Interaction between interfaces in solution of chemical reagents is scientifically and technologically significant. Atomic force microscopy has been used to visualise structures of adsorbed surfactant molecules and measure interaction forces between surfaces with adsorbed molecules. This paper presents the results obtained by the author and associates in three major research strands, including imaging structures of surfactants adsorbed at solid-liquid interfaces, measuring interfacial forces between water-repellent (hydrophobic) surfaces and microhydrodynamic forces between an AFM colloid probe and a soft interface. The research increases our understanding of the role of surfactants and interaction forces in particle technology and interface science

Nguyen, Anh V.

Nguyen, A. V.

University of Queensland eSpace

Seeing structures and measuring forces at nanoscales with atomic force microscopy

 
Acta  Montanistica  Slovaca     Ročník 10 (2005), číslo 1, 12-16 
 
Imaging Structures and Measuring Forces at Nanoscales with Atomic 
Force Microscopy 
 
 
Anh V. Nguyen1  
 
 
Zobrazovanie štruktúr a meranie síl na nanometrickej úrovni atómovo silovou mikroskópiou 
Interaction between interfaces in solution of chemical reagents is scientifically and technologically significant. Atomic force 
microscopy has been used to visualise structures of adsorbed surfactant molecules and measure interaction forces between surfaces with 
adsorbed molecules. This paper presents the results obtained by the author and associates in three major research strands, including imaging 
structures of surfactants adsorbed at solid-liquid interfaces, measuring interfacial forces between water-repellent (hydrophobic) surfaces           
and microhydrodynamic forces between an AFM colloid probe and a soft interface. The research increases our understanding of the role            
of surfactants and interaction forces in particle technology and interface science. 
 
Key words: self-assembled surface structures, interfacial forces, atomic force microscopy. 
 
Introduction 
 
The atomic force microscope (AFM) was invented in 1986 by the Nobel-prize award winner,              
Dr. G. Binnig (Binnig, 1986), and has been used for imaging nanostructures and measuring atomic                   
and molecular forces with conducting and non-conducting surfaces in many fields of nanoscience              
and nanotechnology. AFM can provide the ability to view and understand events as they occur                       
at the molecular level which will increase our understanding of how systems work and lead to new 
discoveries in many fields, including materials science, electrochemistry, polymer science, biophysics, life 
science, and nanobiotechnology. 
In colloid and interface science and technology, AFM has been used to image surfaces with adsorbed 
surface-active agents (surfactants) with nanoscale resolution as well as measure extremely small surface      
and molecular forces (~10-12 N) in the gas and liquid phases. This paper presents some selected outcomes           
of the author research in three major research strands, including: 
1. Structures of self-assembled surface micelles of surfactants adsorbed at solid-liquid interfaces: 
Surfactant adsorption at solid-liquid surfaces is of significant importance in many particulate processes 
ranging from particle separation and enhanced oil recovery to pharmaceutics, paint technology, high 
quality ceramics production, and mineral processing (Shah, 1998). Stability of the dispersions is related 
not only to the amount of adsorbed surfactant but also to the structure of the adsorbed surfactant 
molecules at the interface. The structures depend on the type of surfactants and the properties                
of the water-solid interfaces. Different types of micellar structures have been observed at the surfaces 
for surfactant concentrations around the critical micelle concentration (CMC). 
 
2.  Role of dissolved gas on attraction between hydrophobic surfaces: This attraction, known              
as the hydrophobic force, is critically important for interaction and attachment of hydrophobic particles 
in coal and mineral flotation (Skvarla, 2001), oil droplets in pharmaceutical formulas and biological 
cells (Israelachvili, 1992). However, the origins and models for this attractive force between 
hydrophobic surfaces have been a source of debate since the first direct measurements of this force       
in the early 1980’s (Christenson and Claesson, 2001). Attraction between an AFM hydrophobic probe 
and a flat hydrophobic surface has been studied in water, in water-ethanol mixtures, and in water 
saturated by gases with different solubilities. Experimental results indicate the role of surface stabilized 
submicron-sized bubbles in the hydrophobic attraction.  
 
3.  Microhydrodynamic interaction between a solid particle and a soft interface: Interaction between        
a solid particle and a soft interface such as an air bubble or an oil drop is a key to understanding 
emulsification and particle separation using air bubbles (Nguyen and Schulze, 2004). The AFM colloid 
probe technique has been used to measure hydrodynamic interaction forces between a solid sphere and 
an air bubble, as well as intermolecular and surface forces. 
 
                                                                          
1 Anh V. Nguyen, Discipline of Chemical Engineering, School of Engineering, The University of Newcastle, Callaghan, New South 
Wales 2038, Australia, Phone (612) 4921 6189; Fax (612) 4921 6920; Anh.Nguyen@newcastle.edu.au
   (Recenzovaná a revidovaná verzia dodaná 8. 11. 2005) 
12 
 
Acta  Montanistica  Slovaca     Ročník 10 (2005), mimoriadne číslo 1, 12-16 
Materials and Methods 
 
Surface images of surfactants and forces between a spherical particle and a flat surface were determined 
with the Nanoscope IIIa and PicoForce AFM’s (Digital Instruments/Veeco, USA). The interaction force 
between a surface and a particle was measured using the AFM colloid probe technique (Butt, 1991; Ducker 
et al., 1991). The micron-sized sphere was attached to the end of an AFM microfabricated cantilever. The flat 
surface was mounted to a piezoelectric transducer and was moved relatively to the particle at constant speeds. 
The cantilever deflection due to the surface-particle interaction was measured with a laser beam reflected 
from the cantilever onto a split photodiode. The force measurements with soft surface such as an air bubble 
or an oil drop attached to the piezoelectric transducer were carried out using the same technique. 
The surface interaction forces were determined by multiplying the cantilever deflection by the spring 
constant. The cantilever deflection was determined from the voltage signal of the photodiode employing        
a scaling procedure, which used the linear displacements between the cantilever and the surface.                 
The separation was then determined from the cantilever deflection and the displacement of the piezoelectric 
transducer. 
Imaging self-assembled surfactant structures at water-solid interfaces was carried out using the soft-
contact technique (Manne et al., 1994; Manne and Gaub, 1995). The technique used a microfabricated 
cantilever with a shape tip (~ 10 nm radius) to probe a force, which was just delicate enough to distinguish 
surfactant surface structures without actually disturbing them. The force was then maintained constant during 
the tip scanning over the surface. The change in position of the tip for maintaining the constant force 
produced images of the adsorbed structures at the solid surface. 
Silicon nitride cantilevers (Digital Instruments/Veeco, USA) with nominal spring constant of 0.12 N/m 
were used. The particles and surfaces used in the experiments included glass and silica micro-spheres, silicon 
wafers, microscope glass slides, highly ordered pyrolytic graphite (HOPG), fused alumina surface, and mica. 
The silica and glass surfaces were made hydrophobic by a silanation process. The contact angle measured 
with a sessile drop apparatus was 62 degrees for the silanated surfaces. 
The chemicals used in the experiments included two surfactants (sodium dodecyl sulphate, SDS,         
and cetyl trimethyl ammonium bromide, CTAB) and NaCl for controlling the electrical double-layer 
interaction between the surfaces. Deionized water from a Millipore Milli-Q system was used in cleaning 
glassware and the apparatus, and in preparing all solutions. All the chemicals used were of analytical grade. 
For the measurements with dissolved gases, water was first outgassed by subsequent boiling/freezing under 
low pressure several times and was then saturated by the required gas directly in the AFM liquid cell. 
 
Results and Discussion 
 
Fig. 1 shows a typical force curve for a silicon nitride cantilever approaching a surface in a solution      
of 0,4 mM SDS and 0,1 M NaCl. A force of ~3 nN was used to image the adsorbed monolayer of SDS         
on the graphite surface. The images of SDS adsorbed at hydrophobic graphite surfaces show linear, parallel 
aggregates oriented perpendicular to a symmetry axis and spaced apart by a little over twice the surfactant 
length (Fig. 2A). These observations are consistent with half-cylindrical aggregates wherein molecules 
adjacent to the surface are arranged tail to tail along parallel strips, with the hydrophobic alkyl chains 
oriented parallel to a graphite symmetry axis. At the hydrophilic, negatively charged, mica surface wormy 
micelles were observed. These structures are consistent with a cylindrical structure where the surfactant 
molecules first adsorb through the electrostatic interaction between the oppositely charged head group        
and the mica surface (Fig. 2D). On amorphous 
mineral oxide surfaces, such as alumina        
and silica, spherical structures have been 
observed (Fig. 2B & 2C). Clearly, the surface 
charge of these surface micelles plays             
an important role in their formation and is 
crucial for the modelling of this self-assembly 
process involving the formation of surface 
micellar structures (Paruchuri et al., 2004). 
0
1
2
3
4
5
0 5 10 15 20
Tip-surface separation distance (nm)
In
te
ra
ct
io
n 
fo
rc
e 
(n
N
)  
.
imaging force
adsorbed
layer
graphite surface  
Fig. 1.  Typical force curve for a cantilever (with a 10 nm 
radius tip) approaching a HOPG graphite surface             
in a solution of 0.4 mM SDS and 0.1 M NaCl. 
 
13 
 
Anh V. Nguyen: Imaging Structures and Measuring Forces at Nanoscales with Atomic Force Microscopy 
 
-25
-15
-5
5
0 50 100 150 200
Separation distance (nm)
F/
R
 (m
N
/m
)
Pure ethanol
17% ethanol
Pure water
0 40 80 120 160 200
Separation [nm]
-70
-60
-50
-40
-30
-20
-10
0
Fo
rc
e/
R
ad
iu
s 
[ m
N
/m
]
Fig. 2.  Images (200 nm × 200 nm) of surface surfactant structures obtained by the soft contact AFM technique. A. Hemi-cylindrical 
structures of SDS at a hydrophobic graphite surface. B. & C. Spherical structures of SDS at a positively charged fused alumina surface 
and of CTAB at a negatively charged silica surface. D. Short worm-like cylindrical structures of CTAB at a hydrophilic negatively 
charged mica surface (Paruchuri et al., 2004). 
 
Fig. 3.  AFM results obtained for the scaled 
force, F/R, between hydrophobic glass 
spheres (radius R = 4 µm) and plates            
in water saturated with argon and in ethanol-
water mixtures  (Nguyen et al., 2003b). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Effect of ethanol and argon dissolved in water on the force between hydrophobic (silanated glass) 
surfaces is shown in Fig. 3. The steps in the force curves are due to the formation of bridging gas micro-
bubbles between hydrophobic surfaces (Attard, 1996; Attard et al., 2002). The addition of ethanol to water 
increases the jump-in distance, where the step in the force curve occurs and decreases the strength                
of the measured force, to the extent that for mixtures with a high content of ethanol the step is difficult          
14 
 
Acta  Montanistica  Slovaca     Ročník 10 (2005), mimoriadne číslo 1, 12-16 
to measure. Ethanol decreases the liquid-vapour surface tension, leading to the decrease in the attractive 
force. The influence of gas solubility on the hydrophobic attraction has been investigated with different 
gases, including argon (solubility in water 34,2 mL/L), nitrogen (solubility in water 15,8 mL/L), and helium 
(solubility in water 8,6 mL/L). Solubility of gases in ethanol is also higher than in water. The size                 
of the observed steps on the force curves depends on the solubility of the gas used to saturate water:           
The higher the solubility, the larger the size of the steps. However, the strength and the range of the attractive 
force in the water saturated by the gases appear to be the same, indicating that the gas solubility is not            
a controlling factor. It is important to note that surface roughness and heterogeneities due to crystallites and 
patchy of the adsorbed surfactants can control the formation and stability of submicroscopic gas bubbles       
at the solid-liquid interface. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 4.  Scaled force, F/R, between air 
bubbles (R    250 µm) in deionized 
water and spherical particles with 
different hydrophobicities (contact 
angles). The approach speed between 
the surfaces was 10 µm/s (Nguyen         
et al., 2003a). 
 
 
Typical results for the interaction force with soft interfaces are shown in Fig. 4. The interaction forces 
are indeed of very long range, in particular, at high approach speeds. This long range indicates that             
the hydrodynamic interaction is significant at high approach speeds between a bubble and a particle               
in flotation. The microhydrodynamic resistance force due to the intervening water film between the surfaces 
increases with decreasing separation distance (film thickness) as predicted (Nguyen and Evans, 2002). Due  
to strong attraction between the hydrophobic surfaces, the intervening liquid films between the bubble       
and the hydrophobic particles are not stable, and the rupture of the intervening water film and the attachment       
of the particle to the air bubble take place. For the data shown in Fig. 4 for the polyethylene (PE) particle, 
which was of a greater degree of hydrophobicity (the contact angle was about 80 degrees), the jump-in-
contact interaction and the contact force were extremely strong and the force curves were not fully captured 
in the ‘force window’ of the AFM system. Our force measurements also show that the jump-in separation 
distances increase with approach speeds which supports the existence of submicroscopic bubbles                  
on hydrophobic surfaces, as shown by the steps in Fig. 3 as well as by direct AFM images (Ishida et al., 
2000; Tyrrell and Attard, 2002; Steitz et al., 2003). The bubbles become very unstable at high approach 
speeds and cause the attraction at large separation distances. This feature can complicate the analysis             
of the interaction force at high approach speeds and separation distances. Further systematic investigations    
of he bubble-particle interaction under the influence of both attractive surface and repulsive hydrodynamic 
forces are in progress.  
In the case of the hydrophilic sphere (with contact angle of ~0o) shown in Fig. 4, the rupture                   
of the intervening water film and the attachment of the particle to the air bubble did not take place. At short 
distances, significant deviations of strong repulsive force between hydrophilic spheres from the model 
prediction are related with the deformation of air-water interface due to the particle approach, as well            
as intermolecular and surface forces. 
 
 
15 
 
Anh V. Nguyen: Imaging Structures and Measuring Forces at Nanoscales with Atomic Force Microscopy 
Conclusion 
 
Atomic force microscopy has been used to image structures of surfactants adsorbed at solid surfaces      
in water and measure interaction forces between the surfaces. Depending on the type of surfactants and       
the surface, different types of micellar arrangements at the surfaces have been observed. Dissolved gases 
have an important effect on attraction between hydrophobic surfaces in liquids. Microhydrodynamic 
interactions between a solid particle and a soft interface have been determined for both hydrophobic                    
and hydrophilic solid surfaces. 
 
Acknowledgements: The author gratefully acknowledges         
the Australian Research Council for financial support              
and assistance of his former and current postgraduate students, 
postdoctoral fellows, and colleagues at the Universities             
of Newcastle and Utah (Salt Lake city, USA), including Mr. Glen 
McCann and Mr. Marc Hampton, Drs. Vamsi K. Paruchuri, 
Keqing Fa, Jakub Nalaskowski and Stoyan I. Karakashev,                       
and Professors Geoffrey M. Evans and Jan D. Miller. 
 
 
References 
 
Attard, P.: Bridging Bubbles between Hydrophobic Surfaces. Langmuir, 12(6): 1693-5, 1996. 
Attard, P., Moody, M., P. Tyrrell, J., W., G.: Nanobubbles: the big picture. Physica A: Statistical Mechanics 
and Its Applications (Amsterdam, Netherlands), 314(1-4): 696-705, 2002. 
Binnig, G.: Atomic force microscope and method for imaging surfaces with atomic resolution. US Patent   
No. 4,724,318, 1986. 
Butt, H., J.: Measuring electrostatic, van der Waals, and hydration forces in electrolyte solutions with           
an atomic force microscope. Biophys. J., 60(6): 1438-44, 1991. 
Christenson, H., K., Claesson, P., M.: Direct measurements of the force between hydrophobic surfaces          
in water. Adv. Colloid Interface Sci., 91(3): 391-436, 2001. 
Ducker, W., A., Senden, T., J., Pashley, R., M.: Direct measurement of colloidal forces using an atomic force 
microscope. Nature (London), 353(6341): 239-41, 1991. 
Ishida, N., Inoue, T., Miyahara, M., Higashitani, K.: Nano Bubbles on a Hydrophobic Surface in Water 
Observed by Tapping-Mode Atomic Force Microscopy. Langmuir, 16(16): 6377-6380, 2000. 
Israelachvili, J., N.: Intermolecular and Surface Forces. Academic Press, London, 291 pp, 1992. 
Manne, S., Cleveland, J., P., Gaub, H., E., Stucky, G., D. Hansma, P., K.: Direct Visualization of Surfactant 
Hemimicelles by Force Microscopy of the Electrical Double Layer. Langmuir, 10: 4409-4413, 1994. 
Manne, S., Gaub, H., E.: Science (Washington, D. C.), 270: 1480, 1995. 
Nguyen, A., V., Evans, G., M.: Exact and approximate expressions for resistance coefficients of a colloidal 
sphere approaching a solid surface at intermediate reynolds number. Chem. Eng. Sci., 000: 000, 2002. 
Nguyen, A., V., Nalaskowski, J., Miller, J., D.: A study of bubble-particle interaction using atomic force 
microscopy. Minerals Engineering, 16(11): 1173-1181, 2003a. 
Nguyen, A., V., Nalaskowski, J., Miller, J., D., Butt, H., J.: Attraction between hydrophobic surfaces. Inter. 
J. Miner. Process., 72: 215-225, 2003b. 
Nguyen, A., V., Schulze, H., J.: Colloidal science of flotation. Marcel Dekker, New York, 840 pp, 2004. 
Paruchuri, V., K., Nguyen, A., V., Miller, J., D.: Zeta-potentials of self-assembled surface micelles of ionic 
surfactants adsorbed at hydrophobic graphite surfaces. Colloids and Surfaces, A: Physicochemical      
and Engineering Aspects, 250(1-3): 519-526, 2004. 
Shah, D., O. (Editor): Micelles, Microemulsions, and Monolayers. Marcel Dekker, New York, 1998. 
Skvarla, J.: Hydrophobic interaction between macroscopic and microscopic surfaces. Unification using 
surface thermodynamics. Adv. Colloid Inter. Sci., 91(3): 335-390., 2001. 
Steitz, R. et al.: Nanobubbles and their precursor layer at the interface of water against a hydrophobic 
substrate. Langmuir, 19(6): 2409-2418, 2003. 
Tyrrell, J., W., G., Attard, P.: Atomic force microscope images of nanobubbles on a hydrophobic surface   
and corresponding force-separation data. Langmuir, 18(1): 160-167., 2002. 
16 


Imaging structures and measuring forces at nanoscales with atomic force microscopy

Nguyen V. Anh

Directory of Open Access Journals

 Acta  Montanistica  Slovaca     Ročník 10 (2005), číslo 1, 12-16  Imaging Structures and Measuring Forces at Nanoscales with Atomic Force Microscopy   Anh V. Nguyen1    Zobrazovanie štruktúr a meranie síl na nanometrickej úrovni atómovo silovou mikroskópiou Interaction between interfaces in solution of chemical reagents is scientifically and technologically significant. Atomic force microscopy has been used to visualise structures of adsorbed surfactant molecules and measure interaction forces between surfaces with adsorbed molecules. This paper presents the results obtained by the author and associates in three major research strands, including imaging structures of surfactants adsorbed at solid-liquid interfaces, measuring interfacial forces between water-repellent (hydrophobic) surfaces           and microhydrodynamic forces between an AFM colloid probe and a soft interface. The research increases our understanding of the role            of surfactants and interaction forces in particle technology and interface science.  Key words: self-assembled surface structures, interfacial forces, atomic force microscopy.  Introduction  The atomic force microscope (AFM) was invented in 1986 by the Nobel-prize award winner,              Dr. G. Binnig (Binnig, 1986), and has been used for imaging nanostructures and measuring atomic                   and molecular forces with conducting and non-conducting surfaces in many fields of nanoscience              and nanotechnology. AFM can provide the ability to view and understand events as they occur                       at the molecular level which will increase our understanding of how systems work and lead to new discoveries in many fields, including materials science, electrochemistry, polymer science, biophysics, life science, and nanobiotechnology. In colloid and interface science and technology, AFM has been used to image surfaces with adsorbed surface-active agents (surfactants) with nanoscale resolution as well as measure extremely small surface      and molecular forces (~10-12 N) in the gas and liquid phases. This paper presents some selected outcomes           of the author research in three major research strands, including: 1. Structures of self-assembled surface micelles of surfactants adsorbed at solid-liquid interfaces: Surfactant adsorption at solid-liquid surfaces is of significant importance in many particulate processes ranging from particle separation and enhanced oil recovery to pharmaceutics, paint technology, high quality ceramics production, and mineral processing (Shah, 1998). Stability of the dispersions is related not only to the amount of adsorbed surfactant but also to the structure of the adsorbed surfactant molecules at the interface. The structures depend on the type of surfactants and the properties                of the water-solid interfaces. Different types of micellar structures have been observed at the surfaces for surfactant concentrations around the critical micelle concentration (CMC).  2.  Role of dissolved gas on attraction between hydrophobic surfaces: This attraction, known              as the hydrophobic force, is critically important for interaction and attachment of hydrophobic particles in coal and mineral flotation (Skvarla, 2001), oil droplets in pharmaceutical formulas and biological cells (Israelachvili, 1992). However, the origins and models for this attractive force between hydrophobic surfaces have been a source of debate since the first direct measurements of this force       in the early 1980’s (Christenson and Claesson, 2001). Attraction between an AFM hydrophobic probe and a flat hydrophobic surface has been studied in water, in water-ethanol mixtures, and in water saturated by gases with different solubilities. Experimental results indicate the role of surface stabilized submicron-sized bubbles in the hydrophobic attraction.   3.  Microhydrodynamic interaction between a solid particle and a soft interface: Interaction between        a solid particle and a soft interface such as an air bubble or an oil drop is a key to understanding emulsification and particle separation using air bubbles (Nguyen and Schulze, 2004). The AFM colloid probe technique has been used to measure hydrodynamic interaction forces between a solid sphere and an air bubble, as well as intermolecular and surface forces.                                                                            1 Anh V. Nguyen, Discipline of Chemical Engineering, School of Engineering, The University of Newcastle, Callaghan, New South Wales 2038, Australia, Phone (612) 4921 6189; Fax (612) 4921 6920; Anh.Nguyen@newcastle.edu.au   (Recenzovaná a revidovaná verzia dodaná 8. 11. 2005) 12  Acta  Montanistica  Slovaca     Ročník 10 (2005), mimoriadne číslo 1, 12-16 Materials and Methods  Surface images of surfactants and forces between a spherical particle and a flat surface were determined with the Nanoscope IIIa and PicoForce AFM’s (Digital Instruments/Veeco, USA). The interaction force between a surface and a particle was measured using the AFM colloid probe technique (Butt, 1991; Ducker et al., 1991). The micron-sized sphere was attached to the end of an AFM microfabricated cantilever. The flat surface was mounted to a piezoelectric transducer and was moved relatively to the particle at constant speeds. The cantilever deflection due to the surface-particle interaction was measured with a laser beam reflected from the cantilever onto a split photodiode. The force measurements with soft surface such as an air bubble or an oil drop attached to the piezoelectric transducer were carried out using the same technique. The surface interaction forces were determined by multiplying the cantilever deflection by the spring constant. The cantilever deflection was determined from the voltage signal of the photodiode employing        a scaling procedure, which used the linear displacements between the cantilever and the surface.                 The separation was then determined from the cantilever deflection and the displacement of the piezoelectric transducer. Imaging self-assembled surfactant structures at water-solid interfaces was carried out using the soft-contact technique (Manne et al., 1994; Manne and Gaub, 1995). The technique used a microfabricated cantilever with a shape tip (~ 10 nm radius) to probe a force, which was just delicate enough to distinguish surfactant surface structures without actually disturbing them. The force was then maintained constant during the tip scanning over the surface. The change in position of the tip for maintaining the constant force produced images of the adsorbed structures at the solid surface. Silicon nitride cantilevers (Digital Instruments/Veeco, USA) with nominal spring constant of 0.12 N/m were used. The particles and surfaces used in the experiments included glass and silica micro-spheres, silicon wafers, microscope glass slides, highly ordered pyrolytic graphite (HOPG), fused alumina surface, and mica. The silica and glass surfaces were made hydrophobic by a silanation process. The contact angle measured with a sessile drop apparatus was 62 degrees for the silanated surfaces. The chemicals used in the experiments included two surfactants (sodium dodecyl sulphate, SDS,         and cetyl trimethyl ammonium bromide, CTAB) and NaCl for controlling the electrical double-layer interaction between the surfaces. Deionized water from a Millipore Milli-Q system was used in cleaning glassware and the apparatus, and in preparing all solutions. All the chemicals used were of analytical grade. For the measurements with dissolved gases, water was first outgassed by subsequent boiling/freezing under low pressure several times and was then saturated by the required gas directly in the AFM liquid cell.  Results and Discussion  Fig. 1 shows a typical force curve for a silicon nitride cantilever approaching a surface in a solution      of 0,4 mM SDS and 0,1 M NaCl. A force of ~3 nN was used to image the adsorbed monolayer of SDS         on the graphite surface. The images of SDS adsorbed at hydrophobic graphite surfaces show linear, parallel aggregates oriented perpendicular to a symmetry axis and spaced apart by a little over twice the surfactant length (Fig. 2A). These observations are consistent with half-cylindrical aggregates wherein molecules adjacent to the surface are arranged tail to tail along parallel strips, with the hydrophobic alkyl chains oriented parallel to a graphite symmetry axis. At the hydrophilic, negatively charged, mica surface wormy micelles were observed. These structures are consistent with a cylindrical structure where the surfactant molecules first adsorb through the electrostatic interaction between the oppositely charged head group        and the mica surface (Fig. 2D). On amorphous mineral oxide surfaces, such as alumina        and silica, spherical structures have been observed (Fig. 2B & 2C). Clearly, the surface charge of these surface micelles plays             an important role in their formation and is crucial for the modelling of this self-assembly process involving the formation of surface micellar structures (Paruchuri et al., 2004). 0123450 5 10 15 20Tip-surface separation distance (nm)Interaction force (nN)  .imaging forceadsorbedlayergraphite surface  Fig. 1.  Typical force curve for a cantilever (with a 10 nm radius tip) approaching a HOPG graphite surface             in a solution of 0.4 mM SDS and 0.1 M NaCl.  13  Anh V. Nguyen: Imaging Structures and Measuring Forces at Nanoscales with Atomic Force Microscopy  -25-15-550 50 100 150 200Separation distance (nm)F/R (mN/m)Pure ethanol17% ethanolPure water0 40 80 120 160 200Separation [nm]-70-60-50-40-30-20-100Force/Radius [ mN/m]Fig. 2.  Images (200 nm × 200 nm) of surface surfactant structures obtained by the soft contact AFM technique. A. Hemi-cylindrical structures of SDS at a hydrophobic graphite surface. B. & C. Spherical structures of SDS at a positively charged fused alumina surface and of CTAB at a negatively charged silica surface. D. Short worm-like cylindrical structures of CTAB at a hydrophilic negatively charged mica surface (Paruchuri et al., 2004).  Fig. 3.  AFM results obtained for the scaled force, F/R, between hydrophobic glass spheres (radius R = 4 µm) and plates            in water saturated with argon and in ethanol-water mixtures  (Nguyen et al., 2003b).                  Effect of ethanol and argon dissolved in water on the force between hydrophobic (silanated glass) surfaces is shown in Fig. 3. The steps in the force curves are due to the formation of bridging gas micro-bubbles between hydrophobic surfaces (Attard, 1996; Attard et al., 2002). The addition of ethanol to water increases the jump-in distance, where the step in the force curve occurs and decreases the strength                of the measured force, to the extent that for mixtures with a high content of ethanol the step is difficult          14  Acta  Montanistica  Slovaca     Ročník 10 (2005), mimoriadne číslo 1, 12-16 to measure. Ethanol decreases the liquid-vapour surface tension, leading to the decrease in the attractive force. The influence of gas solubility on the hydrophobic attraction has been investigated with different gases, including argon (solubility in water 34,2 mL/L), nitrogen (solubility in water 15,8 mL/L), and helium (solubility in water 8,6 mL/L). Solubility of gases in ethanol is also higher than in water. The size                 of the observed steps on the force curves depends on the solubility of the gas used to saturate water:           The higher the solubility, the larger the size of the steps. However, the strength and the range of the attractive force in the water saturated by the gases appear to be the same, indicating that the gas solubility is not            a controlling factor. It is important to note that surface roughness and heterogeneities due to crystallites and patchy of the adsorbed surfactants can control the formation and stability of submicroscopic gas bubbles       at the solid-liquid interface.                   Fig. 4.  Scaled force, F/R, between air bubbles (R  250 µm) in deionized water and spherical particles with different hydrophobicities (contact angles). The approach speed between the surfaces was 10 µm/s (Nguyen         et al., 2003a).   Typical results for the interaction force with soft interfaces are shown in Fig. 4. The interaction forces are indeed of very long range, in particular, at high approach speeds. This long range indicates that             the hydrodynamic interaction is significant at high approach speeds between a bubble and a particle               in flotation. The microhydrodynamic resistance force due to the intervening water film between the surfaces increases with decreasing separation distance (film thickness) as predicted (Nguyen and Evans, 2002). Due  to strong attraction between the hydrophobic surfaces, the intervening liquid films between the bubble       and the hydrophobic particles are not stable, and the rupture of the intervening water film and the attachment       of the particle to the air bubble take place. For the data shown in Fig. 4 for the polyethylene (PE) particle, which was of a greater degree of hydrophobicity (the contact angle was about 80 degrees), the jump-in-contact interaction and the contact force were extremely strong and the force curves were not fully captured in the ‘force window’ of the AFM system. Our force measurements also show that the jump-in separation distances increase with approach speeds which supports the existence of submicroscopic bubbles                  on hydrophobic surfaces, as shown by the steps in Fig. 3 as well as by direct AFM images (Ishida et al., 2000; Tyrrell and Attard, 2002; Steitz et al., 2003). The bubbles become very unstable at high approach speeds and cause the attraction at large separation distances. This feature can complicate the analysis             of the interaction force at high approach speeds and separation distances. Further systematic investigations    of he bubble-particle interaction under the influence of both attractive surface and repulsive hydrodynamic forces are in progress.  In the case of the hydrophilic sphere (with contact angle of ~0o) shown in Fig. 4, the rupture                   of the intervening water film and the attachment of the particle to the air bubble did not take place. At short distances, significant deviations of strong repulsive force between hydrophilic spheres from the model prediction are related with the deformation of air-water interface due to the particle approach, as well            as intermolecular and surface forces.   15  Anh V. Nguyen: Imaging Structures and Measuring Forces at Nanoscales with Atomic Force Microscopy Conclusion  Atomic force microscopy has been used to image structures of surfactants adsorbed at solid surfaces      in water and measure interaction forces between the surfaces. Depending on the type of surfactants and       the surface, different types of micellar arrangements at the surfaces have been observed. Dissolved gases have an important effect on attraction between hydrophobic surfaces in liquids. Microhydrodynamic interactions between a solid particle and a soft interface have been determined for both hydrophobic                    and hydrophilic solid surfaces.  Acknowledgements: The author gratefully acknowledges         the Australian Research Council for financial support              and assistance of his former and current postgraduate students, postdoctoral fellows, and colleagues at the Universities             of Newcastle and Utah (Salt Lake city, USA), including Mr. Glen McCann and Mr. Marc Hampton, Drs. Vamsi K. Paruchuri, Keqing Fa, Jakub Nalaskowski and Stoyan I. Karakashev,                       and Professors Geoffrey M. Evans and Jan D. Miller.   References  Attard, P.: Bridging Bubbles between Hydrophobic Surfaces. Langmuir, 12(6): 1693-5, 1996. Attard, P., Moody, M., P. Tyrrell, J., W., G.: Nanobubbles: the big picture. Physica A: Statistical Mechanics and Its Applications (Amsterdam, Netherlands), 314(1-4): 696-705, 2002. Binnig, G.: Atomic force microscope and method for imaging surfaces with atomic resolution. US Patent   No. 4,724,318, 1986. Butt, H., J.: Measuring electrostatic, van der Waals, and hydration forces in electrolyte solutions with           an atomic force microscope. Biophys. J., 60(6): 1438-44, 1991. Christenson, H., K., Claesson, P., M.: Direct measurements of the force between hydrophobic surfaces          in water. Adv. Colloid Interface Sci., 91(3): 391-436, 2001. Ducker, W., A., Senden, T., J., Pashley, R., M.: Direct measurement of colloidal forces using an atomic force microscope. Nature (London), 353(6341): 239-41, 1991. Ishida, N., Inoue, T., Miyahara, M., Higashitani, K.: Nano Bubbles on a Hydrophobic Surface in Water Observed by Tapping-Mode Atomic Force Microscopy. Langmuir, 16(16): 6377-6380, 2000. Israelachvili, J., N.: Intermolecular and Surface Forces. Academic Press, London, 291 pp, 1992. Manne, S., Cleveland, J., P., Gaub, H., E., Stucky, G., D. Hansma, P., K.: Direct Visualization of Surfactant Hemimicelles by Force Microscopy of the Electrical Double Layer. Langmuir, 10: 4409-4413, 1994. Manne, S., Gaub, H., E.: Science (Washington, D. C.), 270: 1480, 1995. Nguyen, A., V., Evans, G., M.: Exact and approximate expressions for resistance coefficients of a colloidal sphere approaching a solid surface at intermediate reynolds number. Chem. Eng. Sci., 000: 000, 2002. Nguyen, A., V., Nalaskowski, J., Miller, J., D.: A study of bubble-particle interaction using atomic force microscopy. Minerals Engineering, 16(11): 1173-1181, 2003a. Nguyen, A., V., Nalaskowski, J., Miller, J., D., Butt, H., J.: Attraction between hydrophobic surfaces. Inter. J. Miner. Process., 72: 215-225, 2003b. Nguyen, A., V., Schulze, H., J.: Colloidal science of flotation. Marcel Dekker, New York, 840 pp, 2004. Paruchuri, V., K., Nguyen, A., V., Miller, J., D.: Zeta-potentials of self-assembled surface micelles of ionic surfactants adsorbed at hydrophobic graphite surfaces. Colloids and Surfaces, A: Physicochemical      and Engineering Aspects, 250(1-3): 519-526, 2004. Shah, D., O. (Editor): Micelles, Microemulsions, and Monolayers. Marcel Dekker, New York, 1998. Skvarla, J.: Hydrophobic interaction between macroscopic and microscopic surfaces. Unification using surface thermodynamics. Adv. Colloid Inter. Sci., 91(3): 335-390., 2001. Steitz, R. et al.: Nanobubbles and their precursor layer at the interface of water against a hydrophobic substrate. Langmuir, 19(6): 2409-2418, 2003. Tyrrell, J., W., G., Attard, P.: Atomic force microscope images of nanobubbles on a hydrophobic surface   and corresponding force-separation data. Langmuir, 18(1): 160-167., 2002. 16 

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Imaging Structures and Measuring Forces at Nanoscales with Atomic Force Microscopy

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Imaging structures and measuring forces at nanoscales with atomic force microscopy

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