Effects of Chemical Heterogeneity and Roughness on Contact Angle Hysteresis: Experimental Study and Modeling
Authors: Robin M. , Paterson A. , Cuiec L. , Yang C. 
Published: December 2007

Abstract
Wettability is an important parameter for oil recovery efficiency ; and its heterogeneity character is more and more considered for actual oil reservoir rocks.
This paper describes an experimental study through contact angles measurement of the influence of the density and spatial distribution of chemical heterogeneities, as well as roughness on water/oil wetting.
This influence has been demonstrated, and a suitable model has been build.

Introduction
This paper describes an experimental study of the influence of the density and of the spatial distribution of chemical heterogeneities, as well as roughness, on wetting phenomena. Chemical heterogeneities were created by using a photosensitive resin and applying a micro lithography technique [after spreading on a glass slide, resin is hardened by heating - oil-wet areas - and exposed to UV light through a mask - water-wet areas].
We have worked with a series of heterogeneous substrates such as horizontal or vertical stripes, checkerboard pattern and small circular oil wetting defects on a water-wet substrate (glass) with increasing density. Dynamic contact angles were measured by the Wilhelmy technique and modeling was performed. This study has been completed by an analysis of the effect of roughness and of the influence of spatial distribution of heterogeneities, at a given coverage rate.

Experimental

Wilhelmy plate technique.
The contact angle was evaluated by measuring the force exerted by a fluid/fluid interface on a solid surface as the latter moves across the interface (immersion and emersion cycles). At any time, the measured force F has three components: weight of the slide, buoyancy, and vertical component of the interfacial tension:
F = Weight + PBuoyancy + l - - cos where denotes the interfacial tension, l the length of the triple line and 0 the angle between the interfacial force and the vertical plane.
The experimental set-up consists of a Mettler AT250 balance (0 - 52g 10 g) and a step-by-step motor (1 step = 0.1 m), driven by a frequency generator. The device allows vertical displacement of 10 cm at variable speed with a resolution of 0.1 m. A speed of 0.03 mm/s was used in most cases. The force is measured vs. sample position.
Spatial distribution of heterogeneities. For a given coverage rate, two spatial distributions of defects were used in order to modify the geometry without changing the "global average wettability":–a uniform distribution, for which the surface is divided into squares. A single defect takes place inside each square. The center of gravity is chosen randomly inside this square. This distribution looks very homogeneous (figure 1 a).–a more random distribution where the coordinates of the center of gravity of each defect (same size as previously) are randomly chosen on the surface (figure lb).
Roughness. A low surface energy solid (PTFE) was used in the present study. Such a solid is well suited for the investigation of several aspects of the wettability problem (Morrow and Mungan). Specially PTFE pieces were designed to study the effect of roughness on contact angle hysteresis. They were made up of several co-centric cylinders, as shown in Figure 2.
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