Droplet based microfluidics is a rapidly growing interdisciplinary field of research combining soft matter physics, biochemistry and microsystems engineering. Its applications range from fast analytical systems or the synthesis of advanced materials to protein crystallization and biological assays for living cells. Precise control of droplet volumes and reliable manipulation of individual droplets such as coalescence, mixing of their contents, and sorting in combination with fast analysis tools allow us to perform chemical reactions inside the droplets under defined conditions. In this paper, we will review available drop generation and manipulation techniques. The main focus of this review is not to be comprehensive and explain all techniques in great detail but to identify and shed light on similarities and underlying physical principles. Since geometry and wetting properties of the microfluidic channels are crucial factors for droplet generation, we also briefly describe typical device fabrication methods in droplet based microfluidics. Examples of applications and reaction schemes which rely on the discussed manipulation techniques are also presented, such as the fabrication of special materials and biophysical experiments.

https://iopscience.iop.org/article/10.1088/0034-4885/75/1/016601/pdf

Droplet based microfluidics

Pendant drop tensiometry offers a simple and elegant solution to determining surface and interfacial tension - a central parameter in many colloidal systems including emulsions, foams and wetting phenomena. The technique involves the acquisition of a silhouette of an axisymmetric fluid droplet, and iterative fitting of the Young-Laplace equation that balances gravitational deformation of the drop with the restorative interfacial tension. Since the advent of high-quality digital cameras and desktop computers, this process has been automated with high speed and precision. However, despite its beguiling simplicity, there are complications and limitations that accompany pendant drop tensiometry connected with both Bond number (the balance between interfacial tension and gravitational forces) and drop volume. Here, we discuss the process involved with going from a captured experimental image to a fitted interfacial tension value, highlighting pertinent features and limitations along the way. We introduce a new parameter, the Worthington number, Wo, to characterise the measurement precision. A fully functional, open-source acquisition and fitting software is provided to enable the reader to test and develop the technique further.

Measurement of surface and interfacial tension using pendant drop tensiometry

Arthur Chickering is Distinguished Professor of Higher Education at Memphis State University. On leave from the Directorship of the Center for the Study of Higher Education at Memphis State, he is Visiting Professor at George Mason University. Zelda Gamson is a sociologist who holds appointments at the John W. McCormack Institute of Public Affairs at the University of Massachusetts-Boston and in the Center for the Study of Higher and Postsecondary Education at the University of Michigan.

Seven Principles for Good Practice in Undergraduate Education: Faculty Inventory. Institutional Inventory.

Dripping and jetting regimes in microfluidic multiphase flows have been investigated extensively, and this review summarizes the main observations and physical understandings in this field to date for three common device geometries: coaxial, flow-focusing and T-junction. The format of the presentation allows for simple and direct comparison of the different conditions for drop and jet formation, as well as the relative ease and utility of forming either drops or jets among the three geometries. The emphasis is on the use of drops and jets as templates for microparticle and microfiber syntheses, and a description is given of the more common methods of solidification and strategies for achieving complex multicomponent microparticles and microfibers.

https://iopscience.iop.org/article/10.1088/0022-3727/46/11/114002/pdf

Dripping and jetting in microfluidic multiphase flows applied to particle and fibre synthesis

Building functional materials for health care and pharmacy from microfluidic principles and Flow Focusing.

Optical imaging was used to measure the free surface deformation due to thermal (Marangoni-buoyant) convection in liquid bridges of 5-cSt silicone oil. We obtained the free surface position averaged over time in both the steady and oscillatory regimes. The deviation of the free surface contour from the corresponding equilibrium shape was determined with an uncertainty of about 2 μm. This deviation grew linearly with the applied temperature difference with a proportionality coefficient depending on the liquid bridge volume at equilibrium. Shrinkage at the upper part of the liquid bridge was slightly greater than bulging at the lower with the sum of the maximum deviations at both parts being about 30 μm near the onset of oscillations. This sum, normalized with the radius of the supporting disks, was of the same order of magnitude as the Capillary number. We observed the influence of thermal expansion, surface tension variation over the free surface, and fluid motion separately. The local mean curvature was also calculated and compared with its value at equilibrium, showing that the hydrodynamic effects were important.

Experimental study of the free surface deformation due to thermal convection in liquid bridges

The linear vibration of axisymmetric liquid bridges of millimetric size was analyzed experimentally. The equilibrium shape and the evolution of the interface position were recorded by a high-speed video camera. To calculate the surface tension, the Theoretical Image Fitting Analysis method was adapted to process images of liquid bridges at equilibrium. An image processing method was developed to obtain both the position of the solid supports delimiting the liquid bridge and the interface deformation as a function of time. The former allows one to accurately measure the inertial force acting on the liquid bridge. The interface deformation was determined at the subpixel level, so that vibrations of very small amplitude could be analyzed. The results are compared with various theoretical approaches in the linear regime.

An experimental analysis of the linear vibration of axisymmetric liquid bridges

The electrohydrodynamic response of low-conductivity pendant drops to a step change in the electric field magnitude was examined both numerically and experimentally. Both the leaky-dielectric and perfect-conductor models were solved in the simulations. Experiments were conducted to precisely measure the drop interface shape as a function of time. The drop oscillated for applied voltages smaller than a critical value which depended on the rest of governing parameters. It stretched and subsequently emitted a microjet from its tip for electric potentials above that critical value. The perfect-conductor model described accurately the oscillations of subcritical drops. This model also provided satisfactory results for the prejetting regime in the supercritical case. We found a good agreement between the leaky-dielectric model and the experiments for the drop-jet transitional region, despite the fact that the tip streaming arose on a time scale much shorter than the electric relaxation time. This result shows the capability of the leaky-dielectric model to describe the flow in this singular region. The numerical simulations allowed us to describe the pressure and velocity fields in the transitional region.

Dynamical behavior of electrified pendant drops

Drop shape techniques, such as axisymmetric drop shape analysis, provide accurate measurements of the interfacial tension from images of pendant drops for a wide variety of experimental conditions. However, these techniques are known to fail when dealing with nearly spherical drop shapes, which may occur, for instance, when working with interfaces between liquids of similar densities and/or under microgravity. We analyzed the advantages of using liquid bridges close to the minimum volume stability limit instead of pendant drops to measure the interfacial tension under different experimental conditions. First, the sensitivity of both configurations to a variation of the interfacial tension is studied numerically as a function of the volume for several Bond numbers B. The results indicate that a liquid bridge close to the minimum volume stability limit is generally more sensitive than a pendant drop of the same volume, especially for small values of the density difference across the interface and/or gravity. This suggests that the use of liquid bridges may extend the range of applicability of drop shape techniques. To explore this possibility, synthetic images of both pendant drops and liquid bridges were generated and then processed by TIFA-AI. The results demonstrated that the use of liquid bridges enhances the range of Bond numbers for which drop shape techniques work satisfactorily. More specifically, similar accuracy is obtained from both configurations for B ~ 10−1, while the use of liquid bridges yields much better results for B ~ 10−2. Finally, experiments were conducted to partially validate the analysis based on synthetic images. Good agreement was found between the values determined from the real and synthetic images.

An analysis of the sensitivity of pendant drops and liquid bridges to measure the interfacial tension

The stability of flow focusing taking place in a converging?diverging nozzle, as well as the size of the resulting microjets, is examined experimentally in this paper. The results obtained in most aspects of the problem are similar to those of the classical plate-orifice configuration. There is, however, a notable difference between flow focusing in nozzles and in the plate-orifice configuration. In the former case, the liquid meniscus oscillates laterally (global whipping) for a significant area of the control parameter plane, a phenomenon never observed when focusing with the plate-orifice configuration. Global whipping may constitute an important drawback of flow focusing with nozzles because it reduces the robustness of the technique.

/pdf/focusing-liquid-microjets-with-nozzles-25w4tur3ii.pdf

C. Ferrera

Papers

Experimental study of the free surface deformation due to thermal convection in liquid bridges

An experimental analysis of the linear vibration of axisymmetric liquid bridges

Dynamical behavior of electrified pendant drops

An analysis of the sensitivity of pendant drops and liquid bridges to measure the interfacial tension

Focusing liquid microjets with nozzles