; compound lesion
A water drop on a superhydrophobic surface that is pinned by wire loops can be reproducibly cut without formation of satellite droplets. Drops placed on low-density polyethylene surfaces and Teflon-coated glass slides were cut with superhydrophobic knives of low-density polyethylene and treated copper or zinc sheets, respectively. Distortion of drop shape by the superhydrophobic knife enables a clean break. The driving force for droplet formation arises from the lower surface free energy for two separate drops, and it is modeled as a 2-D system. An estimate of the free energy change serves to guide when droplets will form based on the variation of drop volume, loop spacing and knife depth. Combining the cutting process with an electrofocusing driving force could enable a reproducible biomolecular separation without troubling satellite drop formation.
Enhancing condensation heat transfer is important for broad applications from power generation to water harvesting systems. Significant efforts have focused on easy removal of the condensate, yet the other desired properties of low contact angles and high nucleation densities for high heat transfer performance have been typically neglected. In this work, we demonstrate immersion condensation on oil-infused micro and nanostructured surfaces with heterogeneous coatings, where water droplets nucleate immersed within the oil. The combination of surface energy heterogeneity, reduced oil-water interfacial energy, and surface structuring enabled drastically increased nucleation densities while maintaining easy condensate removal and low contact angles. Accordingly, on oil-infused heterogeneous nanostructured copper oxide surfaces, we demonstrated approximately 100% increase in heat transfer coefficient compared to state-of-the-art dropwise condensation surfaces in the presence of non-condensable gases. This work offers a distinct approach utilizing surface chemistry and structuring together with liquid-infusion for enhanced condensation heat transfer.