Page 37 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Concept of Gas Wettability and Research Status CHAPTER 1 21

basically results in a super-hydrophobic surface. In addition, this kind of
super-hydrophobic surface has a larger CA and a smaller roll angle.

Normally, the super-hydrophobic surface is produced in two ways. One is con-
structing a rough structure on the surface of the hydrophobic material (CA is
larger than 90 degrees), and the other is to modify materials with low surface
energy on the rough surface. According to the two basic principles, there are
many ways to produce super-hydrophobic surfaces. Most super-hydrophobic
surfaces are produced from a combination of low free surface energy and sur-
face roughness. The most commonly used methods are: layer-by-layer assem-
bly technique, electrochemical reaction and deposition, sol-gel processing,
etching and lithography, template method, and vapor deposition methods.
1. Layer-by-Layer (LbL) assembly technique
The advantage of LbL self-assembly technique developed by Decher’s
research group is that the interaction is conducted on intermolecular
electrostatic interaction and hydrogen bonds to control film thickness
and surface chemical properties at the molecular level. At present, the
super-hydrophobic surface was produced by many research groups by
this method. For example, Zhai et al., produced a pH-sensitive poly
(allylamine hydroch-loride)/poly(acrylic acid), (PAH/PAA) laminated
coating. Acid treatment lends it a honeycomb-like structure. When the
structure is cross-linked, the PAH solution and suspension containing
silica nano particles are dropped into it, so that silica nano particles are
deposited on the surface. Lastly, it turns super-hydrophobic when chem-
ical modification is done with fluoride silane and it is heat-treated.
Zhang et al., combined the LbL technique and electrochemical deposi-
tion technique and produced a super-hydrophobic surface. At present, it
has been proven that the self-assembly technique of a colloidal system
provides appropriate surface roughness. Based on the technique, Ming
et al., prepared a double-layered rough surface made of hook-like parti-
cles. This structure was mainly constructed by a chemical reaction
between amine groups and epoxy groups in silica particles, basically by
a covalent graft amine with a diameter of 70nm and silica particles func-
tionalized by the epoxy groups with a diameter of 700nm, and then
processed by a PDMS chemical modification. This makes the surface
super-hydrophobic.
2. Electrochemical reaction and deposition
This technique has been extensively applied to obtain a super-
hydrophobic surface. For example: Zhang’s research group made use of
an electrochemical deposition technique to deposit a layer of dendritic
gold cluster on conductive glass (ITO), modified by multilayer polymer
electrolyte. Then a layer of dodecane of monomolecular layer was fur-
ther deposited and a super-hydrophobic surface was obtained. Cho’s
research group used sulfated Cu or Cu-Sn alloy coated on the surface of
iron sheets with electrochemical reaction. Then it was processed with
perfluorinated silane, and then a leaf-like metallic super-hydrophobic