1. Droplet migration across interfaces
The key to manipulating microfluidic droplets for interphase transfer lies in precisely controlling the directional movement of droplets, both within and between phases. The methods for realizing the directional movement of droplets in microchannels are mainly divided into active and passive methods, which have their own advantages and disadvantages.
The active method relies on external forces to realize the directional movement of the droplets, such as through the external magnetic field, acoustic field, electric field and so on. The advantage is that the direction and strength of the external field can be adjusted to precisely control the movement direction of the droplet and the size of the driving force, thus realizing a more accurate manipulation of the droplet.
In addition, the active approach provides new ideas for manipulating special fluids (e.g., magnetic fluids). However, this approach requires complex equipment and more manual regulation, thus increasing equipment cost, labor cost and energy consumption.
The passive approach, on the other hand, does not rely on external forces, such as the use of special microchannel structures, fluid dynamics, or the nature of the fluid itself to induce the directional movement of the droplets.
The advantage of the passive approach is that the droplet movement is spontaneous and does not require additional energy input to the system or manual intervention. However, due to the influence of flow field perturbations, the passive approach generally has lower maneuvering accuracy than the active approach and a smaller operating range.
Trans-interfacial migration of droplets in microchannels is characterized by two features: first, the environment in which the droplets are located is changed; second, the process of trans-interfacial migration of droplets is accompanied by the interaction of droplets with the interface. These two characteristics determine the application of droplet transinterfacial migration.
Based on the different environments in which the droplets are located, the trans-interfacial migration of droplets can be used to initiate reactions, which has great potential for application in the fields of gel preparation, drug delivery, and drug slow release.
At the same time, the design of microchannel structures and fluids allows for size sieving and property sieving of droplets, which has a wide range of applications in areas such as cell analysis.
In addition, droplet-interface interactions provide new ideas for the synthesis of spherical materials. For example, during droplet migration across the interface, the droplet-interface interaction provides the curvature required by the cell for the phospholipid bilayer, which provides a new approach to solving the challenge of synthesizing spherical phospholipid vesicles in synthetic biology.
2. Methods for realizing directional movement of microfluidic droplets
1) Proactive approach
a) Imposed sound field
Directional movement of droplets controlled by acoustic fields has become an area of extensive research due to its advantages of being label-free, contact-free, and highly biocompatible. In microchannels, two commonly used acoustic modes are surface acoustic waves and body acoustic waves, respectively.
A surface acoustic wave is a wave that propagates along the surface of a piezoelectric substrate and can be further categorized into surface traveling waves and surface standing waves. Body acoustic waves, on the other hand, are ultrasonic longitudinal waves that propagate inside the material. Both types of waves have important applications in the directional movement of droplets.
The interaction of sound waves with a medium produces two main acoustic phenomena: the acoustic radiation effect and the acoustic flow effect. The migration of droplets is mainly realized by these two phenomena.
The acoustic radiation effect is the physical effect of sound waves reflecting, refracting and scattering with the surface of an object during propagation, while exchanging energy with the object. Macroscopically, it manifests itself in the force exerted by the sound wave on the object, thus pushing the droplet to move.
The acoustic flow effect, on the other hand, refers to the vibration and movement of liquid molecules caused by the propagation of acoustic waves through a liquid, accompanied by a change in the internal pressure of the liquid, which leads to a macroscopic flow of the liquid. This flow also affects the movement of droplets in the fluid.
The acoustic radiation effect and the acoustic flow effect work together to determine the directional movement and flow behavior of the droplets in the acoustic field.
b) Imposed electric field
The directional displacement of a droplet driven by an applied electric field allows for fast, high-throughput and precise control of the measurable charge in the droplet. This approach offers significant advantages, but also has some limitations.
The premise of electric field-driven droplet migration is that there must be sufficient free charge in the droplet, or that the difference in conductivity or dielectric constant between the continuous and dispersed phase fluids is sufficiently large. Only if these conditions are met can the electric field generate sufficient interfacial or volumetric stresses on the droplets to drive the directional movement of the droplets.
Since these stringent conditions limit the applicability of the electric field drive method, the method may not be able to achieve the desired results in some specific application scenarios.
c) Imposed magnetic field
Manipulating the directional movement of droplets in a microfluidic device by means of an applied magnetic field has the advantage of enabling remote manipulation, as well as relatively easy fabrication and integration. Applied magnetic fields face similar problems as applied electric fields, i.e., they are more demanding on the properties of the fluid inside the microchannel, making it difficult to meet certain conditions of biological or chemical compatibility.
In contrast to electric fields, the distribution of magnetic fields in space is not affected by the liquid-liquid interface, and is therefore more conducive to trans-interfacial migration of droplets. Compared to other external fields, magnetic fields are slower to respond and the switching frequency is usually only a few Hz or less.
Depending on the type of magnet used, the magnetically controlled migration of droplets can be categorized into two ways using electromagnets and using permanent magnets. The strength of the magnetic field generated by electromagnets can be controlled by adjusting the current, but excessive heat is generated under high current conditions, which is not favorable for the separation of heat-sensitive substances or most biological samples. Therefore, in microfluidic platforms, permanent magnets have received more attention because they do not generate excessive heat.
2) Passive approach
a) Utilization of fluid dynamics
The use of hydrodynamics to induce radial displacements of droplets in microchannels in specified directions relies mainly on the application of unequal shear forces on both sides of successive relative droplets. The radial displacement of droplets in two-phase flow has been extensively studied.
Under low Reynolds number conditions, droplets typically tend to move to locations where the shear stress is zero (e.g., the center of a symmetric channel). For parallel flow within a microchannel, the presence of a liquid-liquid interface changes the velocity distribution within the channel. When the equilibrium position of a droplet located in one continuous phase is in another continuous phase, the droplet will penetrate the liquid-liquid interface and migrate to the other phase under the effect of shear stress.
This droplet migration mechanism provides a new method for the directional control of liquids in microfluidic channels, as well as a theoretical basis for droplet manipulation in complex fluid environments.
b) Utilization of channel configurations
Although the non-inertial lift plays a dominant role in the cross-interfacial migration of droplets and solves the problems of reagent waste, channel blockage and time-consuming operation to a certain extent, relying on the non-inertial lift alone to drive the cross-interfacial migration of droplets suffers from the limitation of small operation range. For this reason, designing and improving the structure of microchannels has become an effective solution.
A method known as flow-inducing method involves setting up microstructures in microchannels that are in direct contact with the droplets, thereby guiding the direction of flow of the droplets. For example, a microrailing structure formed by a single row of neatly arranged microcolumns allows a continuous phase to pass through, while a droplet cannot pass through due to interfacial tension. Such microstructures have been used in recent years to manipulate the directional migration of droplets, showing good results.
c) Utilization of fluid properties
In the practice and theory of utilizing fluid dynamics and channel configurations to induce droplet migration across interfaces, it is usually assumed that the droplets migrate by overcoming the obstruction of the liquid-liquid interface in the presence of a driving force, and thus the interfacial tension is regarded as a kind of resistance.
In recent years, a method of spontaneous droplet interphase transfer based on the principle of droplet interfacial energy minimum has gradually become a hot research topic. In this method, the interfacial tension instead becomes the driving force for droplet migration across the interface.
By utilizing the property of droplets spontaneously pursuing the lowest state of interfacial energy, droplets move spontaneously during trans-interfacial migration to reach a lower energy state. This spontaneous migration provides a droplet manipulation method without external energy input and opens up a new direction for the development of microfluidic technology.
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