Microfluidics—from fundamental research to industrial applications

Abstract

The advance of microfluidics started in the early 1980s. At the time, researchersrealized that many processes and reactions in chemistry and biology, whichtypically take place on small length scales, can be defined, controlled andunderstood much better when using tools on equally small length scales. Reactionsand reaction kinetics rely on (gradual) concentration differences and microfluidicsprovides the unique possibility to establish exactly such gradients of solutes, ionconcentrations, pH value and so on. Nowadays the variety of specific microfluidicmethods is large. In principle, they can be divided into two groups: (i) monophaseflow, where miscible (e.g. aqueous) fluids are mixed, mostly by diffusion owing tothe laminar flow on small length scales and (ii) multiphase flow, the most prominentexample of which is probably droplet microfluidics, where water-in-oil oroil-in-water emulsions are used to encapsulate chemical or biological systems andseparate them from each other, much like in 'micron-scale test tubes'. Now,30 years later, microfluidic techniques are seriously considered for industrialapplications, although some important steps in the upscaling process are stillmissing.The purpose of this special issue is to shed light on the different aspects inmicrofluidics research starting from fundamental research reaching all the way toindustrial applications. The study by Toma and co-workers takes advantage of thecontrolled diffusive mixing when co-flowing aqueous, miscible solutions. Theycombine microfluidics with optical, spectroscopic and scattering techniques tostudy DNA packing. Nunes et al review the different regimes when replacing oneof the fluids by an oil phase and varying flow rates and device geometries with aparticular emphasis on using multiphase microfluidics for synthesis of particles orfibres. Going into the third dimension by fabricating microfluidic devices withseveral layers, producing emulsions can also be achieved by so-called 'stepemulsification', the physical mechanisms behind which are described by Danglaet al. Tran and co-workers move a considerable step towards applicability ofwater-in-oil emulsions for biological research and review ultrahigh-throughputmethods used for bio-assays. The article by Lagus et al focuses this topicspecifically on single-cell experiments.Whereas it is very popular to use emulsions with drop sizes of a few tens ofmicrometers as 'tiny test tubes' they may also serve as templates for materialsfabrication. Gundabala and co-workers combine both aspects by producingso-called 'celloidosomes', which consist of liquid drops decorated with yeast cellsat the outer interface. Wang et al fabricate microcrawlers that can be thermally setin motion. Finally, Holtze gives a perspective on the possibility to upscale andapply such methods in industry.The choice of papers shows the wide and diverse applicability of microfluidicsin various fields of research. While microfluidics started out as a 'niche' techniquefor very specific applications and as a tool in fundamental soft and biological matterresearch, the advancements made during recent years promise further progress inthe chemical industry, biomedicine and pharmacology. Advantages such as lowsample consumption, single cell accessibility and controlled experimentalparameters in general may in the future be exploited for real industrial sizedapplications.In Journal of Physics D: Applied Physics we find a journal that is ideallypositioned to give applied microfluidics research a wide readership across manydisciplines. In the publication of this special issue we hope to inspire and encouragemicrofluidics researchers, and to promote interdisciplinary collaborations.We would like to thank all of the authors for their excellent contributions to thisspecial issue.