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The Surface Force Apparatus (SFA) allows the forces between two surfaces in vapour or liquid to be measured with a sensitivity of better than l00 nN and a sub-nanometer distance resolution in favourable conditions. It can also be used to measure the refractive index of the medium between the surfaces and to study adsorption from liquids, capillary condensation, viscosity in thin films, and surface deformations caused by surface forces. Molecularly smooth mica surfaces are the primary surfaces used for these measurements, but it is also possible to coat these surfaces with surfactants, polymers, metals, etc. Such alternative surfaces may remain smooth by virtue of the mica substrate. Successful experiments have been carried out with sapphire (alumina) and silica surfaces. A novel technique for chemically modifying the mica has also been developed (see References & Publication section). The Surface Forces Apparatus is a scientific instrument that requires skill and experience to operate. MARK 4 DESCRIPTION SFA ACCESSORIES / SOFTWARE Mk4 Surface Forces Apparatus Customers: ADDITIONAL LABORATORY SERVICES REQUIRED: REFERENCES: CONTACT DETAILS

Multiple-beam interferometry allows you to "see" what is happening between the surfaces and gives you an unambiguous zero of separation

Basic Mark 4 Surface Forces Apparatus

The Surface Forces Apparatus Mark 4 consists of a cylindrical chamber to which a circular top flange is attached. The top flange holds the precision piezoelectric crystal (tube type) assembly and the coarse drive mechanism. Coarse mechanical control of separation is achieved through a low reving, high torque DC motor, attached to a translation stage that positions the shaft, double cantilever force measuring springs, and lower surface, within range of the top surface that is mounted on the piezoelectric tube. This mechanism is useful for most experimental requirements.
Coarse mechanical positioning accuracy is better than 500 Å enabling surfaces to be positioned well within the range of the piezoelectric crystal. The total range of the piezo is approximately 2 µm, being sufficient to accurately measure all but the most repulsive force curves.

The apparatus is simple to clean as the shape allows easy access to all parts of the chamber. By design the chamber is essentially fluid leak free. It is constructed mainly of stainless steel 316 and Al.Alloy with no welded parts. Most mechanical components are machined in-house by precision machinists, ensuring quality control. Included with the basic apparatus is an adjustable mirror, a microscope and stand with prisms for accurate alignment of optics, toolkit, a comprehensive SFA user manual, supplemented by a training videotape showing techniques involved in mica cleaving and gluing as well as general experimental procedures.

The Surface Force Apparatus Mark 4 is first described in "A Device for Measuring the Force and Separation between Two Surfaces down to Molecular Separations" by J. L. Parker, H. K. Christenson and B. W. Ninham, Rev. Sci. Instrum. 60, 3135 (1989). The Mk4 model follows on from the highly sucessful Mk2 model. Thirteen Mk4 SFA instruments have been sold to customers worldwide.

The price of a basic Mk4 Surface Force Apparatus is in the order of $100,000 Australian dollars.

SFA Accessories - A comprehensive range of well developed accessories is offered:

• Variable stiffness double cantilever spring
  This attachment enables the spring constant to be varied during experiments by a factor of greater than 100, from about 100 N m-1 to more than 10,000 N m-1. The attachment is particularly suitable for measuring monotonically attractive force laws, and also when very large short-range repulsive forces are being investigated.
  
  • Mk IV Variable Double Cantilever Spring (photo Tim Thompson)
• Friction attachment Link to detailed web page
  This attachment enables the study and measurement of interfacial frictional forces between surfaces in the field of nanotribology. There is a growing interest in measuring interfacial friction, particulary by research groups in universities and industry. The device will fit existing Mk2 and Mk4 Surface Force Apparatuses, or could be customised for a stand-alone device.
• Capacitance attachment Link to detailed web page
  The distance between the active surfaces is obtained from the capacitance of a cylindrical parallel plate capacitor measured with a variable ratio transformer bridge. The surface separation is made to vary with a magnetic force transducer. Displacements of 0.1 nm can be detected.
• Syringe injection unit
  This allows for small amounts of liquid to be injected into specific regions within the chamber, or as a droplet between the two surfaces, without having to open the chamber.
• Small Volume chamber
  This replaces the normal chamber and is suitable for experiments with opaque or very expensive liquids. The volume is approximately 25 mls.
• Rigid support for lower surface
  This is useful for experiments where spring deflection (force measurement) is not desirable, eg. capillary condensation and absorption.
  
• Attachment for LB Deposition
  This device simplifies LB Deposition onto mica surfaces.
• Differential spring mechanism
  In addition to performing exactly the same function as the coarse mechanical control of separation outlined in the basic apparatus, the differential spring drive mechanism enables fine control of the lower surface over a range of about 15 µm. The differential spring is driven by a second DC motor driven translation stage. This is useful when measuring forces larger than 10 mN/m (i.e. outside the piezoelectric tube range). (Custom made on request)
• Magnetic force transducer
  A force on a magnet attached to the end of the cantilever spring is produced by means of an electric current passed through a coil external to the apparatus. This complements or substitutes for the piezoelectric transducer and enables the application of highly linear and non-hysteretic external forces used for force measurement (Stewart and Christenson, 1990).
• Vacuum / Pressure Version Link
  For investigations at pressures other than atmospheric. Useful for studies of capillary condensation with high vapour pressure liquids, and to provide more accurate control of the chemical potential in vapour adsorption experiments.
• Software Link
  Available for the Apple Mac computer is a program with graphical interface for calculating the electric double layer force and dispersion force between two identical surfaces in electrolyte by means of the non-linear Poisson-Boltzmann equation. This enables the fitting of theoretical values to experimental data.

A demonstration version of the software is available for the Mac. Select the above link.

ADDITIONAL LABORATORY SERVICES and EQUIPMENT REQUIRED :

The following is a checklist of laboratory equipment required to conduct surface force experiments. Most items are commercially available (we do not provide them) :

• Grating spectrometer, e.g. Jarrell-Ash 82-010, 590 g/mm grating (Å/mm at exit slit). The exit slit should be equipped with a microscope eyepiece mounted on a translation stage with a micrometer for the reading of the fringe positions. This is preferable to cranking the grating. The micrometer should capable of giving an output signal that can be displayed on an encoder and the signal recorded by a computer. To direct the light beam into the spectrometer's front inlet slit, a tilt/rotation stage with a right-angle prism sitting on top will need to be mounted with a bracket at the front of the spectrometer.
• White light source, eg 150 W halogen bulb.
• Low Pressure Sodium Vapour Lamp (for viewing Newton's rings). eg Philips SOX E18, 18W
• Mercury pen-ray lamp (for reference spectral lines).
• High Voltage Power Amplifier - to drive the piezoelectric tube which is a capacitance load. The supply should be low noise, highly stable with very linear operation over a wide range of output voltages of ± 500V. It should have facility for an input voltage of ± 10V, and a monitor voltage of ± 10V. eg Trek 601C
• Low voltage laboratory DC power supply - to drive the motor, DC 0-30 V, with facility for remote adjustment by variable resistor.
• Laminar air flow cabinet (Horizontal flow type, HEPA filter) for mica cleaving, gluing and apparatus assembly.
• Clean nitrogen gas supply for cleaning and flushing of chamber.
• Supply of clean and filtered ethanol to fill a Pressure Rinser for cleaning apparatus parts.
• Hot platinum wire cutter for mica. (Available for sale to apparatus purchasers).
• Vacuum evaporator for thin film silvering of mica.
• Temperature control - A SFA experimental room requires a separate air conditioning system with precise temperature control to minimize instrument drifts. Experience has show that a chilled-water air conditioning system with a PI or PID controller is best in order to obtain 0.2 deg C or better control. Standard on/off or multi-step control of cooling is not recommended. Air ducting around the SFA room should be well designed, providing a generous airflow rate with low discharge velocity.
• Mounting of SFA and spectrometer - Low vibration enviroment, two separate stands e.g. Sand filled columns or anti vibration table.
• Syringes, filters, tweezers, general laboratory accessories and instruments.

The following items are optional and are not essential for basic operation of the SFA, although many users find them useful :

• Computer, plus a Data acquisition card 16 Bit A/D, D/A. Interface adaptor box to collect inputs and outputs to/from a data acquisition card. Software will need to be written to interface with components selected.
• Video camera, high quality CCD optimised for low light level imaging, recorder and monitor for recording fringes (optional).

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  Dr Satomi Ohnishi operating the friction device in the SFA darkroom - Applied Maths, Surface Physics Group

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REFERENCES and PUBLICATIONS:
The Surface Forces Apparatus Mark IV is first described in :
"A Device for Measuring the Force and Separation between Two Surfaces down to Molecular Separations" by J. L. Parker, H. K. Christenson and B. W. Ninham : Rev. Sci. Instrum. 60, 3135 (1989).

The following publications describe the theory and applications of multiple-beam interferometry:

J. N. Israelachvili : J. Colloid Interface Sci. 44, 259 (1973).	R. G. Horn and D. T. Smith : Appl. Opt. 30, 59 (1991).
M. T. Clarkson : J. Phys. D 22, 475 (1989).	V. S. Mangipudi : J. Colloid Interface Sci. 175, 484 (1995).

The following is a 'selection' of publications describing various applications of the SFA technique:

Refractive index measurements and adsorption isotherms

P. Kékicheff and O. Spalla: Langmuir 10, 1584 (1994).

J. E. Curry and H. K. Christenson : Langmuir 12, 5729 (1996).

Measurements with a non-mica substrate - silica surfaces
R. G. Horn, D. T. Smith and W. Waller, Chem, Phys. Lett. 162, 404 (1989).

Metal surfaces

J. L. Parker and H. K. Christenson, J. Chem. Phys. 88, 8013 (1988).

J. M. Levins and T. K. Vanderlick : J. Phys. Chem. 99, 5067 (1995).

Forces from dynamic measurements
P. M. Claesson and H. K. Christenson, J. Phys. Chem. 92, 1650 (1988).

Capillary condensation and phase transitions in thin films

H. K. Christenson, J. Fang, and J. N. Israelachvili, J. Phys. Rev. B. 39, 11750 (1989).
H. K. Christenson, Phys. Rev. Lett. 73, 1821 (1994).	H. K. Christenson, Phys. Rev. Lett. 74, 4675 (1995).

Plasma modification of mica

J. L. Parker, D. L. Cho and P. M. Claesson, J. Phys. Chem. 93, 6121 (1989).

J. Wood and R. Sharma : Langmuir 11, 4797 (1995).

Studies of lyotropic liquid crystals
P. Petrov, S. Miklavic, U. Olsson and H. Wennerström : Langmuir 11, 3928 (1995). 

Langmuir-Blodgett deposition

K. Kurihara, T. Kunitake, N. Higashi and M. Nina : Thin Solid Films 210/211, 681 (1992).

P. Berndt, K. Kurihara and T. Kunitake : Langmuir 11, 3083 (1995).

Protein adsorption
P. M. Claesson, E. Blomberg, J. C. Fröberg, T. Nylander and T. Arnebrant : Adv. Colloid Interface Sci. 57, 161 (1995). 

Magnetic force transducer
A. M. Stewart and H. K. Christenson, Measurement Science and Technology 1, 1301 (1990).

Capacitance dilatometry
A. M. Stewart. Measurement Science and Technology 11, 298 - 304 (2000).

Interfacial friction (ANU web page Friction link)
Humidity dependence of interfacial friction between mica surfaces
S. Ohnishi and A. M.Stewart. Langmuir  18 (16), 6140-6146 (2002). Link to Langmuir 2002/Vol 18

Influence of adsorbed/condensed cyclohexane between mica surfaces on stick-slip frictional behavior.
Satomi Ohnishi, Dasikaku Kaneko, A. M. Stewart, V. V. Yaminsky
16th National Biennial Congress of the Australian Institute of Physics, Canberra, 31 Jan. - 4 Feb. (2005) (poster paper).
 

Adhesion and Friction A. M. Homola, J. N. Israelachvili, M. L. Gee and P. M. McGuiggan : J. Tribol. 111, 675 (1989).

J. Peachey, J. van Alsten and S. Granick : Rev. Sci. Instrum. 62, 463 (1991).

Viscosity

D. Y. C. Chan and R. G. Horn : J. Chem. Phys. 83, 5311 (1985).	J. N. Israelachvili, S. J. Kott and L. J. Fetters : J. Polymer Sci. Phys. 27, 489 (1989).
J. N. Israelachvili : J. Colloid Interface Sci. 110, 263 (1986).

Adhesion

H. K. Christenson : J. Phys. Chem. 97, 12034 (1993).

H. K. Christenson : Langmuir 12, 1404 (1996).

Surface deformations

R. G. Horn, J. N. Israelachvili and F. Pribac : J. Colloid Interface Sci. 115, 480 (1987).

D. Maugis and B. Gauthier Manuel : J. Adhesion Sci. Technol. 8, 1311 (1994).

CONTACT DETAILS
Marketed by The Australian National University
Research School of Physical Sciences and Engineering
Canberra ACT 0200, AUSTRALIA
International Ph: +61-2-6125 0105
International Fax: +61-2-6125 0732
E-mail Surface.Forces@rsphysse.anu.edu.au

Link to Applied Maths Research Page

Link to Older pictures of Mk2 SFA

Updated August 2003 AH.