Article Cite This: Langmuir 2018, 34, 2865−2875
pubs.acs.org/Langmuir
Precise Liquid Transport on and through Thin Porous Materials Souvick Chatterjee,† Pallab Sinha Mahapatra,†,‡ Ali Ibrahim,† Ranjan Ganguly,§ Lisha Yu,∥ Richard Dodge,∥ and Constantine M. Megaridis*,† †
Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, United States Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India § Department of Power Engineering, Jadavpur University, Kolkata 700098, India ∥ Corporate Research and Engineering, Kimberly-Clark Corporation, Neenah, Wisconsin 54956, United States ‡
S Supporting Information *
ABSTRACT: Porous substrates have the ability to transport liquids not only laterally on their open surfaces but also transversally through their thickness. Directionality of the fluid transport can be achieved through spatial wettability patterning of these substrates. Different designs of wettability patterns are implemented herein to attain different schemes (modes) of three-dimensional transport in a high-density paper towel, which acts as a thin porous matrix directing the fluid. All schemes facilitate precise transport of metered liquid microvolumes (dispensed as droplets) on the surface and through the substrate. One selected mode features lateral fluid transport along the bottom surface of the substrate, with the top surface remaining dry, except at the initial droplet dispension point. This configuration is investigated in further detail, and an analytical model is developed to predict the temporal variation of the penetrating drop shape. The analysis and respective measurements agree within the experimental error limits, thus confirming the model’s ability to account for the main transport mechanisms.
1. INTRODUCTION Liquid transport using wettability-patterned surfaces is an applicative field with immense importance in several engineering domainsranging from heat pipes1 to propellant management devices for spacecraft,2 soil science,3 personal hygiene products,4 and filtration.5 Applications in all these fields rely on the passive transport (i.e., movement without external energy supply) of liquid on the surface as well as through the porous medium. Development of facile techniques of fabricating materials with different porosity and surface wettability, combined with advancements in harnessing open-surface flows for microfluidic devices, has led to the possibility of incorporating such flows in low-cost, point-of-care (POC) biomedical diagnostics.6 Tuning material wettability spatially to control liquid/solid interactions toward specific microfluidic tasks is relevant not only to impervious substrates (rigid or flexible) but also to porous and fibrous substances. Several studies have been conducted to characterize the roles of surface wettability and bulk properties (e.g., porosity, wicking property, etc.) of the porous structure on fluid transport. Wang et al.7 demonstrated unidirectional fluid transport using a special coating technique that created a wettability gradient along the thickness of the fibrous substrate through selectively different exposure levels of ultraviolet (UV) radiation. The concept was extended for selective permeation of three different fluids (water, soybean oil, and hexadecane) by Zhou et al.8 who further characterized the depth of penetration of the fluid using 3D microcomputed tomography. The operation of such a © 2018 American Chemical Society
porous membrane, fabric, or paper, featuring wettability gradients is dependent on the penetration resistance through such materials; this resistance arises from the coupling effect of the local geometric angle of adjacent fibers and the solid−liquid contact angle (CA).9 The critical breakthrough resistance to overcome for liquid penetration can be increased by reducing the spacing between the fibers or by increasing the wettability gradient along the thickness direction.9 A sheet of porous material, coated on one or both sides, would exhibit unidirectional liquid transport across it, provided the penetration pressure to transport fluid from one side to the other is much greater than the pressure required to force fluid in the reverse direction. Mates et al. showed that the penetration pressure required to transport liquid through such a coated substrate from the hydrophilic to the hydrophobic side significantly exceeds that for a reverse flow.10 Zhou et al.8 also observed that, while such wettability-engineered fabrics displayed unidirectional transport with liquids of higher surface tension (29−50 mN/m), they allowed liquids of lower surface tension (
