Towards combinatorial mixing devices without any pumps by open-capillary channels: fundamentals and applications

Spontaneous Transport Mechanics of Water Droplets under a Synergistic Action of Designed Pattern and Non-Wetting Gradient

The controllable spontaneous transport of water droplets on solid surfaces has a broad application background in daily life. Herein, a patterned surface with two different non-wetting characteristics was developed to control the droplet transport behavior. Consequently, the patterned surface exhibited great water-repellant properties in the superhydrophobic region, and the water contact angle reached 160° ± 0.2°. Meanwhile, the water contact angle on the wedge-shaped hydrophilic region dropped to 22° after UV irradiation treatment. On this basis, the maximum transport distance of water droplets could be observed on the sample surface with a small wedge angle of 5° (10.62 mm), and the maximum average transport velocity of droplets was obtained on the sample surface with a large wedge angle of 10° (218.01 mm/s). In terms of spontaneous droplet transport on an inclined surface (4°), both the 8 μL droplet and 50 μL droplet could move upward against gravity, which showed that the sample surface possessed an obvious driving force for droplet transport. Surface non-wetting gradient and the wedge-shaped pattern provided unbalanced surface tension to produce the driving forces in the process of droplet transport, and the Laplace pressure as well is produced inside the water droplet during this process. This work provides a new strategy to develop a patterned superhydrophobic surface for droplet transport.

Real-time observation of the precursor film for low viscosity liquids spreading on solid substrates by a customized differential laser interference microscopy

While static wettability is well treated with Young's equation via its static contact angle, theoretical analyses for wetting dynamics are not yet reaching consensus due to a singularity of the spreading forces worked at the vapor/liquid/solid contact line. One plausible explanation to overcome the singularity problem is that there is a so-called precursor film spreading outside the apparent contact line. After its first finding in 1919, many researchers have attempted to visualize its shape. However, because its length and thickness are as small as micrometer and nanometer-order, respectively, its visualization still remains a challenging issue especially for low-viscosity liquids. In the present study, we developed a differential laser interference microscope, which has a thickness resolution of approximately 2 nm at the best, and applied it to the wetting front of 10 cSt of silicone oil spreading on a silicon wafer with an almost constant spreading velocity. As a result, the precursor film of 14 µm long and 108 nm thick was clearly visualized. While the macro contact line has a finite advancing contact angle of 4.0°, the gradient of the precursor film surface gradually decreased and converged to ~ 0.1° at the micro-contact angle. The shape of the precursor film was independent of the time after the dropping for the range of 600 s ± 10%, which is consistent to theoretical estimation. The present study demonstrated that our interferometer simultaneously achieved nanometer thickness resolutions, micrometer in-plane spatial resolution, and at least a millisecond temporal resolution with a simple optical setup.

Evaporative and Wicking Functionalities at Hot Airflows of Laser Nano-/Microstructured Ti-6Al-4V Material

A novel multifunctional material with efficient wicking and evaporative functionalities was fabricated using hierarchical surface nano-/microstructuring by femtosecond laser micromachining. The created material exhibits excellent multifunctional performance. Our experiments in a wind tunnel demonstrate its good wicking and evaporative functionalities under the conditions of high-temperature airflows. An important finding of this work is the significantly enhanced evaporation rate of the created material compared with the free water surface. The obtained results provide a platform for the practical implementation of Maisotsenko-cycle cooling technologies for substantially increasing efficiency in power generation, thermal management, and other evaporation-based technologies. The developed multifunctional material demonstrates long-lasting wicking and evaporative functionalities that are resistant to degradation under high-temperature airflows, indicating its suitability for practical applications.

Characterization of Fluidic-Barrier-Based Particle Generation in Centrifugal Microfluidics

The fluidic barrier in centrifugal microfluidic platforms is a newly introduced concept for making multiple emulsions and microparticles. In this study, we focused on particle generation application to better characterize this method. Because the phenomenon is too fast to be captured experimentally, we employ theoretical models to show how liquid polymeric droplets pass a fluidic barrier before crosslinking. We explain how secondary flows evolve and mix the fluids within the droplets. From an experimental point of view, the effect of different parameters, such as the barrier length, source channel width, and rotational speed, on the particles’ size and aspect ratio are investigated. It is demonstrated that the barrier length does not affect the particle’s ultimate velocity. Unlike conventional air gaps, the barrier length does not significantly affect the aspect ratio of the produced microparticles. Eventually, we broaden this concept to two source fluids and study the importance of source channel geometry, barrier length, and rotational speed in generating two-fluid droplets.

Silicone Nanofilament Support Layers in an Open-Channel System for the Fast Reduction of Para-Nitrophenol

Chemical vapor phase deposition was used to create hydrophobic nanostructured surfaces on glass slides. Subsequently, hydrophilic channels were created by sputtering a metal catalyst on the channels while masking the outside. The surface tension gradient between the hydrophilic surface in the channels and the outside hydrophobicity formed the open-channel system. The reduction of para-nitrophenol (PNP) was studied on these devices. When compared to nanostructure-free reference systems, the created nanostructures, namely, silicone nanofilaments (SNFs) and nano-bagels, had superior catalytic performance (73% and 66% conversion to 55% at 0.5 µL/s flow rate using 20 nm platinum) and wall integrity; therefore, they could be readily used multiple times. The created nanostructures were stable under the reaction conditions, as observed with scanning electron microscopy. Transition electron microscopy studies of platinum-modified SNFs revealed that the catalyst is present as nanoparticles ranging up to 13 nm in size. By changing the target in the sputter coating unit, molybdenum, gold, nickel and copper were evaluated for their catalytic efficiency. The relative order was platinum < gold = molybdenum < nickel < copper. The decomposition of sodium borohydride (NaBH₄) by platinum as a concurrent reaction to the para-nitrophenol reduction terminates the reaction before completion, despite a large excess of reducing agent. Gold had the same catalytic rate as molybdenum, while nickel was two times and copper about four times faster than gold. In all cases, there was a clear improvement in catalysis of silicone nanofilaments compared to a flat reference system.

Spontaneous rise in open rectangular channels under gravity

Fluid movement in microfluidic devices, porous media, and textured surfaces involves coupled flows over the faces and corners of the media. Spontaneous wetting of simple grooved surfaces provides a model system to probe these flows. This numerical study investigates the spontaneous rise of a liquid in an array of open rectangular channels under gravity, using the Volume-of-Fluid method with adaptive mesh refinement. The rise is characterized by the meniscus height at the channel center, outer face and the interior and exterior corners. At lower contact angles and higher channel aspect ratios, the statics and dynamics of the rise in the channel center show little deviation with the classical model for capillarity, which ignores the existence of corners. For contact angles smaller than 45°, rivulets are formed in the interior corners and a cusp at the exterior corner. The rivulets at long times obey the one-third power law in time, with a weak dependence on the geometry. The cusp behaviour at the exterior corner transforms into a smooth meniscus when the capillary force is higher in the channel, even for contact angles smaller than 45°. The width of the outer face does not influence the capillary rise inside the channel, and the channel size does not influence the rise on the outer face.

Time Evolution of Precursor Thin Film of Water on Polyelectrolyte Brush

The microscopic wetting behavior of a water film on the line-patterned surface of a polyelectrolyte brush was directly visualized using an optical microscope by dyeing procedures. Surface line patterns of 5 and 5 μm width or 10 and 5 μm width for the polyelectrolyte brush and hydrophobic monolayer, respectively, were prepared by a photolithography process, chemical vapor adsorption method, and surface-initiated polymerization. A droplet of water containing dye was placed on the line-patterned surface. In front of the contact line, a water film with a nanometer-scale thickness, referred to as a precursor film, elongated along the polymer brush line with time. The elongation velocity at the first stage increased as the brush line width increased. On the other hand, at the second stage after the macroscopic contact line stopped moving, the precursor film continued to elongate in proportion to the 0.6 power of time, independent of the brush thickness, line width, and droplet volume.

Viscous dynamics of drops and bubbles in Hele-Shaw cells: Drainage, drag friction, coalescence, and bursting

In this review article, we discuss recent studies on drops and bubbles in Hele-Shaw cells, focusing on how scaling laws exhibit crossovers from the three-dimensional counterparts and focusing on topics in which viscosity plays an important role. By virtue of progresses in analytical theory and high-speed imaging, dynamics of drops and bubbles have actively been studied with the aid of scaling arguments. However, compared with three-dimensional problems, studies on the corresponding problems in Hele-Shaw cells are still limited. This review demonstrates that the effect of confinement in the Hele-Shaw cell introduces new physics allowing different scaling regimes to appear. For this purpose, we discuss various examples that are potentially important for industrial applications handling drops and bubbles in confined spaces by showing agreement between experiments and scaling theories. As a result, this review provides a collection of problems in hydrodynamics that may be analytically solved or that may be worth studying numerically in the near future.

3D printed microfluidic mixer for point-of-care diagnosis of anemia

3D printing has been an emerging fabrication tool in prototyping and manufacturing. We demonstrated a 3D microfluidic simulation guided computer design and 3D printer prototyping for quick turnaround development of microfluidic 3D mixers, which allows fast self-mixing of reagents with blood through capillary force. Combined with smartphone, the point-of-care diagnosis of anemia from finger-prick blood has been successfully implemented and showed consistent results with clinical measurements. Capable of 3D fabrication flexibility and smartphone compatibility, this work presents a novel diagnostic strategy for advancing personalized medicine and mobile healthcare.

Geometry-induced capillary emptying

When a capillary is half-filled with liquid and turned to the horizontal, the liquid may flow out of the capillary or remain in it. For lack of a better criterion, the standard assumption is that the liquid will remain in a capillary of narrow cross-section, and will flow out otherwise. Here, we present a precise mathematical criterion that determines which of the two outcomes occurs for capillaries of arbitrary cross-sectional shape, and show that the standard assumption fails for certain simple geometries, leading to very rich and counterintuitive behavior. This opens the possibility of creating very sensitive microfluidic devices that respond readily to small physical changes, for instance, by triggering the sudden displacement of fluid along a capillary without the need of any external pumping.

3D printed auto-mixing chip enables rapid smartphone diagnosis of anemia

Clinical diagnosis requiring central facilities and site visits can be burdensome for patients in resource-limited or rural areas. Therefore, development of a low-cost test that utilizes smartphone data collection and transmission would beneficially enable disease self-management and point-of-care (POC) diagnosis. In this paper, we introduce a low-cost iPOC^3D diagnostic strategy which integrates 3D design and printing of microfluidic POC device with smartphone-based disease diagnosis in one process as a stand-alone system, offering strong adaptability for establishing diagnostic capacity in resource-limited areas and low-income countries. We employ smartphone output (AutoCAD 360 app) and readout (color-scale analytical app written in-house) functionalities for rapid 3D printing of microfluidic auto-mixers and colorimetric detection of blood hemoglobin levels. The auto-mixing of reagents with blood via capillary force has been demonstrated in 1 second without the requirement of external pumps. We employed this iPOC^3D system for point-of-care diagnosis of anemia using a training set of patients (n_anemia = 16 and n_healthy = 6), which showed consistent measurements of blood hemoglobin levels (a.u.c. = 0.97) and comparable diagnostic sensitivity and specificity, compared with standard clinical hematology analyzer. Capable of 3D fabrication flexibility and smartphone compatibility, this work presents a novel diagnostic strategy for advancing personalized medicine and mobile healthcare.

Scaling crossover in thin-film drag dynamics of fluid drops in the Hele-Shaw cell

We study both experimentally and theoretically the descending motion due to gravity of a fluid drop surrounded by another immiscible fluid in a confined space between two parallel plates, i.e., in the Hele-Shaw cell. As a result, we show a new scaling regime of a nonlinear drag friction in viscous liquid that replaces the well-known Stokes’ drag friction through a clear collapse of experimental data thanks to the scaling law. In the novel regime, the dissipation in the liquid thin film formed between the drop and cell walls governs the dynamics. The crossover of this scaling regime to another scaling regime in which the dissipation inside the droplet is dominant is clearly demonstrated and a phase diagram separating these scaling regimes is presented.

Pattern Formation by Staphylococcus epidermidis via Droplet Evaporation on Micropillars Arrays at a Surface

We evaluate the effect of epoxy surface structuring on the evaporation of water droplets containing Staphylococcus epidermidis (S. epidermidis). During evaporation, droplets with S. epidermidis cells yield to complex wetting patterns such as the zipping-wetting1-3 and the coffee-stain effects. Depending on the height of the microstructure, the wetting fronts propagate circularly or in a stepwise manner, leading to the formation of octagonal or square-shaped deposition patterns.4,5 We observed that the shape of the dried droplets has considerable influence on the local spatial distribution of S. epidermidis deposited between micropillars. These changes are attributed to an unexplored interplay between the zipping-wetting1 and the coffee-stain6 effects in polygonally shaped droplets containing S. epidermidis. Induced capillary flows during evaporation of S. epidermidis are modeled with polystyrene particles. Bacterial viability measurements for S. epidermidis show high viability of planktonic cells, but low biomass deposition on the microstructured surfaces. Our findings provide insights into design criteria for the development of microstructured surfaces on which bacterial propagation could be controlled, limiting the use of biocides.

Rapid capillary filling via ion-water interactions over the nanoscale

Giant frictional resistances are grand challenges against the rapid filling of nanoscale capillaries, as encountered in a wide variety of applications ranging from nature to energy. It is commonly believed that partially wettable charged nanocapillaries fill up considerably slower, compared to completely wettable ones, under the influence of a complex interplay between interfacial tension and electrical interactions. In sharp contrast to this common belief, here we discover a new non-intuitive regime of rapid filling of charged capillaries over the nanometer scale, by virtue of which a partially wettable capillary may fill up comparatively faster than a completely wettable one. We attribute the fundamental origin of this remarkable behavior to ion-water interactions over interfacial scales. The underlying novel electro-hydrodynamic mechanism, as unveiled here, may provide deeper insights into the physico-chemical interactions leading to augmentations in the rates of nanocapillary filling over hydrophobic regimes, bearing far-reaching implications in the transport of biological fluids, enhanced oil recovery, and miniaturized energy harvesting applications.

Capillary Imbibition into Converging Tubes: Beating Washburn's Law and the Optimal Imbibition of Liquids

We consider the problem of capillary imbibition into an axisymmetric tube for which the tube radius decreases in the direction of increasing imbibition. For tubes with constant radius, imbibition is described by Washburn's law (referred to here as the BCLW law to recognize the contributions of Bell, Cameron, and Lucas that predate Washburn). We show that imbibition into tubes with a power-law relationship between the radius and axial position generally occurs more quickly than imbibition into a constant-radius tube. By a suitable choice of the shape exponent, it is possible to decrease the time taken for the liquid to imbibe from one position to another by a factor of 2 compared to the BCLW law. We then show that a further small decrease in the imbibition time may be obtained by using a tube consisting of a cylinder joined to a cone of 3 times the cylinder length. For a given inlet radius, this composite shape attains the minimum imbibition time possible. We confirm our theoretical results with experiments on the tips of micropipettes and discuss the possible significance of these results for the control of liquid motion in microfluidic devices.

Hydraulic transport across hydrophilic and hydrophobic nanopores: Flow experiments with water and n-hexane

We experimentally explore pressure-driven flow of water and n-hexane across nanoporous silica (Vycor glass monoliths with 7- or 10-nm pore diameters, respectively) as a function of temperature and surface functionalization (native and silanized glass surfaces). Hydraulic flow rates are measured by applying hydrostatic pressures via inert gases (argon and helium, pressurized up to 70 bar) on the upstream side in a capacitor-based membrane permeability setup. For the native, hydrophilic silica walls, the measured hydraulic permeabilities can be quantitatively accounted for by bulk fluidity provided we assume a sticking boundary layer, i.e., a negative velocity slip length of molecular dimensions. The thickness of this boundary layer is discussed with regard to previous capillarity-driven flow experiments (spontaneous imbibition) and with regard to velocity slippage at the pore walls resulting from dissolved gas. Water flow across the silanized, hydrophobic nanopores is blocked up to a hydrostatic pressure of at least 70 bar. The absence of a sticking boundary layer quantitatively accounts for an enhanced n-hexane permeability in the hydrophobic compared to the hydrophilic nanopores.