Flow of liquids through paper

Abstract

5 μm 5 μm Cross section of paper Single fibreThe flow of liquids through paper is challenging to model due to the complexity and disordered layout of the fibre matrix. The expanding use and capability of microfluidic paper-based analytical devices (µPADs) and their requirement for precision has increased the need to accurately predict the flow of liquids through paper. Many studies have developed models and revealed some of the physical mechanisms responsible for the flow behaviour, but we still lack a complete understanding, particularly in relation to how the fluid fills the various voids with a wide range of shapes and sizes in the fibre matrix of paper. In the featured article, Chang et al. (J. Fluid Mech., vol. 845, 2018, 36-50) used a combined experimental and theoretical approach to uncover the importance of the liquid filling the intra-fibre pores, showed that this results in deviation from the flow behaviour predicted by the Lucas-Washburn equation and developed a model which accounts for this effect.

Anomalous diffusion in an electrolyte saturated paper matrix

“…This well‐known Lucas‐Washburn law [19,20] in capillaries is a consequence of the interplay of time independence of the mean capillary pressure and a progressively increasing viscous drag in the liquid column. The same universal scaling is observed in the random porous medium of the paper matrix over length and time scales that are substantially larger than the relevant microscopic scales [6,21,22].…”

“…Flow of a liquid, often made visible by a dye, in a complex porous matrix is a very familiar phenomenon [1–5]. A classical yet elusively simple example is the diffusive spreading of a colored liquid on a piece of paper [6]. Other examples are the transfer of ink onto paper in writing [7], the transfer of dye onto fabric in the textile industry [8], and the biochemical detection of diseases or health conditions using bodily fluids in point‐of‐care diagnostic procedures [9–15].…”

Microfluidic through fibrous structures: Recent developments and future trends

“…Some researchers highlighted the importance of intrafiber pores and swelling of the fibers in microfluidic paper‐based analytical devices (μPAD) and microfluidic thread‐based electroanalytical device (μ‐TED).…”

“…According to the Equation , Lucas‐Washburn expression is generally used to predict liquid flow through fibrous materials due to the surface tension. It is clear that wicking height is often linearly proportional to the square root of time …”

“…Microfluidic chips can be manufactured by different techniques such as lithography, micromachining, etching, and 3D printing. Recently some different substrates, eg, papers, treads, and fabrics have been used in various reports as open microfluidic systems. Microfluidic through papers is known as lab on a paper and microfluidic through textiles is known as lab on fiber as well…”