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1.  Electrohydrodynamic Generation and Delivery of Monodisperse Pico-Liter Droplets Using PDMS Microchip 
Analytical chemistry  2006;78(23):8011-8019.
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.
PMCID: PMC2577391  PMID: 17134134
2.  Theoretical Model of Droplet Wettability on a Low-Surface-Energy Solid under the Influence of Gravity 
The Scientific World Journal  2014;2014:647694.
The wettability of droplets on a low surface energy solid is evaluated experimentally and theoretically. Water-ethanol binary mixture drops of several volumes are used. In the experiment, the droplet radius, height, and contact angle are measured. Analytical equations are derived that incorporate the effect of gravity for the relationships between the droplet radius and height, radius and contact angle, and radius and liquid surface energy. All the analytical equations display good agreement with the experimental data. It is found that the fundamental wetting behavior of the droplet on the low surface energy solid can be predicted by our model which gives geometrical information of the droplet such as the contact angle, droplet radius, and height from physical values of liquid and solid.
PMCID: PMC3910366  PMID: 24511297
3.  Temperature-Induced Coalescence of Colliding Binary Droplets on Superhydrophobic Surface 
Scientific Reports  2014;4:4303.
This report investigates the impact of droplet temperature on the head-on collision of binary droplets on a superhydrophobic surface. Understanding droplet collision is critical to many fundamental processes and industrial applications. There are many factors, including collision speed, collision angle, and droplet composition, that influence the outcome of the collision between binary droplets. This work provides the first experimental study of the influence of droplet temperature on the collision of binary droplets. As the droplet temperature increases, the possibility increases for the two droplets to coalesce after collision. The findings in this study can be extended to collision of droplets under other conditions where control of the droplet temperature is feasible. Such findings will also be beneficial to applications that involve droplet collision, such as in ink-jet printing, steam turbines, engine ignition, and spraying cooling.
PMCID: PMC3946014  PMID: 24603362
4.  Computation of transient flow rates in passive pumping micro-fluidic systems 
Lab on a chip  2008;9(1):107-114.
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.
PMCID: PMC2941346  PMID: 19209342
5.  Droplet centrifugation, droplet DNA extraction, and rapid droplet thermocycling for simpler and faster PCR assay using wire-guided manipulations 
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.
PMCID: PMC3526397  PMID: 22947281
Droplet manipulations; Escherichia coli; Peptidase D; Droplet PCR; Rapid PCR
6.  Enhanced Droplet Control by Transition Boiling 
Scientific Reports  2012;2:720.
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.
PMCID: PMC3467560  PMID: 23056912
7.  Spontaneous electrical charging of droplets by conventional pipetting 
Scientific Reports  2013;3:2037.
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.
PMCID: PMC3687225  PMID: 23784001
8.  Behavior of DNA fibers stretched by precise meniscus motion control 
Nucleic Acids Research  2001;29(22):e109.
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.
PMCID: PMC92573  PMID: 11713329
9.  Hierarchically structured superhydrophobic flowers with low hysteresis of the wild pansy (Viola tricolor) – new design principles for biomimetic materials 
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.
PMCID: PMC3148064  PMID: 21977435
anti-adhesive; petal effect; petal structures; polymer replication; superhydrophobic
10.  Multiphase Bioreaction Microsystem with Automated On-Chip Droplet Operation 
Lab on a chip  2010;10(10):1308-1315.
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.
PMCID: PMC2909751  PMID: 20445885
11.  Capillary Electrophoresis Separation in the Presence of an Immiscible Boundary for Droplet Analysis 
Analytical chemistry  2006;78(19):6948-6954.
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.
PMCID: PMC2525566  PMID: 17007519
12.  Droplet Shape Analysis and Permeability Studies in Droplet Lipid Bilayers 
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.
PMCID: PMC3502024  PMID: 22509902
13.  Capillary origami: superhydrophobic ribbon surfaces and liquid marbles 
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.
PMCID: PMC3148044  PMID: 21977426
capillary origami; Cassie; contact angle; superhydrophobic; Wenzel
14.  Minimal Size of Coffee Ring Structure 
The journal of physical chemistry. B  2010;114(16):5269-5274.
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.
PMCID: PMC2902562  PMID: 20353247
15.  Backscattering particle immunoassays in wire-guide droplet manipulations 
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.
PMCID: PMC2596077  PMID: 19014703
16.  Stable superhydrophobic surface of hierarchical carbon nanotubes on Si micropillar arrays 
Nanoscale Research Letters  2013;8(1):412.
It is of great importance to construct a stable superhydrophobic surface with low sliding angle (SA) for various applications. We used hydrophobic carbon nanotubes (CNTs) to construct the superhydrophobic hierarchical architecture of CNTs on silicon micropillar array (CNTs/Si-μp), which have a large contact angle of 153° to 155° and an ultralow SA of 3° to 5°. Small water droplets with a volume larger than 0.3 μL can slide on the CNTs/Si-μp with a tilted angle of approximately 5°. The CNTs growing on planar Si wafer lose their superhydrophobic properties after exposing to tiny water droplets. However, the CNTs/Si-μp still show superhydrophobic properties even after wetting using tiny water droplets. The CNTs/Si-μp still have a hierarchical structure after wetting, resulting in a stable superhydrophobic surface.
PMCID: PMC3874759  PMID: 24098965
Carbon nanotube; Hierarchical architecture; Superhydrophobic surface
17.  Acoustic Droplet Vaporization for the Enhancement of Ultrasound Thermal Therapy 
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.
PMCID: PMC3145358  PMID: 21804749
thermal therapy; ultrasound; microbubble; compound lesion
18.  Exhaled droplets due to talking and coughing 
Journal of the Royal Society Interface  2009;6(Suppl 6):S703-S714.
Respiratory infections can be spread via ‘contact’ with droplets from expiratory activities such as talking, coughing and sneezing, and also from aerosol-generating clinical procedures. Droplet sizes predominately determine the times they can remain airborne, the possibility of spread of infectious diseases and thus the strategies for controlling the infections. While significant inconsistencies exist between the existing measured data on respiratory droplets generated during expiratory activities, a food dye was used in the mouth during measurements of large droplets, which made the expiratory activities ‘unnatural’. We carried out a series of experiments using glass slides and a microscope as well as an aerosol spectrometer to measure the number and size of respiratory droplets produced from the mouth of healthy individuals during talking and coughing with and without a food dye. The total mass of respiratory droplets was measured using a mask, plastic bag with tissue and an electronic balance with a high precision. Considerable subject variability was observed and the average size of droplets captured using glass slides and microscope was about 50–100 µm. Smaller droplets were also detected by the aerosol spectrometer. More droplets seemed to be generated when a food dye was used.
PMCID: PMC2843952  PMID: 19812073
droplets; talking; coughing; airborne infection
19.  Submicron Dropwise Condensation under Superheated and Rarefied Vapor Condition 
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.
PMCID: PMC2978260  PMID: 20942412
20.  The Investigation of Intermediate Stage of Template Etching with Metal Droplets by Wetting Angle Analysis on (001) GaAs Surface 
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.
PMCID: PMC3211836
Droplet epitaxy; Local droplet etching; Quantum dots; Atomic force microscopy; Molecular beam epitaxy
21.  Control of initiation, rate, and routing of spontaneous capillary-driven flow of liquid droplets through microfluidic channels on SlipChip 
Langmuir  2012;28(3):1931-1941.
This paper describes the use of capillary pressure to initiate and control the rate of spontaneous liquid-liquid flow through microfluidic channels. In contrast to flow driven by external pressure, flow driven by capillary pressure is dominated by interfacial phenomena and is exquisitely sensitive to the chemical composition and geometry of the fluids and channels. A step-wise change in capillary force was initiated on a hydrophobic SlipChip by slipping a shallow channel containing an aqueous droplet into contact with a slightly deeper channel filled with immiscible oil. This action induced spontaneous flow of the droplet into the deeper channel. A model predicting the rate of spontaneous flow was developed based on the balance of net capillary force with viscous flow resistance, using as inputs the liquid-liquid surface tension, the advancing and receding contact angles at the three-phase aqueous-oil-surface contact line, and the geometry of the devices. The impact of contact angle hysteresis, the presence or absence of a lubricating oil layer, and adsorption of surface-active compounds at liquid-liquid or liquid-solid interfaces were quantified. Two regimes of flow spanning a 104-fold range of flow rates were obtained and modeled quantitatively, with faster (mm/s) flow obtained when oil could escape through connected channels as it was displaced by flowing aqueous solution, and slower (micrometer/s) flow obtained when oil escape was mostly restricted to a μm-scale gap between the plates of the SlipChip (“dead-end flow”). Rupture of the lubricating oil layer (reminiscent of a Cassie-Wenzel transition) was proposed as a cause of discrepancy between the model and the experiment. Both dilute salt solutions and complex biological solutions such as human blood plasma could be flowed using this approach. We anticipate that flow driven by capillary pressure will be useful for design and operation of flow in microfluidic applications that do not require external power, valves, or pumps, including on SlipChip and other droplet- or plug-based microfluidic devices. In addition, this approach may be used as a sensitive method of evaluating interfacial tension, contact angles and wetting phenomena on chip.
PMCID: PMC3271727  PMID: 22233156
Lab on a chip  2008;8(6):898-906.
Electrowetting-on-dielectric (EWOD) actuation enables digital (or droplet) microfluidics where small packets of liquids are manipulated on a two-dimensional surface. Due to its mechanical simplicity and low energy consumption, EWOD holds particular promise for portable systems. To improve volume precision of the droplets, which is desired for quantitative applications such as biochemical assays, existing practices would require near-perfect device fabricaion and operation conditions unless the droplets are generated under feedback control by an extra pump setup off of the chip. In this paper, we develop an all-electronic (i.e., no ancillary pumping) real-time feedback control of on-chip droplet generation. A fast voltage modulation, capacitance sensing, and discrete-time PID feedback controller are integrated on the operating electronic board. A significant improvement is obtained in the droplet volume uniformity, compared with an open loop control as well as the previous feedback control employing an external pump. Furthermore, this new capability empowers users to prescribe the droplet volume even below the previously considered minimum, allowing, for example, 1:x (x < 1) mixing, in comparison to the previously considered n:m mixing (i.e., n and m unit droplets).
PMCID: PMC2769494  PMID: 18497909
23.  Persisting Water Droplets on Water Surfaces† 
The journal of physical chemistry. B  2010;114(44):14020-14027.
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.
PMCID: PMC3208511  PMID: 20961076
24.  Hydrophobic-induced Surface Reorganization: Molecular Dynamics Simulations of Water Nanodroplet on Perfluorocarbon Self-Assembled Monolayers 
Soft matter  2010;6(8):1644-1654.
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.
PMCID: PMC2877516  PMID: 20514368
25.  Synthetic cell division system: Controlling equal vs. unequal divisions by design 
Scientific Reports  2013;3:3475.
Cell division is one of the most fundamental and evolutionarily conserved biological processes. Here, we report a synthetic system where we can control by design equal vs. unequal divisions. We synthesized a micro-scale inverse amphipathic droplet of which division is triggered by the increase of surface to volume ratio. Using this system, we succeeded in selectively inducing equal vs. unequal divisions of the droplet cells by adjusting the temperature or the viscosity of the solvent outside the droplet cell accordingly. Our synthetic division system may provide a platform for further development to a system where intracellular contents of the parent droplet cell could be divided into various ratios between the two daughter droplet cells to control their functions and fates.
PMCID: PMC3858794  PMID: 24327069

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