Rapid evaporation-driven chemical pre-concentration and separation on paper

Abstract

Airflow-enhanced evaporation is investigated as a method for rapid chemical pre-concentration on a thin porous substrate. The mechanism is described by combining 1D models of capillary rise, chromatography and pervaporation concentration. It is shown that the effective length of the column can be shorter than its actual length, allowing concentrate to be held at a stagnation point and then released for separation, and that the Péclet number, which determines concentration performance, is determined only by the substrate properties. The differential equations are solved dynamically, and it is shown that faster concentration can be achieved during capillary filling.Experiments are carried out using chromatography paper in a ducted airflow and concentration is quantified by optical imaging of water-soluble food dyes. Good agreement with the model is obtained, and concentration factors of ≈ 100 are achieved in 10 mins using Brilliant Blue FCF.Partial separation of Brilliant Blue from Tartrazine is demonstrated immediately following concentration, on a single un-patterned substrate. The mechanism may provide a method for improving the sensitivity of lab-on-paper devices.

Microfluidic Evaporation, Pervaporation, and Osmosis: From Passive Pumping to Solute Concentration

“…Various strategies were developed over the past 20 years to design and integrate such evaporation surfaces in order to enhance the efficiency of evaporation-based microfluidic pumps. One can cite, for instance, microporous hydrophilic membranes, filter papers, − hydrophilic porous materials as shown in Figure , microfabricated arrays of micropores, , or even simply splitting the main channel into an array of open channels with small transverse dimensions, , as illustrated in Figure .…”

“…This flow in turn increases the pressure drop associated with viscous forces and can modify the imbibition dynamics. In the case of a simple strip of paper partially immersed in a water tank, for example, evaporation can stop the dynamics of liquid imbibition at a given position, ,,, as detailed in section .…”

“…This evaporation-driven flow possibly stops the liquid front at a fixed position H e smaller than the length of the paper strip. This steady regime occurs when the capillary driving force (capillary pressure P c ) is exactly balanced by the evaporation-induced viscous dissipation in the porous matrix. ,, The length H e of the wetted surface in this steady regime is given by where κ is the permeability (m 2 ) of the porous layer, η is the viscosity of the liquid, and u e is the solvent volumetric rate of evaporation per unit area (m/s) …”

Achieving asymmetry and trapping in diffusion with spatiotemporal metamaterials

“…However, it can be shown that diffusion phenomena in homogeneous materials obey a parity symmetry, which means that diffusion, in homogeneous media, occurs identically along the positive and negative directions of a given axis 2 . One way to break such a symmetry has been shown to be the use of a moving medium; for instance, showing an asymmetric diffusion of chemical solutions within airflow 3 . Therefore, let us pose the following question: can we design an effectively homogenized material in which diffusion would have a spatially directional preference?…”

Microfluidic Time-Delay Valve Mechanism on Paper-Based Devices for Automated Competitive ELISA

“…Multiple-step competitive ELISA for Imidacloprid detection is successfully conducted with a similar sensitivity to the ELISA procedures done in plates. The device can be further improved and extended its capability of performing detection for clinical or environmental analytes [33][34][35][36].…”