PHYSICAL REVIEW E 85, 016314 (2012) Spreading profile of evaporative liquid drops in thin porous layer W. Y. Chong, 1,* K. S. Lim, 1 W. H. Lim, 1 S. W. Harun, 1,2 F. R. Mahamd Adikan, 1,2 and H. Ahmad 1 1 Photonic Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia 2 Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia (Received 13 May 2011; revised manuscript received 13 October 2011; published 19 January 2012; corrected 22 May 2012) Spreading of evaporative liquid drops in a thin porous layer has been studied. The entire spreading process can be divided into three distinct phases according to the change of the wetted porous region size. The first phase is characterized by the expansion of the wetted porous region and shrinking of the liquid drop. Contact line pinning is observed in the wetted porous region in the second phase even with the liquid drop totally absorbed into the porous layer. The third phase sees the shrinkage of the wetted porous region until it is not observable. Based on these observations, a model is devised to simulate the spreading of a liquid drop under the studied conditions. Partial differential equations are used to describe the relation between liquid drop volume and other important parameters of a fluid flow, including maximum wetted region diameter achieved, time taken to reach each spreading process phase, and evaporation rate. Calculated results are in good agreement with the experimental data. DOI: 10.1103/PhysRevE.85.016314 PACS number(s): 47.56.+r, 64.70.fm I. INTRODUCTION Many applications involving liquid drop adhesion, such as spray coating, painting, and ink jet printing, involve liquid contact not with a homogeneous flat surface but with a thin porous layer. Concurrence of infiltration, redistribution, and evaporation of a liquid drop in a thin porous layer is a common phenomenon observed for water balance in natural porous media. Therefore, it is important to understand fluid flow within a liquid drop as well as in the porous layer, which in turn determines the distribution of solid particles within the liquid drop after the drying process. Fluid flow within an evaporating liquid drop on a solid surface can be ascribed to contact line pinning of the liquid drop due to capillary flow of liquid from the interior to the periphery of the drop to compensate for higher evaporation rate at the periphery [1,2]. This effect results in solute accumulation at the periphery of a liquid drop after drying, popularly termed the “coffee-ring” effect. The study was then developed to consider imbibitions of a liquid drop into a porous media and the fluid flow within. This has prompted studies of fluid flow in such realistic conditions. Starov et al. have been major contributors in the studies of fluid flow in porous layer and have considered nonevaporative liquid drop spreading in both a porous layer saturated with the same liquid and later in a dry porous layer [3,4]. Spreading of an aqueous liquid drop on porous layers has later been studied by considering drop volume conservation during the spreading process [5,6]. Evaporation of liquid from thick porous slabs of the same as well as different porosity has been reported by Shokri et al. [7,8] and Bechtold et al. [9]. It is found that evaporation from a thick porous layer, namely a sand column, involves con- tinuous capillary flow of liquid from wetted zones below the surface [10]. This work studies fluid flow of an evaporative liquid drop in a thin porous layer. Experimental work has been performed to observe spread dynamics of an aqueous liquid drop on a thin * wuyi80@yahoo.com porous layer. Based on observation, an analytical solution of liquid drop spreading on a thin porous layer with consideration of liquid evaporation is postulated. It is found that the analytical result agrees very well with experimental observation. This paper is arranged in seven sections. A description of the experimental setup used to observe liquid drop spreading is presented in Sec. II. Preliminary observation of liquid flow in a porous layer is discussed in Sec. III. From the observation, the principle of evaporative fluid flow in a thin porous layer is proposed in Sec. IV. Section V discusses an analytical solution of fluid flow according to the proposed principles. Comparison of experimental results with the predictions of the model discussed in Sec. V is presented in Sec. VI. Conclusion of this work is made in Sec. VII. II. EXPERIMENTAL SETUP The experimental setup for this work is shown in Fig. 1. De-ionized (DI) water was used as the liquid drop as it is a common solvent for many inorganic chemicals. The use of DI water also allows a straightforward relation between liquid weight and volume, where 1 g of DI water has a volume of 1 cm 3 . A drop of DI water with known volume was applied on a thin porous layer using a micropipette. The porous layer used is a heat-treated silica soot layer deposited on silicon wafer via flame hydrolysis deposition. The porous layer thickness  is 25 ± 1 μm with porosity of 75 ± 5% and an average pore size of 2 μm. Evolution of the liquid drop and spreading of the wetted porous region was recorded using a CCD camera capturing images from the side and top position, respectively. The evaporation of DI water was monitored by measuring the change in weight of the sample at time intervals of 5 s throughout the spreading, using a digital analytical balance with 0.1 mg precision. Focus was given to the spreading of the wetted porous region as the evolution of a water drop spreading on a solid surface is well established. The measurements were done in a clean room where the ambient temperature 016314-1 1539-3755/2012/85(1)/016314(8) ©2012 American Physical Society