Microfabricated integrated circuits revolutionized computation by vastly reducing the space, labor, and time required for calculations. Microfluidic systems hold similar promise for the large-scale automation of chemistry and biology, suggesting the possibility of numerous experiments performed rapidly and in parallel, while consuming little reagent. While it is too early to tell whether such a vision will be realized, significant progress has been achieved, and various applications of significant scientific and practical interest have been developed. Here a review of the physics of small volumes (nanoliters) of fluids is presented, as parametrized by a series of dimensionless numbers expressing the relative importance of various physical phenomena. Specifically, this review explores the Reynolds number Re, addressing inertial effects; the Peclet number Pe, which concerns convective and diffusive transport; the capillary number Ca expressing the importance of interfacial tension; the Deborah, Weissenberg, and elasticity numbers De, Wi, and El, describing elastic effects due to deformable microstructural elements like polymers; the Grashof and Rayleigh numbers Gr and Ra, describing density-driven flows; and the Knudsen number, describing the importance of noncontinuum molecular effects. Furthermore, the long-range nature of viscous flows and the small device dimensions inherent in microfluidics mean that the influence of boundaries is typically significant. A variety of strategies have been developed to manipulate fluids by exploiting boundary effects; among these are electrokinetic effects, acoustic streaming, and fluid-structure interactions. The goal is to describe the physics behind the rich variety of fluid phenomena occurring on the nanoliter scale using simple scaling arguments, with the hopes of developing an intuitive sense for this occasionally counterintuitive world.

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Microfluidics: Fluid physics at the nanoliter scale

Changes in portlandite morphology with solvent composition: Atomistic simulations and experiment

We discuss in this review how the roughness of a solid impacts its wettability. We see in particular that both the apparent contact angle and the contact angle hysteresis can be dramatically affected by the presence of roughness. Owing to the development of refined methods for setting very well-controlled micro- or nanotextures on a solid, these effects are being exploited to induce novel wetting properties, such as spontaneous filmification, superhydrophobicity, superoleophobicity, and interfacial slip, that could not be achieved without roughness.

Wetting and Roughness

The importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs

Dipstick and lateral-flow formats have dominated rapid diagnostics over the last three decades. These formats gained popularity in the consumer markets due to their compactness, portability and facile interpretation without external instrumentation. However, lack of quantitation in measurements has challenged the demand of existing assay formats in consumer markets. Recently, paper-based microfluidics has emerged as a multiplexable point-of-care platform which might transcend the capabilities of existing assays in resource-limited settings. However, paper-based microfluidics can enable fluid handling and quantitative analysis for potential applications in healthcare, veterinary medicine, environmental monitoring and food safety. Currently, in its early development stages, paper-based microfluidics is considered a low-cost, lightweight, and disposable technology. The aim of this review is to discuss: (1) fabrication of paper-based microfluidic devices, (2) functionalisation of microfluidic components to increase the capabilities and the performance, (3) introduction of existing detection techniques to the paper platform and (4) exploration of extracting quantitative readouts via handheld devices and camera phones. Additionally, this review includes challenges to scaling up, commercialisation and regulatory issues. The factors which limit paper-based microfluidic devices to become real world products and future directions are also identified.

Paper-based microfluidic point-of-care diagnostic devices

Penetration of Liquids into Cylindrical Capillaries.---The rate of penetration into a small capillary of radius $r$ is shown to be: $\frac{\mathrm{dl}}{\mathrm{dt}}=\frac{P({r}^{2}+4\ensuremath{\epsilon}r)}{8\ensuremath{\eta}l}$, where $P$ is the driving pressure, $\ensuremath{\epsilon}$ the coefficient of slip and $\ensuremath{\eta}$ the viscosity. By integrating this expression, the distance penetrated by a liquid flowing under capillary pressure alone into a horizontal capillary or one with small internal surface is found to be the square root of ($\frac{\ensuremath{\gamma}\mathrm{rt}\ifmmode\cdot\else\textperiodcentered\fi{}cos\ensuremath{\theta}}{2\ensuremath{\eta}}$), where $\ensuremath{\gamma}$ is the surface tension and $\ensuremath{\theta}$ the angle of contact. The quantity ($\frac{\ensuremath{\gamma}cos\ensuremath{\theta}}{2\ensuremath{\eta}}$) is called the coefficient of penetrance or the penetrativity of the liquid.Penetration of Liquids into a Porous Body.---(1) Theory. If a porous body behaves as an assemblage of very small cylindrical capillaries, the volume which penetrates in a time $t$ would be proportional to the square root of ($\frac{\ensuremath{\gamma}t}{\ensuremath{\eta}}$). (2) Experiments with mercury, water and other liquids completely verify the theoretical deductions.Dynamic capillary method of measuring surface tension is described. It possesses certain advantages on the static method of capillary rise.

The Dynamics of Capillary Flow

https://www.pnas.org/content/pnas/7/4/115.full.pdf

Note on a Method of Determining the Distribution of Pore Sizes in a Porous Material

Theory of the method.—The pore volume is measured by allowing the gas which fills the pores to expand into a measured volume and measuring the accompanying fall in pressure.

Applicability.—The method is in general applicable to pieces of any size and shape and to all classes of porous bodies or materials.

A new porosimeter.—For rapid work with shaped test pieces a new porosimeter is described. The new instrument measures accurately both pore volume and bulk volume. A complete porosity determination can be made in 5 minutes. No weighing is required. The results on ceramic bodies arc reproducible to one unit in the first decimal place of the per cent porosity. Considerably higher accuracy than this can be secured if desired.

Results.—The results obtained with the new method are in all cases higher, in a number of cases very much higher, than those obtained by the methods of liquid absorption in current use. For fired bodies the same results are obtained with dry air, hydrogen or helium as the pore filling gas. The results show conclusively that complete filling of all the pores in a reasonable time can not be secured by any of the current methods using a liquid as the pore filling agent.

Edward W. Washburn

Papers

The Dynamics of Capillary Flow

Note on a Method of Determining the Distribution of Pore Sizes in a Porous Material

Porosity: vi. determination of porosity by the method of gas expansion*

The laws of “concentrated” solutions. vi. the general boiling-point law.

The measurement of electrolytic conductivity. i. the theory of the design of conductivity cells.