Soft matter principles of microfluidics

Techniques in microfluidics have played an increasing role in the study, science and engineering of soft matter, for a variety of reasons. Microfluidics and soft matter tend to operate within a shared regime of physico-chemical parameter space: the effects of confinement, screened electrostatic interactions and the dominance of viscous damping, rather than inertial turbulence, in governing the evolution of flowing systems. Science studied in one realm can often illuminate or inspire developments in another. To highlight this point, microfluidic systems provide incredible control over system geometry and material conditions, such that physico-chemical effects relevant to soft matter systems can be isolated and investigated in microfluidic systems with precision.

Secondly, microfluidic systems provide a wide variety of advantages for the study of soft matter. In particular, the low-Reynolds numbers inherent in most microfluidic systems lead to laminar flows that can be predicted and designed in a rational fashion. Even in systems where inertia does play the key role (e.g., where inertia drives suspended particles to marginate and interact hydrodynamically¹), flows are still typically laminar and can thus be designed. Conversely, the strong shear rates possible within microfluidic channels can be exploited to drive strong non-Newtonian effects, even at moderate-to-small Reynolds numbers,^2,3 so that elastic effects (and elastic turbulence) can be unambiguously disentangled from those of inertial turbulence. Additionally, microfluidic systems handle sample volumes that are, by nature, very small: fluid metering systems have been developed for the study of protein crystallization that reliably mete out samples with precise volumes in the tens of picoliters.⁴ Furthermore, monodisperse droplets of one fluid can be formulated and suspended in an immiscible carrier fluid, enabling hundreds of distinct ‘microreactors’ to be created per second, while consuming very little sample. In addition to probing solubility⁵ and chemical kinetics,⁶ emulsification microfluidics combined with passive, bead-based microrheology enables the high-throughput screening of the rheology of various formulations of soft materials.⁷ The quasi-two dimensional nature of microfluidic systems enables a direct visualization of microstructural processes within soft materials—both dilute and concentrated—that would be next to impossible in bulk materials. Examples here include the flow of foams, emulsions, suspensions and polymers.

Finally, microfluidics provide tight control over local flow and physico-chemical conditions that allow novel particles and materials to be synthesized. For example, the type, rate and rheological character of the flows (e.g., pure shear, extension and various mixtures) can be effectively designed through clever choice of the microchannel geometry. Additionally, the solvent and solution conditions can be precisely controlled, including deliberately introducing a spatially non-uniform distribution of solute or solution, or incorporating slowly changing solutions.

Altogether, this special issue highlights many of the advantages and capabilities that microfluidics offer to the study of soft matter.

Probing the properties of suspensions

Several papers employ microfluidic systems to probe, through direct visualization, the dynamics of colloidal suspensions in flow. Kumar and Graham (DOI: 10.1039/C2SM25943E) review the physical mechanisms behind margination and segregation in flows of suspension mixtures, including whole blood, wherein leukocytes and platelets are known to segregate to the boundaries of the flow. Pandey and Conrad (DOI: 10.1039/C2SM25935D) use suspended polymers as depletants to tune the attractive interactions between suspended colloids under flowing conditions and directly visualize a yielding flow profile. Uspal and Doyle (DOI: 10.1039/C2SM25931A) look at quasi-2D suspensions and, in particular, focus on the effects of this confinement upon the collective dynamics of small clusters.

These and other studies critically involve the effects of confinement, which occurs in soft matter and in biological systems. Bartolo and Aarts (DOI: 10.1039/C2SM26157J) provide an opinion piece to describe the delicate and rich interplay between geometric confinement, physical and statistical phenomena. For example, Sbragaglia et al. (DOI: 10.1039/C2SM26167G) provide theoretical studies on the confined flows of non-ideal fluids and interrogate the existence of a cooperative length scale and non-local rheology of soft, glassy materials.

Suspensions of emulsion drops behave in many ways like colloids, while introducing a variety of distinct phenomena. Bremond and Bibette (DOI: 10.1039/C2SM25923K) review key issues in emulsion science and the ways in which microfluidics have been used to study them. Krebs et al. (DOI: 10.1039/C2SM26122G) study shear-induced coalescence by looking at droplet collisions in microchannels and Skhiri et al. (DOI: 10.1039/C2SM25934F) look at emulsion permeation—the exchange of species between individual emulsion droplets—and how surface-active proteins retard this exchange. Along these lines, Lee et al. (DOI: 10.1039/C2SM26044A) examine the effects that surfactant interfaces have on the flow inside of, and external to, an emulsion drop held fixed in a Hele–Shaw flow. Kotula and Anna (DOI: 10.1039/C2SM25970B) examine the dynamics of nanoparticle adsorption onto bubbles and Ma et al. (DOI: 10.1039/C2SM25833A) visualize the dynamics of aqueous foams injected into model porous media and their efficacy in expelling oil from the pores.

New functional materials

Microfluidic and millifluidic devices open the road to the creation of new functional materials. The small scales inherent in microfluidic devices enables rapid cooling and heating and perfect control of the temperature and residence time during the chemical reaction allows a low polydispersity and high monomer conversion for exothermic polymerization reactions, which is not possible in conventional batch mode syntheses.⁸ The ability to produce monodisperse drops of a controlled size and shape has enabled a wide variety of creative techniques to synthesize novel structures. For example, Sin et al. (DOI: 10.1039/C2SM25950H) make monodisperse droplets containing block copolymers, which self-assemble into precise morphologies depending on the solution; by carefully tuning the relative rates of, for example, solvent extraction and self-assembly, they develop microspheres with hierarchically porous morphologies. Additionally, Duncanson et al. (DOI: 10.1039/C2SM25694K) incorporate bubbles to synthesize monodisperse microspheres with size-tunable pores and Adams et al. (DOI: 10.1039/C2SM25953B) synthesize multi-component double-emulsions. Tadmouri (DOI: 10.1039/C2SM25933H) emulsify molten metal alloys, then cool them under different degrees of confinement to form metal spheres, ellipsoids and rods. Finally, Anton et al. (DOI: 10.1039/C2SM25357G) exploit the precise control afforded by microfluidics to develop a new method for the nanoprecipitation of 100 nm particles for drug encapsulation and Huang et al. (DOI: 10.1039/C2SM26126J) describe mechanically robust hydrogel structures for cell culture in microfluidics.

Surfaces and interfaces

Free fluid interfaces, even between miscible fluids, exhibit rich dynamics as well. Cubaud and Mason (DOI: 10.1039/C2SM25902H) review viscous instabilities as co-flowing streams of different viscosities are driven through converging and diverging channels and Darvishi and Cubaud (DOI: 10.1039/C2SM25932J) explore the combination of viscous and capillary instabilities. Bonhomme et al. (DOI: 10.1039/C2SM25552A) demonstrate a novel method for wet-spinning fibers using co-flowing streams to drawn alginate solutions into thin threads and subsequently crosslink the alginate with calcium dissolved in the external streams to form fibers whose physical properties can be tuned.

Liquid–air interfaces introduce, at times, complex dynamics on this small scale, as capillary stress can be quite large for drops and bubbles. Visser et al. (DOI: 10.1039/C2SM26323H) use high-speed imaging to examine the impact of microdroplets on solid surfaces, while Guemas et al. (DOI: 10.1039/C2SM26230D) highlight the coupling between rheology and surface properties during the impact of droplets. Chauvet et al. (DOI: 10.1039/C2SM25982F) probe the capillary filling of nanoslits and ways in which the pinning of contact lines can cause the formation of persistent bubbles. This contact-line pinning gives rise to the so-called ‘coffee-ring’ stain,⁹ which Augustine et al. (DOI: 10.1039/C2SM26103K) show can be suppressed using electro-wetting.

New tools

Finally, several reviews describe powerful techniques that can be incorporated into microfluidic systems to probe soft matter. Electrical effects have been used extensively throughout the histories of both soft matter and microfluidics. Barabati and Kirby (DOI: 10.1039/C2SM26121A) review the various electrokinetic tools available to soft matter researchers. Electrokinetic effects were central to the first genome sequencing and to genomic and proteomic separations. Mai et al. (DOI: 10.1039/C2SM26036K) review the various microfluidic systems that have been developed to study the detailed dynamics of DNA (and, by extension, other polymers). Lastly, Badilita et al. (DOI: 10.1039/C2SM26065D) review recent advances in nuclear magnetic resonance imaging—in particular, the increasingly small interrogation volumes and short acquisition times and their incorporation into microfluidic systems—and its use for the study of soft matter.

We believe this Soft Matter special issue nicely highlights the confluence of microfluidic systems and soft matter and the complementary ways in which the study of one can inform the other. Microfluidics has opened new paths in soft matter and soft matter has driven new developments in microfluidics. It is our sincere hope that the combination of scientific investigation, synthesis strategies and powerful experimental techniques included here will clarify the special advantages that microfluidic systems have for the study and manipulation of soft matter and will motivate and inspire future studies.

Annie Colin, Université Bordeaux, France
Todd M. Squires, University of California, Santa Barbara, USA
Lyderic Bocquet, Université Lyon, France

References

1. D. Di Carlo, Lab Chip, 2009, 9, 3038–3046.
2. L. E. Rodd, T. P. Scott, D. V. Boger, J. J. Cooper-White and G. H. McKinley, XIVth International Congress on Rheology, Seoul, South Korea, 2004.
3. T. M. Squires and S. R. Quake, Rev. Mod. Phys., 2005, 77, 977–1026.
4. C. L. Hansen, E. Skordalakes, J. M. Berger and S. R. Quake, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 16531–16536.
5. C. Hansen, M. O. A. Sommer and S. R. Quake, Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 14431–14436.
6. H. Song and R. F. Ismagilov, J. Am. Chem. Soc., 2003, 125, 14613–14619.
7. K. M. Schultz and E. M. Furst, Lab Chip, 2011, 11, 3802–3809.
8. C. Rosenfeld, C. Serra, C. Brochon and G. Hadziioannou, Chem. Eng. Sci., 2007, 62, 5245–5250.
9. R. Deegan, O. Bakajin, T. Dupont, G. Huber, S. Nagel and T. Witten, Nature, 1997, 389, 827–829.

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