Spontaneous Capillary Flow Research Articles

An “Ion Harvester” Battery in Soil Empowered by a Microfluidic Pump and Interlayer Confinement within Micro‐Swiss‐Rolls

AbstractA significant challenge arises in powering distributed tiny sensors that gather high‐resolution soil data are crucial for advancing digital and geospatial technologies in the field. Traditional batteries and solar cells are unsuitable due to size mismatches. Microbial fuel cells lack stability in delivering constant power. Soil naturally contains redox‐active ions, such as Zn2+ and Mn2+, which can be harvested and utilized for electrochemical energy storage. To facilitate access to soluble ions, a microfluidic pump based on micro‐Swiss‐roll structures via micro‐origami is developed. Spontaneous capillary flow allows for efficient liquid extraction. The micro‐Swiss‐roll architecture shows significant improvements in redox reversibility and charge transfer within micrometer‐thick interlayer space. Notably, the interference of Zn0/2+ and Mn(II)/Mn(IV) reactions is suppressed. Consequently, the battery using a micro‐Swiss‐roll electrode shows high voltage and energy efficiency at high currents up to 10 µA, with cycling stability of 300 times. A twin‐micro‐Swiss‐roll architecture of 0.385 ± 0.006 mm2 is used for energy storage in wet soil, yielding over 40 times more energy as compared to soil‐microbial fuel cells. A conceptual demonstration of an integrated irrigation water sensor and micro‐Swiss‐roll battery highlights the potential of microbattery development in addressing the critical challenge of powering tiny sensors in the soil.

Filter paper as electrolyte flow transport using vaporized methanol as fuel in a microfluidic fuel cell: Experimental and numerical simulation

In this work, we present a design of a paper-based microfluidic fuel cell (μFC), which employs the spontaneous capillary flow of reactant solutions in a filter paper to accomplish passive conveyance of the fuel and oxidant. This self-pumping device uses methanol vapor as a fuel. The gas phase in the microfluidic fuel cell increases the fuel supply to the anode due to a higher diffusion coefficient of 1.5 ​× ​10−5 ​m2 ​s−1 compared with 5 ​× ​10−9 ​m2 ​s−1 for liquid phase. An air-breathing cathode is incorporated to paper-based μFC through which atmospheric oxygen is continuously supplied. The paper-based μFC performance is studied by polarization curves and chronoamperometry to determinate the power output and stability. Peak power of 1.49 ​mW and a stable current of 1.35 ​mA at 0.35 ​V for 28 ​h can be achieved with this prototype under room temperature. To interpret the device performance a numerical model is developed and validated with the experimental polarization curve. The fuel and oxidant concentration profiles in the electrodes from the model demonstrates a constant species availability at the cathode and anode and explains the stable current obtained in the experimental measurements. Subsequently, a stack of four MμFCFP was developed and evaluated in both series and parallel connections. In the parallel configuration, a maximum open circuit potential (OCP) of 0.69 ​V with a maximum current and power output of 34.53 ​mA and 4.14 ​mW are delivered, respectively. Conversely, in the series connection, a total current of 7.35 ​mA, an OCP of 2.39 ​V and a maximum power of 3.57 ​mW are reached. As a proof of concept, the stack successfully operates a 3 green LEDs array, each requiring a 2.1–2.5 ​V and 4.2–5 ​mW power to function, for a continuous duration of 3 ​h.

Single-cell trapping and retrieval in open microfluidics

Among various single-cell analysis platforms, hydrodynamic cell trapping systems remain relevant because of their versatility. Among those, deterministic hydrodynamic cell-trapping systems have received significant interest; however, their applications are limited because trapped cells are kept within the closed microchannel, thus prohibiting access to external cell-picking devices. In this study, we develop a hydrodynamic cell-trapping system in an open microfluidics architecture to allow external access to trapped cells. A technique to render only the inside of a polydimethylsiloxane (PDMS) microchannel hydrophilic is developed, which allows the precise confinement of spontaneous capillary flow in the open-type microchannel with a width on the order of several tens of micrometers. Efficient trapping of single beads and single cells is achieved, in which trapped cells can be retrieved via automated robotic pipetting. The present system can facilitate the development of new single-cell analytical systems by bridging between microfluidic devices and macro-scale apparatus used in conventional biology.

Potential and motion design of droplets in a wetting-programable nanoporous surface

In this study, we design a wetting-programable method to create a surface channel for droplets. Superhydrophilic channels can be designed and erased reversibly by selectively masking the wetting and prewetting process on superhydrophobic silicone surfaces with microgroove and nanoporous structures fabricated using a micromachining technique. We found that droplets in this flow channel present a complex wetting behavior. The surface that undergoes infiltration by wetting and prewetting transitions from being a superhydrophobic surface to becoming a superhydrophilic surface. To understand the dynamics of droplet exploration, we measured the spreading distance of the droplet’s leading edge and the spontaneous capillary flow (SCF). The final spreading length of the droplet’s leading edge was also recorded. We found that increasing the prewetting volume enhances the final spreading distance of the droplet’s leading edge and the average SCF speed. Prewetting minimizes the apparent contact angle and pinning, promoting superhydrophilicity and increasing the droplet mobility. To further enhance our control over liquid transport, we incorporated vertical vibration as an active approach to regulate the programmed transport of droplets. We observed a substantial extension in the final spreading distance of the droplet’s leading edge. This extension was attributed to the effective overcoming of the pinning phenomena caused by the application of vertical vibration. With the above-mentioned methods, we achieve guided motion and multi-droplet sequence mixing of droplets with the designed channel shapes. Our results suggest how the wetting behavior dynamics of a multi-level structural surface are affected by prewetting liquid and vertical vibration. This provides insight into the wetting mechanism on porous superhydrophobic/superhydrophilic material.

An open microfluidic design for contact angle measurement

Spontaneous capillary flow in open microchannels is a phenomenon driven by surface energies. The contact angle that the liquid forms with the channel's substrate material and the cross-section of the microchannel decide whether liquid from a connected reservoir will automatically fill the channel or not. In this work we show how this behavior can be used to design a passive contact angle measurement device (CAMD) based on parabolic open microgrooves. To that end, we present a theory of open capillary flow in such microgrooves and compare the results to minimal energy surface simulations. Additionally, we discuss that the condition for capillary flow of curved microchannels is essentially equal to the condition for their straight counterparts having the same cross-section. Lastly, we present two demonstrators of our CAMD made out of micromilled poly(methyl methacrylate). The devices consist of five open microchannels with different cross-sections which are connected to a common liquid reservoir. We show how the behavior of a liquid placed into that reservoir can be used to evaluate the contact angle between the liquid and the substrate material. A comparison to conventional contact angle goniometry shows that our approach is able to successfully estimate contact angles with an accuracy of 10° by design which can be improved by employing a greater number of microchannels. Since our devices were automatically designed and can be tuned to specific applications, this provides an easy approach to include contact angle measurement into existing lab-on-a-chip devices.

Characterization of liquid flow and electricity generation in a glass channel based evaporation-driven electrokinetic energy conversion device

Evaporation-driven spontaneous capillary flow presents a promising approach for driving electrolytes through electrically charged channels and pores in electrokinetic energy conversion devices. However, there are no literature reports of detailed flow visualization in these systems and/or experimental observations relating the liquid velocity and evaporation rate to the generated voltage and current. In this manuscript, we describe such a visualization study for a glass channel based electrokinetic energy conversion device with one of its channel terminals left open to ambient air for facilitating the evaporation process. Fluorescence microscopy was used to measure the liquid velocity in the electrokinetic energy conversion channel by observing the advancement of an electrolyte solution dyed with a neutral tracer. The accumulation of the same dye tracer was also imaged at the open terminal of this glass conduit to estimate the rate of solvent evaporation, which was found to be consistent with the flow velocity measurements. Additionally, an electrochemical analyzer was employed to record the electrical voltage and current produced by the device under different operating conditions. The highest electrical power output was derived in our experiments upon flowing de-ionized water through a 1 μm deep channel, which also produced the fastest liquid velocity in it. Moreover, the energy conversion efficiency of our device was observed to increase for shallower channels and lower ionic strength electrolytes, consistent with previous literature reports on electrokinetic energy conversion platforms.

Investigation of spontaneous capillary flow and heat transfer characteristics in parallel microchannels based on phase-field model

• The mechanism of liquid surface oscillation in capillary flow is revealed. • Effect of channel structure on capillary flow and heat transfer is discussed. • A novel heat sink based on parallel capillary microchannels is constructed. • Non-circular heat sink obtains better capillary flow and heat transfer performance. Passive heat dissipation based on capillary flow has excellent applications in the thermal management of electronic devices and spacecraft due to its advantages of spontaneity and high stability. However, the conflicting relationship between capillary pressure and permeability in conventional sintered capillary wick severely limits its heat dissipation limit, and the complex and tortuous flow path is the key limiting factor. In this study, a novel close-packed capillary microchannel is constructed to provide an unobstructed flow path, and based on the phase-field model, the capillary flow and heat transfer characteristics of capillary microchannels with different structures are investigated. First, for the single channel, the oscillatory mechanism of phase interface in the initial stage of capillary flow, which has received little attention before, is revealed, which is mainly caused by the reconstruction of the advancing meniscus. This provides more nuanced insights into the initiating mechanism of capillary flow. In the noncircular channel, due to the change in curvature of the liquid surface at the acute angle and the expansion of the adsorption layer on the wall surface, the capillary velocity and heat transfer capacity improve compared to circular channels of equal hydraulic diameter. In addition, the effects of channel diameter and wettability on capillary flow and heat transfer are discussed. Capillary microchannels with better wettability and larger diameter achieve better capillary flow and heat transfer capabilities. On this basis, the parallel capillary microchannels inside the block are analyzed, and the non-circular channels achieve a higher capillary velocity while ensuring capillary pressure. Furthermore, the design of the noncircular section obtains a smaller thermal resistance and greater heat transfer efficiency, where the triangular microchannels show the best results. For the same hydraulic diameter and total block area, the performance decreases continuously as the inner angle of the channel increases. These results provide new ideas and a theoretical basis for designing high-performance passive heat sinks based on capillary flow.

Facile Fabrication of Flexible Polymeric Membranes with Micro and Nano Apertures over Large Areas.

Freestanding, flexible and open through-hole polymeric micro- and nanostructured membranes were successfully fabricated over large areas (>16 cm2) via solvent removal of sacrificial scaffolds filled with polymer resin by spontaneous capillary flow. Most of the polymeric membranes were obtained through a rapid UV curing processes via cationic or free radical UV polymerisation. Free standing microstructured membranes were fabricated across a range of curable polymer materials, including: EBECRYL3708 (radical UV polymerisation), CUVR1534 (cationic UV polymerisation) UV lacquer, fluorinated perfluoropolyether urethane methacrylate UV resin (MD700), optical adhesive UV resin with high refractive index (NOA84) and medical adhesive UV resin (1161-M). The present method was also extended to make a thermal set polydimethylsiloxane (PDMS) membranes. The pore sizes for the as-fabricated membranes ranged from 100 µm down to 200 nm and membrane thickness could be varied from 100 µm down to 10 µm. Aspect ratios as high as 16.7 were achieved for the 100 µm thick membranes for pore diameters of approximately 6 µm. Wide-area and uniform, open through-hole 30 µm thick membranes with 15 µm pore size were fabricated over 44 × 44 mm2 areas. As an application example, arrays of Au nanodots and Pd nanodots, as small as 130 nm, were deposited on Si substrates using a nanoaperture polymer through-hole membrane as a stencil.

Wicking dynamics into two-rail open channel with periodical branches

Wicking into open channels with branches is frequently adopted in multiple assays for medical testing. The penetration velocity plays a key role in improving efficiency. However, it is significantly reduced in the bifurcation microfluidic systems. As a result, understanding and enhancing wicking dynamics in bifurcation capillary flow is imperative. Capillary imbibition into a two-rail channel with periodical branches is investigated by many-body dissipative particle dynamics. The influences of the branch width and surface wettability on the penetration velocity and imbibition flow rate are examined. Four different types of spontaneous capillary flows are observed, including no invasion into branches, reduction in the penetration velocity, stick-slip motion, and inter-branch gap covered by liquid. Four regimes are identified accordingly, the phase diagram relates the flow behavior to the branch width, and surface wettability is established. As the branch width is significantly large (small gap), the penetration velocity is found to be greater than that without branches. It is attributed to the partial covering behavior, which leads to the effective width more than the main channel width.

A condition for spontaneous capillary flow in open microgrooves

In this work, we investigate the behaviour of liquids in symmetric open microgrooves and give a criterion for spontaneous capillary flow. To that end, we use a two-dimensional model and analyse the liquid morphologies minimizing the Gibbs energy of the system. We find that the condition of a flat liquid surface, which was hitherto assumed, is indeed the solution minimizing the Gibbs energy, so that it can safely be accepted to investigate whether open capillaries fill spontaneously. Furthermore, we find a condition for spontaneous capillary flow that depends on the cross-section of the channel alone. We use the findings to derive the critical contact angle, below which spontaneous capillary flow happens, for three examples including V-grooves, Gaussian grooves, and lenticular grooves.

Molecular dynamics study of spontaneous capillary flow and heat transfer in nanochannels

Passive cooling technology has become increasingly used in microelectronic devices, with capillary force being the most commonly used driving method. In this study, the capillary flow and heat transfer mechanisms and characteristics in nanochannels are studied based on molecular dynamics (MD), which have rarely been explored in previous studies. The capillary speed increases as the solid-liquid interaction intensity (εwl) increased, but when the wall is completely wetted, the capillary speed slows slightly an increasing εwl. By introducing wall slip, the Lucas-Washburn (L-W) equation is modified and a better fit is obtained. Furthermore, the effects of εwl, the temperature difference of the solid-liquid and surface nanostructure on capillary flow and heat transfer are analyzed. The increase in εwl and wall temperature enhances the capillary speed and heat transfer, but a continuous increase in the wall temperature will accelerate liquid evaporation and cause climb stagnation. The introduction of nanostructures slows the capillary speed, but can enhance the solid-liquid heat transfer. Finally, the microscopic mechanism of the above findings is analyzed by potential energy and density analyses. The reduction of the liquid potential energy at the solid-liquid interface can accelerate the capillary speed to a certain extent, but when the potential energy is too low, the liquid atoms will gather near the wall surface, blocking the capillary climbing. The expansion of the low potential energy region and the high mass density region enhances the solid-liquid heat transfer.

Perfusable micro-vascularized 3D tissue array for high-throughput vascular phenotypic screening

Microfluidic organ-on-a-chip technologies have enabled construction of biomimetic physiologically and pathologically relevant models. This paper describes an injection molded microfluidic platform that utilizes a novel sequential edge-guided patterning method based on spontaneous capillary flow to realize three-dimensional co-culture models and form an array of micro-vascularized tissues (28 per 1 × 2-inch slide format). The MicroVascular Injection-Molded Plastic Array 3D Culture (MV-IMPACT) platform is fabricated by injection molding, resulting in devices that are reliable and easy to use. By patterning hydrogels containing human umbilical endothelial cells and fibroblasts in close proximity and allowing them to form vasculogenic networks, an array of perfusable vascularized micro-tissues can be formed in a highly efficient manner. The high-throughput generation of angiogenic sprouts was quantified and their uniformity was characterized. Due to its compact design (half the size of a 96-well microtiter plate), it requires small amount of reagents and cells per device. In addition, the device design is compatible with a high content imaging machine such as Yokogawa CQ-1. Furthermore, we demonstrated the potential of our platform for high-throughput phenotypic screening by testing the effect of DAPT, a chemical known to affect angiogenesis. The MV-IMPACT represent a significant improvement over our previous PDMS-based devices in terms of molding 3D co-culture conditions at much higher throughput with added reliability and robustness in obtaining vascular micro-tissues and will provide a platform for developing applications in drug screening and development.

Spontaneous Imbibition in Paper-Based Microfluidic Devices: Experiments and Numerical Simulations.

Microfluidic paper-based analytical devices (μPADs) have quickly been an excellent choice for point-of-care diagnostic platforms ever since they appeared. Because capillary force is the main driving force for the transport of analytes in μPADs, low spontaneous imbibition rates may limit the detection sensitivity. Therefore, quantitative understanding of internal spontaneous capillary flow progress is requisite for designing sensitive and accurate μPADs. In this work, experimental and numerical studies have been performed to investigate the capillary flow in a typical filter paper. We use light-transmitting imaging technology to study wetting saturation changes in the paper. Our experimental results show an obvious transition of a saturated wetting front into an unsaturated wetting front as the imbibition proceeds. We find that the single-phase Darcy model considerably overestimates the temporal wetting penetration depths. Alternatively, we use the Richards equation together with the two-phase flow material properties that are obtained from the image-based pore-network modeling of the filter paper. Moreover, we have considered a dynamic term in the capillary pressure due to strong wetting dynamics in spontaneous imbibition. As a result, the numerical predictions of spontaneous imbibition in the paper are significantly improved. Our studies provide insights into the development of a quantitative spontaneous imbibition model for μPADs applications.

Anomalous spontaneous capillary flow of water through graphene nanoslits: Channel width-dependent density

Unexpectedly fast flow (up to 1 m/s) of water in graphene nanoslits was reported in experiments, revealing the breakdown of Washburn’s equation with no-slip condition. In this work, the imbibition dynamics of water into graphene nanoslits, ranging in width from one to ten atomic planes, is explored via Molecular Dynamics. The density of imbibed water in the nanoslit is analyzed and it depends on the channel width. Moreover, the interfacial tensions, equilibrium contact angle of water on the graphene sheet, and slip lengths of steady flow in nanoslits are evaluated. The slip length is found to decrease with increasing the channel width. The analyses of spontaneous capillary flow indicate that the time evolution of the penetration length follows Washburn’s equation qualitatively and the flow rates are comparable to the experimental results. However, the dependence of the imbibition rate on the channel width does not agree with Washburn’s equation at all. As the channel width-dependent slip length is introduced, the imbibition dynamics can be reasonably described by Washburn’s equation. The channel width-dependent behavior can be explained by the variation of the water state with the channel width, which can be realized by the changes of water densities and hydrogen bonds with the channel width.

Investigation of Spontaneous Capillary Flow and Heat Transfer Characteristics in Parallel Microchannels Based on Phase-Field Model

Investigation of Spontaneous Capillary Flow and Heat Transfer Characteristics in Parallel Microchannels Based on Phase-Field Model

Imbibition dynamics in an open-channel capillary with holes

Spontaneous capillary flow is generally described by Washburn’s equation. However, the modification must be made for imbibition dynamics in an open channel. Moreover, the presence of the holes in the capillary for flow division or bifurcation complicates the outcome of the imbibition process. In this work, capillary imbibition into a two-rail channel with/without holes is investigated by many-body dissipative particle dynamics (MDPD). The influences of channel height (h), channel width (w), hole size (wh), and surface wettability (θY) on the imbibition rate are examined. For two-rail channel without holes, the simulation results agree reasonably with Washburn’s equation with the correction 1-(h/w)/cosθY. The penetration rate grows with reducing the channel height or enlarging the channel width. However, in the presence of holes, the wicking dynamics is slowed down and four types of outcomes are observed: “covering”, “separation”, “pass/stop”, and “stoppage”. These states depend on the size and (symmetric or asymmetric) position of the rectangular hole. Finally, the phase diagrams which vary with θY, wh/h, and w/h are constructed. The domains of those states agree reasonably with the equations acquired simply from the analyses of surface free energy.

Anchor-IMPACT: A standardized microfluidic platform for high-throughput antiangiogenic drug screening.

In vitro models are becoming more advanced to truly present physiological systems while enabling high-throughput screening and analysis. Organ-on-a-chip devices provide remarkable results through the reconstruction of a three-dimensional (3D) cellular microenvironment although they need to be further developed in terms of multiple liquid patterning principle, material properties, and scalability. Here we present a 3D anchor-based microfluidic injection-molded plastic array culture platform (Anchor-IMPACT) that enables selective, space-intensive patterning of hydrogels using anchor-island for high-throughput angiogenesis evaluation model. Anchor-IMPACT consists of a central channel and an anchor-island, integrating the array into an abbreviated 96-well plate format with a standard microscope slide size. The anchor-island enables selective 3D cell patterning without channel-to-channel contact or any hydrogel injection port using an anchor structure unlike conventional culture compartment. The hydrogel was patterned into defined regions by spontaneous capillary flow under hydrophilic conditions. We configured multiple cell patterning structures to investigate the angiogenic potency of colorectal cancer cells in Anchor-IMPACT and the morphological properties of the angiogenesis induced by the paracrine effect were evaluated. In addition, the efficacy of anticancer drugs against angiogenic sprouts was verified by following dose-dependent responses. Our results indicate that Anchor-IMPACT offers not only a model of high-throughput experimentation but also an advanced 3D cell culture platform and can significantly improve current in vitro models while providing the basis for developing predictive preclinical models for biopharmaceutical applications.

Capillary-flow dynamics in open rectangular microchannels

Abstract

Injection molded open microfluidic well plate inserts for user-friendly coculture and microscopy.

Open microfluidic cell culture systems are powerful tools for interrogating biological mechanisms. We have previously presented a microscale cell culture system, based on spontaneous capillary flow of biocompatible hydrogels, that is integrated into a standard cell culture well plate, with flexible cell compartment geometries and easy pipet access. Here, we present two new injection molded open microfluidic devices that also easily insert into standard cell culture well plates and standard culture workflows, allowing seamless adoption by biomedical researchers. These platforms allow culture and study of soluble factor communication among multiple cell types, and the microscale dimensions are well-suited for rare primary cells. Unique advances include optimized evaporation control within the well, manufacture with reproducible and cost-effective rapid injection molding, and compatibility with sample preparation workflows for high resolution microscopy (following well-established coverslip mounting procedures). In this work, we present several use cases that highlight the usability and widespread utility of our platform including culture of limited primary testis cells from surgical patients, microscopy readouts including immunocytochemistry and single molecule fluorescence in situ hybridization (smFISH), and coculture to study interactions between adipocytes and prostate cancer cells.

Capillary Flow in Open Microgrooves: Bifurcations and Networks.

Open capillary flows are increasingly used in biotechnology, biology, thermics, and space science. So far, the dynamics of capillary flows has been studied mostly for confined channels. However, the theory of open microfluidics has considerably progressed during the last years, and an expression for the travel distance has been derived, generalizing the well-known theory of Lucas, Washburn, and Rideal. This generalization is based on the use of the average friction length and generalized Cassie angle. In this work, we successively study the spontaneous capillary flow in uniform cross section open rounded U-grooves-for which methods to determine the friction lengths are proposed-the flow behavior at a bifurcation, and finally flow in a simple-loop network. We show that after a bifurcation, the Lucas-Washburn-Rideal law needs to be adapted and the relation between the travel distance and time is more complicated than the square root of time dependency.