Free-Boundary Microfluidics - Robert Keane
Introduction
Microfluidics is a useful field for manipulating fluids and interfaces at the microscale. However, most of the designs are usually made in close chambers. Free-Boundary Microfluidics (FBM) is an alternative to this approach and offers a similar way to manipulate fluids but using continuous droplets or jets in spacious open environments. [1] FBM is incredibly useful for its greater versatility over closed chamber microfluidics. [1] FBM is split into different groups based on the main mechanisms at play in the system being used. [1] These four groups are hydrodynamic, electrohydrodynamic, interfacial tension based systems, and acoustics based systems. [1] A qualitative description of these groups and some of the specific techniques which arise are shown in Figure 1. These types of free-boundary microfluidics are discussed in greater detail below.
Hydrodynamics
Hydrodynamics-based FBM are methods driven hydrodynamically and consist of flow focusing, co-flow, free jets, and cross flow. [1] These techniques differ in how the continuous and dispersed phase flow next to each other. [1] These FBM techniques can generate droplets of jets either actively or passively. [1]
Flow Focusing
In flow-focusing, usually one phase known as a continuous phase surrounds a dispersed phase. [1] Once this occurs, the dispersed phase is able to break off into droplets and separate. In closed containers, this usually takes the form of droplet makers and is limited by the size of the containers and flow rate. [1] For FBM, the dispersed phase can consist of either a gas or liquid and is dictated by the velocity applied in the continuous phase and the chemical and physical properties of the individual fluids. [1] Some examples of flow focusing in FBM include stable cone-jetting using 3D flow focusing [2], gas-driven flow-focusing spinning [3], multiplex coaxial flow focusing for multicompartment janus microcapsule production [4], and active gas-driven flow focusing for controlled jet length. [5] Flow focusing is useful because it can be used for precise atomization of highly viscous fluids due to its high shear force. [1]
Co-flow
Co-flow based FBM is similar to flow-focusing FBM but instead the direction of the shearing of the dispersed phase by the continuous phase is in the direction of the dispersed phase without a significant velocity gradient. [1] Co-flow based FBM involves systems with co-flow designs which exist in free space which can take the form of gas or liquid driven flow and are modulated by pressure and flow rate. [1] Some example of co-flow FBM include air-assisted single cell printing [6], aerosol jet printing [7], composite droplet makers with inner layer gas driving the fluid [8], and composite nanofibers prepared by the 3D co-flow gas shearing strategy. [9] Co-flow is a useful technique because it can use programmable pulsed airflow to shear fluids and enable the patterning of soft materials with multi-scale and multi-interface properties [1]
Free-Jet
Free jet FBM does not depend on the shear driving force but on the role of pressure and the ability of gravity to overcome capillary forces. [1]. These jets are simpler to manipulate without an outer driving fluid. [1] Mechanical perturbations and external disturbances are used to influence the breakup of a jet. [1] One useful type of microfluidics which arise from this is in-air microfluidics which enables the in-flight formation of droplets, fibers, and particles. [10] An illustration of in-air microfluidics is shown in Figure 2. Other examples of free jet FBM include architected micromaterials from in-air polymerization [11] and multifunctional janus microparticle makers using in-air microfluidics. [12]
Cross flow
Cross-flow FBM uses an angle between the dispersed and continuous phases in order to dictate the direction of the flow. [1] For FBM, the driving medium is either liquid or gas depending on the configuration. [1] Cross flow has been most often associated with droplet fabrication [13] but new air flow assisted printing can allow the creation of new and interesting hydrogel structures like spherical helices and saddles just by modifying the position of the airflow needle and the airflow pressure. [14] Other examples of cross-flow FBM include spinning micro-pipette droplet generation [15] and asymmetrical oscillation for droplet generation. [16]
Electrohydrodynamics
Electrohydrodynamic (EHD) based microfluidics have origins in Rayleigh’s work on the action of droplet instability under electric fields. [1] However, only recently in the last two decades has research in EHD really ramped up. EHD involves using an electric field coupled with surface tension and viscoelastic forces. [1] The electric field provides the force for jetting, the surface tension hinders the droplet surface change to maintain surface energy, and the viscoelastic force determines the rheology exhibited by the jet. [1] Three important techniques which have been focused on include electrospray, electrohydrodynamic inkjet printing, and electrospinning. [1]
Electrospray
Electrospray is a key technique for producing monodisperse micro-nano droplets and capsules. [1] The typical coaxial electrospray system consists of a charged coaxial nozzle, a grounded electrode, an ink dispensing device, and DC power supply. [1] Usually, two immiscible inks form an emulsified meniscus at the outlet of the nozzle which grows until gravity forces out the droplet. [1] The electrostatic force helps to accelerate this dripping into jetting which can be modified and controlled. [1] This concept has led to interesting EHD droplet and particle production including one example which involves the formation of a Taylor Cone and a resulting charged thin jet. [17]
EHD Inkjet Printing
EHD Inkjet Printing builds off similar concepts in electrospray. Unlike typical printing methods where the power source is provided by a deformation of a fluid chamber, EHD Inkjet printing is done by using the force from the electric field to induce droplets at the nozzle outlet and form a Taylor cone. [1] The Taylor cone then produces a thin jet which can be used to print at higher resolutions and with greater viscosities. [1]18
Electrospinning
Electrospinning differs from electrospray in that it employs inks which have a greater viscoelasticity like polymer melts and thicker solutions. [1] As a result, unlike EHD droplet techniques, the forces from the electric field have to overcome rheological behaviors of the jet in addition to surface tension and viscoelastic forces. [1] The large viscoelasticies suppress the Rayleigh Plateau Instability and allow for the jet to keep a continuous straight line shape instead of breaking into droplets. [1] This allows for the creation of fibers. [1] Electrospinning has many uses including for multi-material 3D printing [19] which is shown in Figure 3.
Interfacial Tension
Interfacial based FBM systems rely on manipulating interfacial action to get certain droplets or emulsions. [1] The three primary ways of modifying the interfacial action are single-axial shearing, oblique shearing, and interfacial spinning. [1]
Single-axial Interfacial Shearing
Single-axial interfacial shearing has been used to produce microdroplets [20] and double emulsions. [21]. The process works by moving a needle periodically across an interface between gas and liquid and producing microdroplets when the liquid bridge is broken when leaving the interface. [20][21] Double emulsions can be formed similarly using a coaxial needle to cross a gas-liquid interface and producing dispersed microdroplets with two phases and the resulting droplets collect in a receiving pool of the liquid. [20][21] This is shown schematically in Figure 4.
Oblique Interfacial Shearing
Oblique interfacial shearing is different from single-axial shearing in that instead of choosing to insert the appropriate needle vertically, it is instead inserted at an angle to allow for the oblique shearing. [22]These microdroplets form in the open receiving pool after vibrating the associated capillary across the air-liquid interface. [22] This method can be used to produce double emulsions as well and the produced diameters are similar to that of vertical interface shearing. [23]
Dynamic Interfacial Spinning
Dynamic interfacial spinning differs from the other two interfacial methods in that it produces microfibers instead of microdroplets. [1] By simply adjusting the distance from the needle end and air liquid interface, one can produce microfibers of a specified size. [1] These fibers have periodic knots and slender joints which are controlled by the vibration frequency of the associated nozzle at the air liquid interface. [24] One group also developed a method for coaxial interfacial microfiber generation by using the vibrating nozzle at the interface of air and a coagulating bath. [25]
Acoustics
Acoustic microfluidics have gained interest for their ability to provide non-contact operations using only mild pressure waves. [1] Acoustic waves are the main driving force in these systems. [1] Three main acoustic based FBM are acoustic droplet injection, acoustophoretic printing, and vibrating sharp-tip capillary emulsification. [1]
Acoustic Droplet Ejection
Micromachined acoustically actuated microdroplet ejectors have been made and explored in order to overcome challenges presented in other forms of inkjet printing. [1] Some of these challenges include processing more viscous polymers, avoiding damage of high stress to sensitive inks, and balancing the tradeoff between clogging of the nozzle and the resolution of the print. [1] In these acoustic base droplet ejectors, a sine wave signal is input to interdigital transducers and surface acoustic waves are generated and leak into the fluid medium at some angle. [1] These waves travel until they interfere to form a focal point and a strong acoustic radiation force exists at this location. [26] Within a specified viscosity range, the droplet is ejected when the focal point is located on the fluid surface and the acoustic radiation is enough to overcome the surface tension of the droplet. [26] Some examples of acoustic droplet ejectors include systems of acoustic picoliter droplet ejection array for single-cell encapsulation drop-on-demand print [26], nozzle-free surface acoustic wave droplet printers [27], and drop-on demand acoustic ejectors modeled on Lamb wave transducers. [28]
Acoustophoretic Inkjet Printing
Commercial printing often faces issues with printing highly viscous fluids. [1] A high frequency of nozzle squeezing is necessary in order to create deformations to provide inkjet pressure which results in a high shear rate before ejection. [1] This becomes problematic if the fluid is a shear thickened fluid. Acoustophoretic printing is designed to overcome this limitation by decoupling the ink flow from any droplet ejection. [1] The printer is made up of a sound source, a resonator with a single subwavelength ultrasonic cavity, a tapered capillary nozzle, and a ink dispensing system. [1] The resonator produces acoustic standing waves which exert strong acoustic pressure at specified localized zones. [1] This pressure is able to work with gravity to overcome the surface tensions and detach the hanging droplet. [1] The properties of non-Newtonian inks have a minimal effect on this inkjet and are able to work much more effectively. [1]
Vibrating Sharp-Tip Capillary Emulsification
This type of acoustic FBM consists of a piezoelectric transducer on a glass slide connecting substrate and a sharp-tip capillary nozzle on a fixed stage with the tip immersed in the continuous liquid phase. [1] It works by generating a pair of counter-rotating vortex acoustic streams near the vibrating sharp edges of the channel from acoustic energy dissipation. This allows for fluid to be pumped in the microchannels when the sharp edge is replaced by a sharp tip nozzle. [1] This design allows for a wide range of droplet sizes and dynamic droplet generation. [1] It has been used for particle manipulation and microfluid mixing and allows for controllable fluid to be pumped out and pinched off into microdroplets. [1]
Challenges and Outlook
Despite the wealth of opportunities that are available with free-boundary microfluidics, they are not without their challenges. They still exist in early development and there are some issues to overcome. There are four main issues which need to be built upon. [1] The first issue is that for advanced FBM technologies the behaviors of fluids evolves greatly in the free boundary environment and this makes it challenging to set the movement and shape of them. [1] Additionally, it is also difficult to produce emulsions and jets at high throughput rates. [1] Potential solutions to address this will need to incorporate forces such as electric and acoustic forces and will also need to provide more precise microfluidic devices. [1] The second challenge is system-level design and integration of platforms of FBM. [1] Despite the successes and efforts towards commercializing these types of microfluidics, these platforms have not been extensively industrialized which makes it challenging to standardize. [1] Additionally, this prevents popularization of such devices. [1] Significant work needs to be done in both industry and academia to try and reduce costs in order to develop greater mass production. [1] The third issues is the need for the development of novel materials. [1] There is lack of materials currently available and appropriate for free boundary microfluidics. [1] Materials need to be developed which have enhanced functionality, superior performance, affordability, and improved bio-compatibility. [1] The field will likely need to head in this direction as smart materials and stimuli responsive materials become more necessary for future development. [1] Developing these materials may not be simple as properties of shear thinning and non-Newtonian fluids will influence how these devices work. [1] The last difficulty involves where new applications will arise during the development of FBM. While there has been a wide array of new mechanisms and devices that have been made, there is certainly room for more exploration with FBM. [1] As research continues to develop it will become clearer where new areas will desire the applications which FBM can provide.
Ultimately, FBM provides a newer type of microfluidics which utilizes fluids in the form of jets and droplets in free environments. Its advantages include the ability to regulate the size, structure, and morphology of advanced materials. [1] Additionally, it also allows for the ability to use high-speed airflow or a highly viscous solution as the driving fluid, better suitability for handling high surface tension, high density, and molten fluids, easier parallel and diverse processing of droplet or jet templates, and is more suitable for manufacturing. [1] Overall, FBM will continue to develop and will likely become a promising direction for microfluidics in the future.
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