Evaporation dynamics of liquid marbles at elevated temperatures

Using Hybrid Coating to Fabricate Highly Stable and Expandable Transparent Liquid Marbles

Liquid marbles (LMs) are microliter-scale droplets coated with hydrophobic solid particles. The particle size and hydrophobicity of the surface coating determine their properties, such as transparency, expandability, and resistance to evaporation and coalescence, one or more of which can be critical to their application as microreactors. This study reports the use of a mixture of two different hydrophobic powders for fabrication of LMs for colorimetric assays: trichloro(1H,1H,2H,2H-perfluorooctyl) silane-linked silica gel (modified silica gel (MSG), particle size: 40-75 μm) and hexamethyldisilazane-linked fumed silica (modified fumed silica (MFS), average aggregate length: 200-300 nm). The hybrid coating mixture (MIX) prepared by mixing these MSG and MFS powders in a ratio of 3:7 (w/w), respectively, contained particles of different sizes as well as different hydrophobicity as the silane linked to MSG is more hydrophobic than the one linked to MFS. LMs fabricated using MIX as the surface coating were characterized and compared to LMs coated with MSG or MFS alone. It was observed that MIX LMs were comparable to the MFS LMs in transparency (higher than the MSG LMs), expandability (more than 20 times their initial volume), and stability against evaporation (for more than 4 h at 78% relative humidity at 26 °C). However, in terms of resistance to coalescence, the MIX LMs showed a resistance comparable to that of MSG LMs, much higher than that of MFS LMs. Further experiments demonstrated that it is the presence of the particles of different sizes (MSG particles are ∼100 times larger than MFS) that improves the resistance to coalescence rather than the higher hydrophobicity of the MSG. Three different colorimetric assays were performed in the MIX LMs, and the results obtained were comparable in accuracy and precision to those obtained in a standard polystyrene microwell plate system. Low quantities of the analytes could be detected and quantified, as evidenced by the limit of detection (alkaline phosphatase (AP): 0.18 μg/mL; bovine serum albumin (BSA): 2.28 μg/mL; and chymotrypsin: 3.69 μM) and limit of quantification (AP: 0.59 μg/mL; BSA: 12.29 μg/mL; and chymotrypsin: 7.59 μM) values. Color intensities in LMs were quantified using a smartphone application, which provides the added benefit of an instrument-free approach. These findings highlight the potential of using LMs stabilized with mixtures of nano- and microparticles as robust, versatile microreactors for portable and sensitive colorimetric assays, paving the way for more accessible and efficient diagnostic tools.

Biodegradable Polymers for Micro Elastofluidics

Micro elastofluidics is an emerging research field that encompasses characteristics of conventional microfluidics and fluid-structure interactions. Micro elastofluidics is expected to enable practical applications, for instance, where direct contact between biological samples and fluid handling systems is required. Besides design optimization, choosing a proper material is critical to the practical use of micro elastofluidics upon interaction with biological interface and after its functional lifetime. Biodegradable polymers are one of the most studied materials for this purpose. Micro elastofluidic devices made of biodegradable polymers possess exceptional mechanical elasticity, excellent bio compatibility, and structural degradability into non-toxic products. This article provides an insightful and systematic review of the utilization of biodegradable polymers in digital and continuous-flow micro elastofluidics.

Impact of coating particles on liquid marble lifetime: reactor engineering approach to evaporation

Liquid marbles are soft matter objects characterised by a liquid droplet enclosed within a hydrophobic particle coating, preventing wetting. This distinctive structure serves as active sites for solid-liquid-gas reactions. However, the impact the chosen coating material has on liquid marble stability, particularly regarding the number of coating layers and material wetting, remains uncertain. There is a need for a modelling approach to predict the overall lifetime considering these coating characteristics. This study reveals that for PTFE liquid marbles evaporating at ambient temperature, smaller coating particles (250 nm) extend their lifetime by forming a multilayered coating. Conversely, using larger particle sizes (200 μm) results in the formation of monolayer liquid marbles with shorter lifetimes than their equivalent naked droplets. Additionally, a higher number of particle layers and a larger contact angle generally enhance the liquid marble's lifetime. For multilayered liquid marbles comprised of smaller particles (250 nm), the particle contact angle is found to have a more significant impact than the number of layers on lifetime extension, whereas the opposite holds true for larger particle sizes (20 μm). A modelling approach using the reactor engineering method for liquid marble evaporation demonstrates excellent agreement with experimental results, yielding an R2 of 0.996. The implementation of this specific model, capable of assessing lifetime across various physical modifications, will enhance our understanding of liquid marble properties before their application in biomedical, microreactor, and green technologies.

Liquid Marbles and Drops on Superhydrophobic Surfaces: Interfacial Aspects and Dynamics of Formation: A Review

Liquid marbles (LMs) are droplets encapsulated with powders presenting varied roughness and wettability. These LMs have garnered a lot of attention due to their dual properties of leakage-free and quick transport on both solid and liquid surfaces. These droplets are in a Cassie-Baxter wetting state sitting on both roughness and air pockets existing between particles. They are also reminiscent of the state of a drop on a superhydrophobic (SH) surface. In this review, LMs and bare droplets on SH surfaces are comparatively investigated in terms of two aspects: interfacial and dynamical. LMs present a fascinating class of soft matter due to their superior interfacial activity and their remarkable stability. Inherently hydrophobic powders form stable LMs by simple rolling; however, particles with defined morphologies and chemistries contribute to the varied stability of LMs. The factors contributing to this interesting robustness with respect to bare droplets are then identified by tests of stability such as evaporation and compression. Next, the dynamics of the impact of a drop on a hydrophobic powder bed to form LMs is studied vis-à̀-vis that of drop impact on flat surfaces. The knowledge from drop impact phenomena on flat surfaces is used to build and complement insights to that of drop impact on powder surfaces. The maximum spread of the drop is empirically understood in terms of dimensionless numbers, and their drawbacks are highlighted. Various stages of drop impact-spreading, retraction and rebound, splashing, and final outcome-are systematically explored on both solid and hard surfaces. The implications of crater formation and energy dissipations are discussed in the case of granular beds. While the drop impact on solid surfaces is extensively reviewed, deep interpretation of the drop impact on granular surfaces needs to be improved. Additionally, the applications of each step in the sequence of drop impact phenomena on both substrates are also identified. Next, the criterion for the formation of peculiar jammed LMs was examined. Finally, the challenges and possible future perspectives are envisaged.

Experimental and Theoretical Investigation on the Dynamic Response of Ferrofluid Liquid Marbles to Steady and Pulsating Magnetic Fields

Liquid marbles are droplets enwrapped by a layer of hydrophobic micro/nanoparticles. Due to the isolation of fluid from its environment, reduction in evaporation rate, low friction with the surfaces, and capability of manipulation even on hydrophilic surfaces, liquid marbles have attracted the attention of researchers in digital microfluidics. This study investigates the manipulation of ferrofluid liquid marbles (FLMs) under DC and pulse width-modulated (PWM) magnetic fields generated by an electromagnet for the first time. At first, the threshold of the magnetic field for manipulating these FLMs is studied. Afterward, the dynamic response of the FLMs to the DC magnetic field for different FLM volumes, coil currents, and initial distances of FLM from the coil is studied, and a theoretical model is proposed. By applying the PWM magnetic field, it is possible to gain more control over the manipulation of the FLMs on the surface and adjust their position more accurately. Results indicate that with a decrease in FLM volume, coil current, and duty cycle, the FLM step length decreases; hence, FLM manipulation is more precise. Under the PWM magnetic field, it is observed that FLM movement is not synchronous with the generated pulse, and even after the coil is turned off, FLMs keep their motion. In the end, with proper adjustment of the electromagnet pulse width, launching of FLMs at a distance farther than the coil is observed.

Effect of surface roughness on the solar evaporation of liquid marbles

HYPOTHESIS

Nanostructured materials are widely used for solar energy harvesting and conversion due to their excellent photothermal properties. It is generally accepted that the better the light absorption ability, the better the photothermal conversion efficiency.

EXPERIMENT

A series of experiments in solar evaporation of liquid marbles (LMs) by coating the droplets with Fe3O4, Ni nanoparticles (NPs) and carbon nanotubes (CNTs) are conducted.

FINDINGS

Conversely, we found that the surface roughness of solar absorber plays a significant role in solar evaporation rather than the light absorption. The results disclose that the Fe3O4 NPs with the lowest absorptivity has the largest roughness on drop surface, while that of CNTs show the opposite properties. The evaporation dynamics of LMs are featured with dome or constant spherical collapse with different roughness. Such dynamic difference arises from the mechanical competition between the capillary force and interparticle interaction. Besides, the strong light-harvesting and near-field radiation enabled by the rough surfaces enhance the solar evaporation. The Fe3O4-LM shows the highest evaporation rate of 6.55 μg/s, which is 1.09 and 1.30 times larger than that of Ni-LM and CNT-LM, respectively. Numerical analysis reveals that the rough surface with stacking arrangement of NPs greatly enhances the light-induced electromagnetic field and heat concentration over the interface, leading to a plasmon-coupling boundary with high temperature for the fast evaporation. Uncovering these properties could be of much help for developments of heatable miniature evaporators or reactors and their counterparts, permitting a broad range of processes with precise temperature and kinetic control.

Preparation of Phase Change Melt Marbles with High Thermal Stability by Spontaneous Shrinkage and Encapsulation

Liquid marbles (LMs) are widely used in the fields of microfluids, gas sensitivity equipment, and microreactors. However, the thermal stability of the encapsulated liquid poses difficulty to the high-temperature stability of LMs. In this study, polar phase-change materials (PCMs) with high melting points were used as the encapsulated liquid of LMs. According to the required temperature, suitable PCMs were selected as the core and encapsulated by hydrophobic SiO₂ particles to form melt marbles (MMs). The types of PCMs used to prepare the MMs include erythritol, elemental sulfur, urea, and molten salts. Based on the premixed melting method, a series of MMs with high melting points and thermal stability were successfully developed. The highest acceptable temperature of the MMs exceeded 323 °C, and the evaporation rate of erythritol MMs was less than 1% at 140 °C in 8 h. Thus, the MMs maintained their excellent stability through multiple phase transitions. In the molten state, the MMs exhibited the properties of bounce ability, cuttability, and deformation resistance. The performance of the PCMs in energy storage and release during phase transition demonstrates their potential applications in the field of heat storage.

Fabrication of bioinspired edible liquid marble with phase transition and tunable water barrier property

Based on aphid wax-honeydew marble and the hydrophobic wax structure of lotus and its derived applications with superareophilic and superhydrophobic properties, edible carnauba wax and beeswax particles were mixed and utilized to mimic lotus wax and wrap liquid, thus forming liquid marbles (LMs). Through the utilization of continuous production system (CPS), wax as an interfacial surfactant, water and solid, air-phase or mixed-phase marble content was produced. The edible liquid marble (ELM) could encapsulate water and food droplets. Edible solid marble (ESM) and edible solid hollow marbles (ESHMs) could be fabricated by applying pectin or syrup. Moreover, through the heating of wax powders with different melting temperatures, stable tablets and hollow capsules could be produced. The wax powder as interfacial surfactant could firmly bind with pectin through hydrogen bonds on ESM. The edible LMs can therefore be applied for residue reduction, corrosion reduction, biohazard prevention and cleaning in the food industry. The other phase LMs could act as novel tools in the pharmaceutical and food industries with the above-mentioned water transport, preservation, sustained releasing and selective releasing abilities.

GRAPHIC ABSTRACT

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s42242-021-00158-z.

Loop-Mediated Isothermal Amplification in a Core-Shell Bead Assay for the Detection of Tyrosine Kinase AXL Overexpression

The upregulated expression of tyrosine kinase AXL has been reported in several hematologic and solid human tumors, including gastric, breast, colorectal, prostate and ovarian cancers. Thus, AXL can potentially serve as a diagnostic and prognostic biomarker for various cancers. This paper reports the first ever loop-mediated isothermal amplification (LAMP) in a core-shell bead assay for the detection of AXL gene overexpression. We demonstrated simple instrumentation toward a point-of-care device to perform LAMP. This paper also reports the first ever use of core-shell beads as a microreactor to perform LAMP as an attempt to promote environmentally-friendly laboratory practices.

Multifunctional liquid marbles to stabilize and transport reactive fluids

The self-organized transport and delivery of reactive liquids without spillage or loss of activity have been among the most daunting challenges for a long time. In this direction, we employ the concept of forming "liquid marbles" (LMs) to encapsulate and transport reactive hydrogen peroxide (H2O2) coated with functional microparticles. For example, peroxide marbles coated with a toner ink display remote-controlled magnetotactic movement inside a fluidic medium, thus overcoming the weaknesses associated with use of the bare droplets. Interestingly, in such a scenario, the coating of the marbles could also be removed or reformed by bringing the magnet towards or away from the marble. In this way, this process could ensure an on-demand remotely guided coating on the peroxide droplet or its removal. The liquid marbles carrying peroxide solutions are found to preserve the activity of the peroxide and exhibit a low evaporation rate compared with the uncoated peroxide fuel. Interestingly, oil droplets floating on the water could be recovered by introducing the armoured LMs into water under magnetic guidance. Further, the functionalized marbles could be employed as suicide bags for the on-demand delivery of reactive materials in targeted locations. Preliminary research on the antibacterial activity of such liquid marbles has proven to be effective in bacterial killing, which may create new avenues for emerging antibacterial and antibiofilm applications. Finally, such functionalized LMs have been employed to investigate the effects of surface charge on attachment of recombinant Escherichia coli bacteria expressing green fluorescent protein and monitoring the real-time imaging of bacterial death attached to the marble surface.

Liquid marble-based digital microfluidics - fundamentals and applications

Liquid marbles are droplets with volume typically on the order of microliters coated with hydrophobic powder. Their versatility, ease of use and low cost make liquid marbles an attractive platform for digital microfluidics. This paper provides the state of the art of discoveries in the physics of liquid marbles and their practical applications. The paper first discusses the fundamental properties of liquid marbles, followed by the summary of different techniques for the synthesis of liquid marbles. Next, manipulation techniques for handling liquid marbles are discussed. Applications of liquid marbles are categorised according to their use as chemical and biological reactors. The paper concludes with perspectives on the future development of liquid marble-based digital microfluidics.

Interfacial Crystallization within Liquid Marbles

We report interfacial crystallization in the droplets of saline solutions placed on superhydrophobic surfaces and liquid marbles filled with the saline. Evaporation of saline droplets deposited on superhydrophobic surface resulted in the formation of cup-shaped millimeter-scaled residues. The formation of the cup-like deposit is reasonably explained within the framework of the theory of the coffee-stain effect, namely, the rate of heterogeneous crystallization along the contact line of the droplet is significantly higher than in the droplet bulk. Crystallization within evaporated saline marbles coated with lycopodium particles depends strongly on the evaporation rate. Rapidly evaporated saline marbles yielded dented shells built of a mixture of colloidal particles and NaCl crystals. We relate the formation of these shells to the interfacial crystallization promoted by hydrophobic particles coating the marbles, accompanied with the upward convection flows supplying the saline to the particles, serving as the centers of interfacial crystallization. Convective flows prevail over the diffusion mass transport for the saline marbles heated from below.

Elemental Sulfur-Stabilized Liquid Marbles: Properties and Applications

Sulfur-stabilized liquid marbles were readily prepared by rolling water droplets on a sulfur (S₈) powder bed. Because of the construction of a gel layer on the surface of liquid marbles, the resulting liquid marbles have shape-designable characteristics. The effects of rolling time and volume of droplets on the deformability of sulfur-stabilized liquid marbles were investigated along with their mechanical stability and lifetime. The capability of sulfur-stabilized liquid marbles to be deformed at different pH values enables these liquid marbles to act as microreservoirs with desired shapes for aqueous solutions. Immersing the sulfur-stabilized liquid marbles into organic liquids leads to an increase in the liquid marbles' lifetime, and thereby they can survive at the interface of aqueous-organic two-phased systems for a long time. Finally, the applications of sulfur-stabilized liquid marbles as photocatalytic microreactors, electrochemical microcells, and monodisperse Pickering-like emulsions were demonstrated.

Redox-responsive functionalized hydrogel marble for the generation of cellular spheroids

Liquid marbles (LMs) have recently shown a great promise as microbioreactors to construct self-supported aqueous compartments for chemical and biological reactions. However, the evaporation of the inner aqueous liquid core has limited their application, especially in studying cellular functions. Hydrogels are promising scaffolds that provide a spatial environment suitable for three-dimensional cell culture. Here, we describe the fabrication of redox-responsive hydrogel marbles (HMs) as a three-dimensional cell culture platform. The HMs are prepared by introducing an aqueous mixture of a tetra-thiolated polyethylene glycol (PEG) derivative, thiolated gelatin (Gela-SH), horseradish peroxidase, a small phenolic compound, and human hepatocellular carcinoma cells (HepG2) to the inner aqueous phase of LMs. Eventually, HepG2 cells are encapsulated in the HMs then immersed in culture media, where they proliferate and form cellular spheroids. Experimental results show that the Gela-SH concentration strongly influences the physicochemical and microstructure properties of the HMs. After 6 days in culture, the spheroids were recovered from the HMs by degrading the scaffold, and examination showed that they had reached up to about 180 μm in diameter depending on the Gela-SH concentration, compared with 60 μm in conventional HMs without Gela-SH. After long-term culture (over 12 days), the liver-specific functions (secretion of albumin and urea) and DNA contents of the spheroids cultured in the HMs were elevated compared with those cultured in LMs. These results suggest that the developed HMs can be useful in designing a variety of microbioreactors for tissue engineering applications.

Core-Shell Beads Made by Composite Liquid Marble Technology as A Versatile Microreactor for Polymerase Chain Reaction

Over the last three decades, the protocols and procedures of the DNA amplification technique, polymerase chain reaction (PCR), have been optimized and well developed. However, there have been no significant innovations in processes for sample dispersion for PCR that have reduced the amount of single-use or unrecyclable plastic waste produced. To address the issue of plastic waste, this paper reports the synthesis and successful use of a core-shell bead microreactor using photopolymerization of a composite liquid marble as a dispersion process. This platform uses the core-shell bead as a simple and effective sample dispersion medium that significantly reduces plastic waste generated compared to conventional PCR processes. Other improvements over conventional PCR processes of the novel dispersion platform include increasing the throughput capability, enhancing the performance and portability of the thermal cycler, and allowing for the contamination-free storage of samples after thermal cycling.

Contactless sensing of liquid marbles for detection, characterisation & computing

Liquid marbles (LMs) are of growing interest in many fields, including microfluidics, microreactors, sensors, and signal carriers. The generation of LMs is generally performed manually, although there has recently been a burst of publications involving 'automatic marble makers'. The characteristics of a LM is dependent on many things, including how it is generated, it is therefore important to be able to characterise LMs once made. Here is presented a novel contactless LM sensor, constructed on a PCB board with a comb-like structure of 36 interlacing electrical traces, 100 μm wide and 100 μm apart. This cheap, scalable, and easy to use sensor exploits the inherent impedance (comprised of the electrical resistance, capacitive reactance and inductive reactance) of different LMs. With it, parameters of a LM can be easily determined, without interfering with the LM. These parameters are (1) particle size of the LM coating, (2) the concentration of a NaCl solution used as the LM core, and (3) the volume of the LM. Additionally, due to the comb-like nature of the sensor, the accurate positioning (down to the inter-trace spacing) of the LM can be ascertained. The new sensor has been shown to work under both static and dynamic (mobile) conditions. The capacitance of a LM was recorded to be 0.10 pF, which compares well with the calculated value of 0.12 pF.

Influence of water evaporation/absorption on the stability of glycerol-water marbles

The porous shell structure of liquid marbles allows liquid vapor to enter in/out of the liquid marbles, leading to the deformation/collapse of liquid marbles, which limits their application as miniature reactors for long-term chemical reactions. In this study, to prevent volatilization and maintain long-term stability, stable liquid marbles were fabricated by encapsulating glycerol/water droplets using superhydrophobic cellulose nanocrystals. The influence of water evaporation and absorption on the stability of aqueous glycerol marbles at different relative humidities (RHs) was investigated. At the same RH, the evaporation/absorption rates of the liquid marbles decreased on increasing the glycerol concentration. For the liquid marbles with the same glycerol volume concentration, the evaporation rates decreased with the increase in RH. The liquid marbles exhibited higher evaporation/absorption resistance compared with pure naked liquid droplets.

Liquid marbles as biochemical reactors for the polymerase chain reaction

The polymerase chain reaction (PCR) is a popular and well-established DNA amplification technique. Technological and engineering advancements in the field of microfluidics have fuelled the progress of polymerase chain reaction (PCR) technology in the last three decades. Advances in microfluidics-based PCR technology have significantly reduced the sample volume and thermal cycling time. Further advances led to novel and accurate techniques such as the digital PCR. However, contamination of PCR samples, lack of reusability of the microfluidic PCR platforms, complexity in instrumentation and operation remain as some of the significant drawbacks of conventional microfluidic PCR platforms. Liquid marbles, the recently emerging microfluidic platform, could potentially resolve these drawbacks. This paper reports the first liquid marble based polymerase chain reaction. We demonstrated an experimental setup for the liquid-marble based PCR with a humidity-controlled chamber and an embedded thermal cycler. A concentrated salt solution was used to control the humidity of the PCR chamber which in turn reduces the evaporation rate of the liquid marble. The successful PCR of microbial source tracking markers for faecal contamination was achieved with the system, indicating potential application in water quality monitoring.

On-Demand Coalescence and Splitting of Liquid Marbles and Their Bioapplications

Coalescence and splitting of liquid marbles (LMs) are critical for the mixture of precise amount precursors and removal of the wastes in the microliter range. Here, the coalescence and splitting of LMs are realized by a simple gravity-driven impact method and the two processes are systematically investigated to obtain the optimal parameters. The formation, coalescence, and splitting of LMs can be realized on-demand with a designed channel box. By selecting the functional channels on the device, gravity-based fusion and splitting of LMs are performed to mix medium/drugs and remove spent culture medium in a precise manner, thus ensuring that the microenvironment of the cells is maintained under optimal conditions. The LM-based 3D stem cell spheroids are demonstrated to possess an approximately threefold of cell viability compared with the conventional spheroid obtained from nonadhesive plates. Delivery of the cell spheroid to a hydrophilic surface results in the in situ respreading of cells and gradual formation of typical 2D cell morphology, which offers the possibility for such spheroid-based stem cell delivery in regenerative medicine.

An automated on-demand liquid marble generator based on electrohydrodynamic pulling

Liquid marble is a recently emerging digital microfluidic platform with a wide range of applications. Conventional liquid marbles are synthesized by coating liquid droplets with a thin layer of hydrophobic powder. Existing and emerging applications of liquid marbles require a contamination-free synthesis of liquid marbles with a high degree of reproducibility of their volume. Despite this requirement, the synthesis of liquid marbles has been still carried out manually. Manual production of liquid marbles leads to inconsistent volume and the possibility of contamination. The synthesis of liquid marbles with submicroliter volume is difficult to achieve and prone to large errors. This paper discusses the design and development of the first automated on-demand liquid marble generator with submicroliter capability. The device utilizes electrohydrodynamic pulling of liquid droplets on to a hydrophobic powder bed and subsequently coats them with the hydrophobic powder to synthesize liquid marbles of a desired volume.

Manipulation of a floating liquid marble using dielectrophoresis

A liquid marble is a microliter-sized droplet coated with hydrophobic powder. The porous coating prevents the liquid content from being in direct physical contact with its surroundings, making the liquid marble perfectly non-wetting. On the one hand, the non-wetting ability allows the liquid marble to float and move across a liquid surface with little resistance. On the other hand, the porosity enables gas exchange between the liquid marble and its surroundings. These properties allow the liquid marble to serve as a bioreactor platform for important applications such as cell culture. Liquid marbles floating on a free liquid surface prevent evaporation due to the high humidity near the liquid surface. Moving a floating liquid marble allows for stirring and mixing inside the liquid marble. This paper reports a novel technique for manipulating a floating liquid marble using dielectrophoresis. A relatively simple setup can move liquid marbles of various sizes across the water surface at high speeds. We also present an analytical model to model and accurately predict the motion of the floating liquid marble. The technique reported here potentially allows for high-throughput and efficient handling of floating liquid marbles as a digital microfluidics platform.

Digital polymerase chain reaction technology - recent advances and future perspectives

Digital polymerase chain reaction (dPCR) technology has remained a "hot topic" in the last two decades due to its potential applications in cell biology, genetic engineering, and medical diagnostics. Various advanced techniques have been reported on sample dispersion, thermal cycling and output monitoring of digital PCR. However, a fully automated, low-cost and handheld digital PCR platform has not been reported in the literature. This paper attempts to critically evaluate the recent developments in techniques for sample dispersion, thermal cycling and output evaluation for dPCR. The techniques are discussed in terms of hardware simplicity, portability, cost-effectiveness and suitability for automation. The present paper also discusses the research gaps observed in each step of dPCR and concludes with possible improvements toward portable, low-cost and automatic digital PCR systems.