Surface forces apparatus
Encyclopedia
The surface force apparatus (SFA) is a scientific instrument
and technique pioneered by D. Tabor, R.H.S. Winterton, J.N. Israelachvili
in the early 1970s at Cambridge University. By the mid-70's Israelachvili had adapted the original design to operate in liquids, notably aqueous solutions, while at the Australian National University
.
In this instrument, two surfaces are carefully moved towards and retracted from one another, all the while measuring their interaction force. One surface is held by a cantilever
ed spring, and the deflection of the spring is used to calculate the force being exerted. This technique uses piezoelectric positioning elements (in addition to conventional motors for coarse adjustments), and senses the distance between the surfaces using optical interferometry
. Using these sensitive elements, the device can resolve distances to within 0.1 nanometer, and force
s at the 10–8 N level. This extremely sensitive technique can be used to measure electrostatic forces, elusive van der Waals forces, and even hydration or solvation forces. SFA is in some ways similar to using an atomic force microscope
to measure interaction between a tip (or molecule adsorbed onto the tip) and a surface. The SFA, however, is more ideally suited to measuring surface-surface interactions, and can measure much longer-range forces more accurately. The SFA technique is quite demanding, however, and only a handful of labs worldwide have functional instruments.
In the SFA method two smooth cylindrically curved surfaces whose cylindrical axes are positioned at 90° to each other are made to approach each other in a direction normal to the axes. The distance between the surfaces at the point of closest approach varies between a few micrometers to a few nanometers and down to contact. When the two curved cylinders have the same radius of curvature, R, this so-called 'crossed cylinders' geometry is mathematically equivalent to the interaction between a flat surface and a sphere of radius R. Using the crossed cylinder geometry makes alignment much easier, enables testing of many different surface regions for better statistics, and also enables angle-dependent measurements to be taken. A typical setup involves R = 1 cm. Position measurements are typically made with a white-light source and by analyzing the fringes of equal chromatic order (FECO) (although use of a laser
is also possible). The substrate for the surfaces or molecules of interest is generally mica coated with a semi-reflective layer of silver. This optical setup enables determination of the distance between the two surfaces. Mica is used because it is extremely flat, easy to work with, and optically transparent. Any other material or molecule of interest can be coated or adsorbed onto the mica layer.
Early experiments measured the force between mica
surfaces in air or vacuum
. The technique has been extended, however, to enable an arbitrary vapor
or solvent
to be introduced between the two surfaces. In this way, interactions in various media can be carefully probed, and the dielectric constant
of the gap between the surfaces can be tuned.. Moreover, use of water
as a solvent enables the measurement of interactions between biological molecules (such as lipid
s in biological membrane
s or protein
s) in their native environment. In a solvent environment, SFA can even measure the oscillatory solvation and structural forces arising from the packing of individual layers of solvent molecules. It can also measure the electrostatic 'double layer' forces between charged surfaces in an aqueous medium with electrolyte
.
The SFA has more recently been extended to perform dynamic measurements, thereby determining viscous
and viscoelastic properties of fluids, friction
al and tribological
properties of surfaces, and the time-dependent interaction between biological structures.
Measuring instrument
In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item...
and technique pioneered by D. Tabor, R.H.S. Winterton, J.N. Israelachvili
Jacob Israelachvili
Jacob Israelachvili is a professor of chemical engineering and materials at the University of California, Santa Barbara . Israelachvili received his Ph.D. in Physics from Christ's College, Cambridge in 1972, and joined UCSB in 1986. His research has involved study of molecular and interfacial forces...
in the early 1970s at Cambridge University. By the mid-70's Israelachvili had adapted the original design to operate in liquids, notably aqueous solutions, while at the Australian National University
Australian National University
The Australian National University is a teaching and research university located in the Australian capital, Canberra.As of 2009, the ANU employs 3,945 administrative staff who teach approximately 10,000 undergraduates, and 7,500 postgraduate students...
.
In this instrument, two surfaces are carefully moved towards and retracted from one another, all the while measuring their interaction force. One surface is held by a cantilever
Cantilever
A cantilever is a beam anchored at only one end. The beam carries the load to the support where it is resisted by moment and shear stress. Cantilever construction allows for overhanging structures without external bracing. Cantilevers can also be constructed with trusses or slabs.This is in...
ed spring, and the deflection of the spring is used to calculate the force being exerted. This technique uses piezoelectric positioning elements (in addition to conventional motors for coarse adjustments), and senses the distance between the surfaces using optical interferometry
Optical interferometry
Optical interferometry combines two or more light waves in an opticalinstrument in such a way that interference occurs between them.Early interferometers used white light sources and also monochromatic light from atomic sources...
. Using these sensitive elements, the device can resolve distances to within 0.1 nanometer, and force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
s at the 10–8 N level. This extremely sensitive technique can be used to measure electrostatic forces, elusive van der Waals forces, and even hydration or solvation forces. SFA is in some ways similar to using an atomic force microscope
Atomic force microscope
Atomic force microscopy or scanning force microscopy is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit...
to measure interaction between a tip (or molecule adsorbed onto the tip) and a surface. The SFA, however, is more ideally suited to measuring surface-surface interactions, and can measure much longer-range forces more accurately. The SFA technique is quite demanding, however, and only a handful of labs worldwide have functional instruments.
In the SFA method two smooth cylindrically curved surfaces whose cylindrical axes are positioned at 90° to each other are made to approach each other in a direction normal to the axes. The distance between the surfaces at the point of closest approach varies between a few micrometers to a few nanometers and down to contact. When the two curved cylinders have the same radius of curvature, R, this so-called 'crossed cylinders' geometry is mathematically equivalent to the interaction between a flat surface and a sphere of radius R. Using the crossed cylinder geometry makes alignment much easier, enables testing of many different surface regions for better statistics, and also enables angle-dependent measurements to be taken. A typical setup involves R = 1 cm. Position measurements are typically made with a white-light source and by analyzing the fringes of equal chromatic order (FECO) (although use of a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
is also possible). The substrate for the surfaces or molecules of interest is generally mica coated with a semi-reflective layer of silver. This optical setup enables determination of the distance between the two surfaces. Mica is used because it is extremely flat, easy to work with, and optically transparent. Any other material or molecule of interest can be coated or adsorbed onto the mica layer.
Early experiments measured the force between mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
surfaces in air or vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
. The technique has been extended, however, to enable an arbitrary vapor
Vapor
A vapor or vapour is a substance in the gas phase at a temperature lower than its critical point....
or solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
to be introduced between the two surfaces. In this way, interactions in various media can be carefully probed, and the dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
of the gap between the surfaces can be tuned.. Moreover, use of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
as a solvent enables the measurement of interactions between biological molecules (such as lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s in biological membrane
Biological membrane
A biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...
s or protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s) in their native environment. In a solvent environment, SFA can even measure the oscillatory solvation and structural forces arising from the packing of individual layers of solvent molecules. It can also measure the electrostatic 'double layer' forces between charged surfaces in an aqueous medium with electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
.
The SFA has more recently been extended to perform dynamic measurements, thereby determining viscous
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
and viscoelastic properties of fluids, friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
al and tribological
Tribology
Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear...
properties of surfaces, and the time-dependent interaction between biological structures.