2723
Role of Surface Forces in the Kinetics of Film Thinning and
Contact Angle Formation
Mohit Gupta, Kaiwu Huang, Roe-Hoan Yoon
Center for Advanced Separation Technologies, Virginia Tech, Blacksburg, VA
ABSTRACT: When an air bubble approaches a flat surface in water, a thin liquid film (TLF) is formed in
between. Its stability is controlled by the surface forces, which in turn determine the disjoining pressure (Π)
in units of N/m2. If Π 0, the film ruptures at a critical film thickness (hc), forming a finite contact angle (θ).
In the present work, we have measured the surface forces by monitoring the changes in bubble curvature and
analyzing the results using the Young-Laplace equation. We have studied the role of different surface forces,
particularly that of EDL force, on the kinetics of film thinning and contact angle formation.
INTRODUCTION
During the early part of his career, Professor D.W.
Fuerstenau of U.C. Berkely focused on the adsorption of
surfactant and/or collector molecules to mineral surfaces.
His work showed that the adsorption mechanisms are
closely related to the structure of the electrical double lay-
ers (EDLs) formed around mineral particles suspended in
the aqueous phase. His work helped the industry develop
powerful and selective collectors for more efficient flotation
separation (Fuerstenau and Pradip, 2005). While his work
in this area of research may be characterized as microscopic,
there was another part of his work that had not received as
much attention, i.e., the macroscopic interaction between
bubbles and particles. He showed that flotation recover-
ies of quartz and molybdenite reached the maxima at the
points of zero charge (PZCs) of the minerals (Fuerstenau,
1957 Chander and Fuerstenau, 1972). The authors attrib-
uted this finding to the role of the EDL around air bubbles
playing a role in bubble-particle interactions in flotation.
Derjaguin and Dukhin (1961) modeled bubble-particle
interactions using the DLVO theory, which considers the
EDL and van der Waals (vdW) forces. The authors thought
that flotation is controlled by the attractive vdW force and
the repulsive EDL force and developed a flotation criterion,
A
d}
C 1
2 f
m (1)
in which ε is the dielectric permittivity of the solution, d
is the double-layer thickness, ψ1 is the doublelayer poten-
tial, and A represents the attractive force due to the van
der Waals interaction. For small particles C" is equal to
1. In the absence of any information on the hydrophobic
force at the time, the authors thought that bubble-particle
interaction was driven by the vdW force, which is repul-
sive in bubble-particle interactions. Regardless, Derjaguin
and Dukhin (1961) were the first to recognize that flota-
tion kinetics is controlled by the surface forces involved in
bubble-particle interactions.
The authors of the present investigation have been
measuring surface forces using the atomic force microscope
(AFM) and the surface force apparatus (SFA) with particu-
lar interest in the forces between hydrophobic surfaces in
water. However, the subject matter has been controversial
as many investigators detected the formation of nano-scale
air bubbles during the measurement, while others reported
Role of Surface Forces in the Kinetics of Film Thinning and
Contact Angle Formation
Mohit Gupta, Kaiwu Huang, Roe-Hoan Yoon
Center for Advanced Separation Technologies, Virginia Tech, Blacksburg, VA
ABSTRACT: When an air bubble approaches a flat surface in water, a thin liquid film (TLF) is formed in
between. Its stability is controlled by the surface forces, which in turn determine the disjoining pressure (Π)
in units of N/m2. If Π 0, the film ruptures at a critical film thickness (hc), forming a finite contact angle (θ).
In the present work, we have measured the surface forces by monitoring the changes in bubble curvature and
analyzing the results using the Young-Laplace equation. We have studied the role of different surface forces,
particularly that of EDL force, on the kinetics of film thinning and contact angle formation.
INTRODUCTION
During the early part of his career, Professor D.W.
Fuerstenau of U.C. Berkely focused on the adsorption of
surfactant and/or collector molecules to mineral surfaces.
His work showed that the adsorption mechanisms are
closely related to the structure of the electrical double lay-
ers (EDLs) formed around mineral particles suspended in
the aqueous phase. His work helped the industry develop
powerful and selective collectors for more efficient flotation
separation (Fuerstenau and Pradip, 2005). While his work
in this area of research may be characterized as microscopic,
there was another part of his work that had not received as
much attention, i.e., the macroscopic interaction between
bubbles and particles. He showed that flotation recover-
ies of quartz and molybdenite reached the maxima at the
points of zero charge (PZCs) of the minerals (Fuerstenau,
1957 Chander and Fuerstenau, 1972). The authors attrib-
uted this finding to the role of the EDL around air bubbles
playing a role in bubble-particle interactions in flotation.
Derjaguin and Dukhin (1961) modeled bubble-particle
interactions using the DLVO theory, which considers the
EDL and van der Waals (vdW) forces. The authors thought
that flotation is controlled by the attractive vdW force and
the repulsive EDL force and developed a flotation criterion,
A
d}
C 1
2 f
m (1)
in which ε is the dielectric permittivity of the solution, d
is the double-layer thickness, ψ1 is the doublelayer poten-
tial, and A represents the attractive force due to the van
der Waals interaction. For small particles C" is equal to
1. In the absence of any information on the hydrophobic
force at the time, the authors thought that bubble-particle
interaction was driven by the vdW force, which is repul-
sive in bubble-particle interactions. Regardless, Derjaguin
and Dukhin (1961) were the first to recognize that flota-
tion kinetics is controlled by the surface forces involved in
bubble-particle interactions.
The authors of the present investigation have been
measuring surface forces using the atomic force microscope
(AFM) and the surface force apparatus (SFA) with particu-
lar interest in the forces between hydrophobic surfaces in
water. However, the subject matter has been controversial
as many investigators detected the formation of nano-scale
air bubbles during the measurement, while others reported