A microfluidic device based on an evaporation-driven micropump.
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Typical results show that with 1 to 61 pores (diameter = 250 μm, pitch = 500 μm) flow rates of 7.3 × 10(-3) to 1.2 × 10(-1) μL/min are achieved.The results are theoretically analyzed using an evaporation model that includes an evaporation correction factor.The theoretical and experimental results are in good agreement.
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PubMed Central - PubMed
Affiliation: Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands.
ABSTRACT
In this paper we introduce a microfluidic device ultimately to be applied as a wearable sweat sensor. We show proof-of-principle of the microfluidic functions of the device, namely fluid collection and continuous fluid flow pumping. A filter-paper based layer, that eventually will form the interface between the device and the skin, is used to collect the fluid (e.g., sweat) and enter this into the microfluidic device. A controllable evaporation driven pump is used to drive a continuous fluid flow through a microfluidic channel and over a sensing area. The key element of the pump is a micro-porous membrane mounted at the channel outlet, such that a pore array with a regular hexagonal arrangement is realized through which the fluid evaporates, which drives the flow within the channel. The system is completely fabricated on flexible polyethylene terephthalate (PET) foils, which can be the backbone material for flexible electronics applications, such that it is compatible with volume production approaches like Roll-to-Roll technology. The evaporation rate can be controlled by varying the outlet geometry and the temperature. The generated flows are analyzed experimentally using Particle Tracking Velocimetry (PTV). Typical results show that with 1 to 61 pores (diameter = 250 μm, pitch = 500 μm) flow rates of 7.3 × 10(-3) to 1.2 × 10(-1) μL/min are achieved. When the surface temperature is increased by 9.4°C, the flow rate is increased by 130 %. The results are theoretically analyzed using an evaporation model that includes an evaporation correction factor. The theoretical and experimental results are in good agreement. No MeSH data available. |
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Mentions: To estimate the evaporation rate, we first consider the evaporation through a single pore. The configuration is shown in Fig. 2a: the liquid is pinned at the edges of the pore and forms a circular surface with curvature defined by the angle θ. This case can be approximately described by the theory presented by Picknett and Bexon (Picknett and Bexon 1977). They present a model that describes evaporation from a surface bounded by a circle (which can be curved like a lens or a drop) based on solving the diffusion equation, through an analogy with electrostatic potential theory. This theory has been used to describe the evaporation of sessile drops, as shown in Fig. 2b, which is similar. The resulting equation reads (see also (Semenov et al. 2011)):2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\left(\frac{\mathbf{dV}}{\mathbf{dt}}\right)}_{\mathbf{single}}=-2\frac{\pi \mathbf{D}\mathbf{M}}{\rho}\left[\mathbf{c}\left({\mathbf{T}}_{\mathbf{w}}\right)-\mathbf{H}\mathbf{c}\left({\mathbf{T}}_{\mathbf{a}}\right)\right]\mathbf{F}\left(\theta \right)\mathbf{a} $$\end{document}dVdtsingle=−2πDMρcTw−HcTaFθawhere D is the diffusivity of vapor at ambient temperature, a is the radius of the bounding circle (i.e., the drop radius in case of sessile drops), M is the molecular mass of the vapor molecules, ρ is the density of the liquid. c(Tw) and c(Ta) are the saturated water molar vapor concentration in air at the liquid–gas interface and ambience, respectively. c(Tw) and c(Ta) depend sensitively on the temperature, and approximate values are given in the appendix. H is the relative humidity at the ambience. F(θ) is a function of the contact angle θ indicated in Fig. 3, which is given by (Picknett and Bexon 1977; Schönfeld et al. 2008).Fig. 3 |
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Affiliation: Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands.
No MeSH data available.