Moving droplets between closed and open microfluidic systems

Droplet manipulation on an adjustable closed-open digital microfluidic system utilizing asymmetric EWOD

The closed-open digital microfluidic (DMF) system offers a versatile and powerful platform for various applications by combining the advantages of both closed and open structures. The current closed-open DMF system faces challenges in scaling up due to electrode structural differences between closed and open regions. Here we developed an adjustable closed-open DMF platform by utilizing the modified slippery liquid-infused porous surfaces (SLIPS) with asymmetric electrowetting on dielectric (AEWOD) as a hydrophobic dielectric layer. The consistent electrode structures of the bottom printed circuit board (PCB) electrode array on both the closed and open regions, and the utilization of a transparent acrylic with floating potential as the top plate allow a low-cost and easily scalable closed-open DMF system to be achieved. The impacts of applied voltage, parallel plate spacing, electrode switching interval, and electrode driving strategies on various droplet manipulations were investigated. The results show that the optimal plate spacings range from 340-510 μm within the closed region. Meanwhile, we also studied the influence of the thickness, geometry, and position of the top plate on the droplet movement at the closed-open boundary. Through force analysis and experimentation, it is found that a thin top plate and a bevel of ∼4° can effectively facilitate the movement of droplets at the boundary. Finally, we successfully achieved protein staining experiments on this platform and developed a customized smartphone application for the accurate detection of protein concentration. This innovative closed-open DMF system provides new possibilities for future applications in real-time biological sample processing and detection.

Research progress of electrode shapes in EWOD-based digital microfluidics

Digital microfluidics (DMF) is an innovative technology used for precise manipulation of liquid droplets. This technology has garnered significant attention in both industrial applications and scientific research due to its unique advantages. Among the key components of DMF, the driving electrode plays a role in facilitating droplet generation, transportation, splitting, merging, and mixing. This comprehensive review aims to present an in-depth understanding of the working principle of DMF particularly focusing on the Electrowetting On Dielectric (EWOD) method. Furthermore, it examines the impact of driving electrodes with varying geometries on droplet manipulation. By analyzing and comparing their characteristics, this review offers valuable insights and a fresh perspective on the design and application of driving electrodes in DMF based on the EWOD approach. Lastly, an assessment of the development trend and potential applications of DMF concludes the review, providing an outlook for future prospects in the field.

Droplet Transportation through an Orifice on Electrode for Digital Microfluidics Modulations

A digital microfluidic modular interface (chip-to-chip interface) which possesses an electrode with an orifice to vertically transport core-shell droplets is presented. The electrodes were geometrically designed to promote droplet deformation and suspension. The droplets were then applied with an electrical potential for insertion into and passage through the orifice. The concepts were tested with three types of droplets at the volume of 0.75~1.5 μL, which is usually difficult to transfer through an orifice. The integration of electrowetting on dielectric (EWOD) with paper-based microfluidics was demonstrated: the droplet could be transported within 10 s. More importantly, most of the core droplet (~97%) was extracted and passed through with only minimal shell droplets accompanying it.

Integrated Full-Range Droplet Actuation for Inkjet-Printed Digital Microfluidic Chip on Flexible Substrates

Flexible printed electronic technology makes it possible to fabricate low-cost digital microfluidic (DMF) chips. Inkjet printing on flexible substrates is one of the most cost-effective fabrication processes for DMF chips. Based on inkjet printing technology and simplified coating methods of dielectric and hydrophobic layers, we fabricated low-cost flexible DMF chips (FDMFCs) on PET sheet and on matte photo paper. The surface quality, conductivity and spatial output resolution of the silver lines under different number of printings, different line widths and line gaps on the two types of FDMFCs were comprehensively analyzed. The traditional square dispensing electrodes were optimized to reduce the volume error of the droplets generated during the repeated dispensing operations. Droplets can be driven to implement all the operations on various configurations of FDMFCs by electrowetting-on-dielectric, including closed configuration, open configuration, hybrid configuration composed of closed and open regions on a single chip, and open curved configuration, which are defined as full-range droplet actuation. The droplet motion between closed and open regions in two position modes of the top plate was deeply studied. Droplet operation experiments prove that the motion performance of droplets can be comparable to that of chips processed by traditional technology, and the rapid prototyping technology of the FDMFCs can make the performance of droplet operation mainly depend on the conductivity of the electrode layer and the electrode gap and greatly weaken the influence of the substrate surface quality on the FDMFCs.

Actuation of a Nonconductive Droplet in an Aqueous Fluid by Reversed Electrowetting Effect

Manipulation of a conductive droplet by electrowetting has been a popular topic in microfluidics whereby wettability of the droplet on a solid surface is increased by applying a voltage between the conductive droplet and the insulated surface. However, the opposite phenomenon, e.g., decreasing the wettability of a nonconductive droplet and increasing its contact angle (CA) by the reversed electrowetting (REW) effect, has been scarcely reported. Such a process involves not only the transient dynamics of droplet dewetting but also a critical condition for droplet detachment from the adhesive surface. In this work, actuation of a nonconductive droplet in an aqueous surrounding fluid by REW is studied experimentally. Silicone oil is used for the actuated droplet, and filtered water is used as the surrounding fluid. The solid substrate is made of a glass substrate coated with an indium tin oxide (ITO) film and then deposited by a dielectric layer of Parylene C. Potential difference is applied between the substrate and the surrounding fluid, eliminating the disturbance from the top needle on the motion of the droplet. Three different regimes are identified in the full range of operation. An underactuated regime occurs at low applied voltages, in which the CA of the droplet shows a monotonic increase with the increase of voltage (V). The friction coefficient of the contact line decreases with V before the CA saturation (V_s) but shows little change when V > V_s. At high voltages, the contact line of the sessile droplet is contracted excessively by REW. The droplet shows oscillation, and it refers to the overactuated regime. A combined time scale is proposed, and it verifies that the viscous dissipation of the contact line and liquid inertia show comparable contributions in the droplet dynamics. At sufficiently high voltages, the droplet is rejected completely from the surface. A critical equation for the threshold voltage of droplet detachment is built, and its validity is confirmed by experimental results.

Influence of liquid bridge formation process on its stability in nonparallel plates

The formation of a liquid bridge in non-parallel plates is very common and the stability (whether or not it can move spontaneously) of such liquid bridges has been studied a lot for industry, e.g. in printing applications. It is generally considered that the liquid bridge stability is determined by Contact Angle (CA), Contact Angle Hysteresis (CAH), the position of the liquid bridge (represented as P) and the dihedral angle (θ) between non-parallel plates. The stability equation is θ = f(CA, CAH, P). Since P is a process quantity, which is difficult to determine, so it is also difficult to obtain the critical equation for the stability of the liquid bridge. In the previous study (J. Colloid Interface Sci., 2017, 492, 207–217), based on the fitting simulation results, the critical equation about CA, CAH and θ is obtained, as θ = f(CA, CAH). However, in some special cases, the results are still biased (e.g. the weak hydrophilic situation). In this paper, unlike simulation, we get the critical equation θ = f(CA, CAH) from a theoretical point of view. For the first time, by in-depth analysis of the process of liquid bridge formation, the theoretical calculation equation of P is obtained as P = f(CA, CAH, θ). And then, combining the equations θ = f(CA, CAH, P) and P = f(CA, CAH, θ), the theoretical equation is obtained. A lot of simulations and experiments were performed to verify our theoretical equation. Furthermore, comparing our equation with the previous equation, it was found that our equation is more consistent with the experimental results (error less than 0.2°). Finally, the importance of considering the liquid bridging process (the function of P) for stability analysis is illustrated by comparing the results with those not considered (the difference is more than 20% in some cases). The outputs of this paper provide in-depth theoretical support for the analysis and application of liquid bridges.

The effect of liquid bridge formation process on its stability was discussed to obtain the theoretical equation for determining the stability of the liquid bridge.

Detection of cancer antigens (CA-125) using gold nano particles on interdigitated electrode-based microfluidic biosensor

Integrating microfluidics with biosensors is of great research interest with the increasing trend of lab-on-the chip and point-of-care devices. Though there have been numerous studies performed relating microfluidics to the biosensing mechanisms, the study of the sensitivity variation due to microfluidic flow is very much limited. In this paper, the sensitivity of interdigitated electrodes was evaluated at the static drop condition and the microfluidic flow condition. In addition, this study demonstrates the use of gold nanoparticles to enhance the sensor signal response and provides experimental results of the capacitance difference during cancer antigen-125 (CA-125) antigen-antibody conjugation at multiple concentrations of CA-125 antigens. The experimental results also provide evidence of disease-specific detection of CA-125 antigen at multiple concentrations with the increase in capacitive signal response proportional to the concentration of the CA-125 antigens. The capacitive signal response of antigen-antibody conjugation on interdigitate electrodes has been enhanced by approximately 2.8 times (from 260.80 to 736.33 pF at 20 kHz frequency) in static drop condition and approximately 2.5 times (from 205.85 to 518.48 pF at 20 kHz frequency) in microfluidic flow condition with gold nanoparticle-coating. The capacitive signal response is observed to decrease at microfluidic flow condition at both plain interdigitated electrodes (from 260.80 to 205.85 pF at 20 kHz frequency) and gold nano particle coated interdigitated electrodes (from 736.33 to 518.48 pF at 20 kHz frequency), due to the strong shear effect compared to static drop condition. However, the microfluidic channel in the biosensor has the potential to increase the signal to noise ratio due to plasma separation from the whole blood and lead to the increase concentration of the biomarkers in the blood volume for sensing.

Motion of droplets into hydrophobic parallel plates

Due to the superior operability and good anti-interference, the prospect of controlling microdroplets using a parallel plate structure (PPS) is very promising. However, in practical applications, droplets in such structures are often affected by various factors, resulting in deformation, evaporation, stress rupture and other phenomena, leading to equipment failure. Therefore, how to simply and effectively transfer liquid droplets to PPS to maintain the stable and efficient operation of the system has become an urgent problem to be solved. In this paper, a simple and effective ratchet-like strategy (relaxing and squeezing actions) is introduced to transfer droplets. To analyze the mechanism of the strategy and optimize the control, we conduct this study from three aspects. First, the droplet movement trend is obtained by analyzing the pressure between SPS and PPS. Second, the reasons why the droplet can achieve this inward motion are investigated. Through theoretical analysis, which is also proven by simulations and experiments, we creatively put forward that the asymmetric change of the contact angle (CA) induced by the asymmetric structure is the fundamental cause of this kind of motion. Due to the asymmetric change of the contact angle, the CA in the PPS will reach the advancing angle first in the squeezing process, and the CA in the SPS will reach the receding angle first in the relaxing process, thus causing the inward movement of the droplet. Third, to optimize this strategy, the effects of the following governing parameters are researched individually based on the corresponding simulations and experiments: the control parameters (the initial gap width of the PPS H₀ and the amount of squeezing and relaxing of ΔH) and the thickness of the top plate. Subsequently, an optimized ratchet-like cycle is achieved. In summary, these findings not only provide a new method by which to realize the movement of droplets toward hydrophobic PPSs but also creatively point out the cause of the ratchet strategy, which can be applied in many microfluidics fields.

A simple and effective ratchet-like strategy is introduced to transfer droplets. We creatively put forward that the asymmetric change of the contact angle induced by the asymmetric structure is the fundamental cause of this kind of motion.

Self-Propulsion and Shape Restoration of Aqueous Drops on Sulfobetaine Silane Surfaces

The motion of droplets on typical surfaces is generally halted by contact line pinning associated with contact angle hysteresis. In this study, it was shown that, on a zwitterionic sulfobetaine silane (SBSi)-coated surface, aqueous drops with appropriate solutes can demonstrate hysteresis-free behavior, whereas a pure water drop shows spontaneous spreading. By adding solutes such as polyethylene glycol, 2(2-butoxy ethoxy) ethanol, or sodium n-dodecyl sulfate, an aqueous drop with a small contact angle (disappearance of spontaneous spreading) was formed on SBSi surfaces. The initial drop shape was readily relaxed back to a circular shape (hysteresis-free behavior), even upon severe disturbances. Moreover, it was interesting to observe the self-propulsion of such a drop on horizontal SBSi surfaces in the absence of externally provided stimuli. The self-propelled drop tends to follow a random trajectory, and the continuous movement can last for at least 10 min. This self-propelled random motion can be attributed to the combined effects of the hysteresis-free surface and the Marangoni stress. The former comes from the total wetting property of the surface, while the latter originates from surface tension gradient due to fluctuating evaporation rates along the drop border.

A hydrophobic ionic liquid compartmentalized sampling/labeling and its separation techniques in polydimethylsiloxane microchip capillary electrophoresis

This paper demonstrates a novel compartmentalized sampling/labeling method and its separation techniques using a hydrophobic ionic liquid (IL)-1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imidate (BmimNTf₂ )-as the immiscible phase, which is capable of minimizing signal losses during microchip capillary electrophoresis (MCE). The MCE device consists of a silica tube connected to a straight polydimethylsiloxane (PDMS) separation channel. Poly(diallyldimethylammonium chloride) (PDDAC) was coated on the inner surface of channel to ease the introduction of IL plugs and enhance the IL wetting on the PDMS surface for sample releasing. Electroosmotic flow (EOF)-based sample compartmentalization was carried out through a sequenced injection into sampling tubes with the following order: leading IL plug/sample segment/terminal IL plug. The movement of the sample segment was easily controlled by applying an electrical voltage across both ends of the chip without a sample volume change. This approach effectively prevented analyte diffusion before injection into MCE channels. When the sample segment was manipulated to the PDDAC-modified PDMS channel, the sample plug then was released from isolation under EOF while IL plugs adsorbed onto channel surfaces owing to strong adhesion. A mixture of flavin adenine nucleotides (FAD) and flavin mononucleotides (FMN) was successfully separated on a 2.5 cm long separation channel, for which the theoretical numbers of plates were 15 000 and 17 000, respectively. The obtained peak intensity was increased 6.3-fold over the corresponding value from conventional electrokinetic injection with the same sampling time. Furthermore, based on the compartmented sample segment serving as an interim reactor, an on-chip fluorescence labeling is demonstrated.

Droplet-based Biosensing for Lab-on-a-Chip, Open Microfluidics Platforms

Low cost, portable sensors can transform health care by bringing easily available diagnostic devices to low and middle income population, particularly in developing countries. Sample preparation, analyte handling and labeling are primary cost concerns for traditional lab-based diagnostic systems. Lab-on-a-chip (LoC) platforms based on droplet-based microfluidics promise to integrate and automate these complex and expensive laboratory procedures onto a single chip; the cost will be further reduced if label-free biosensors could be integrated onto the LoC platforms. Here, we review some recent developments of label-free, droplet-based biosensors, compatible with "open" digital microfluidic systems. These low-cost droplet-based biosensors overcome some of the fundamental limitations of the classical sensors, enabling timely diagnosis. We identify the key challenges that must be addressed to make these sensors commercially viable and summarize a number of promising research directions.

Utilization of Cavity Vortex To Delay the Wetting Transition in One-Dimensional Structured Microchannels

Frictional resistance across rough surfaces depends on the existence of slip on the liquid-gas interface; therefore, prolonging the existence of liquid-gas interface becomes relevant. In this work, we explore manipulation of the cavity shape in order to delay the wetting transition. We propose that liquid-driven vortices generated in the air cavity dissipate sufficient energy to delay the Cassie-Wenzel transition. Toward this, we fabricated cavities on the side walls of a polydimethylsiloxane-based microchannel for easy visualization and analysis of the dynamics of the liquid-gas interface. Two distinct flow regimes are identified in the experimental envelope. In the first regime, the liquid-gas interface is found to be protruding into the flow field, thus increasing the pressure drop at low Reynolds number. In the second regime, flow rate and geometry-based wetting transitions are established at moderate to high Reynolds numbers. We then investigate the effect of different cavity shapes (square, trapezoidal, and U-shape) in delaying the wetting transition by manipulating liquid-driven vortices. Out of the shapes considered in this study, trapezoidal cavities perform better than cavities with vertical walls in delaying the wetting transition due to geometrical squeezing of vortices toward the liquid-gas interface. Numerical simulations corroborate the experimental findings in that cavities with inclined walls exert more force on the liquid-gas interface, thus delaying their wetting transition. The proposed method being passive in nature appears more attractive than previous active methods.