The version of the surface force balance developed in our group and shown schematically below is capable of measuring both normal and shear forces between surfaces. The usual substrates used are atomically smooth mica surfaces, or mica on which different molecules or materials have been deposited. The surface separations can be measured to an absolute accuracy of 1 - 2 ┼ in the range from contact to several thousand ňngstroms by multiple beam interferometry. .
The two mica sheets are mounted on cylindrical quartz lenses in a crossed-cylinder configuration (inset). The distance D between them is measured - to ▒1-2┼ - via optical fringes (see picture below) arising from interference of white light passing through the sheets. The top lens is mounted on a sectored piezoelectric tube, PZT, as indicated. The PZT is magnified on the right, to illustrate (not to scale) the sideways motion induced when opposing sectors experience equal and opposite potentials. The bending of the vertical leaf springs S1 changes the capacitance of an air gap G and can be measured to ▒2┼: this yields the shear force between the surfaces. The bending of the horizontal leaf-spring S2, to similar accuracy, yields the normal forces.
Characteristic patterns of optical interference fringes observed in an experiment. The curved shape of the fringes in the left-hand figure corresponds to the shape of the contact region between the surfaces (equivalent to a sphere on a flat). The flat tips of the curves in the right-hand figure show the flattening which occurs when the surfaces come into adhesive molecular contact and distort the initially curved surfaces. The vertical lines are standard spectroscopic lines of the mercury lamp used for calibration. The wavelength can be measured to 0.2┼, corresponding to a resolution of 1 - 2 ┼ in measuring surface separation.
The balance is designed in particular to probe shear forces with extreme sensitivity and resolution: for example, it can measure normal and shear stresses that are a factor of some 5,000 - 10,000 smaller than can be achieved with conventional scanning probe methods (such as atomic force microscopy of friction force microscopy). Characteristic traces taken directly from a shear force measurement are shown below:
The top part of each trace shows the back-and-forth motion of the top surface, and the bottom part is the corresponding shear force between the surfaces. A) The surfaces are in strong molecular contact, and the frictional force exceeds the shear force at all points: the surfaces move together without sliding. B) The surfaces are separated by 4 molecular layers of cyclohexane, and when the shear force exceeds the frictional force the surfaces slide by a stick-slip mechanism as the thin films undergoes consecutive melting and freezing. C) The surfaces, coated with polymer brushes, are strongly compressed are some D = 120 - 130┼ apart. The shear force between the surfaces is out of phase with the sliding motion as the molecules on each surface drag past each other.
Traces of the shear forces between surfaces with polymer brushes, shown below, demonstrate how the brushes reduce friction dramatically by a factor of around 1000-fold.
Trace a shows the stick-slip pattern typical for sliding between two surfaces under a given load, with a friction coefficient of around 0.6. On attaching polymer brushes to the surfaces (cartoon), the friction force on sliding, trace b, is so small that it is within the noise level of our apparatus, as shown in the inset below.
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