John L. Parker

Device for measuring the force and separation between two surfaces down to molecular separations

We present an apparatus for measuring the force as a function of distance between surfaces at separations down to the order of molecular dimensions. The device is a simplified yet improved version of the surface force apparatus first developed by Israelachvili and Adams [J. Chem. Soc. Faraday Trans. 1, 74, 975 (1978)]. It gives the same measurement resolution and has the same possibilities of studying various phenomena in thin films such as friction, viscosity, adhesion, and phase transitions. It offers improved performance with regard to control of surface separation and increased versatility by virtue of variable chamber dimensions and geometry.

Measurements of the forces between a metal surface and mica across liquids

Results of direct measurements of the interactions between a silver surface and a mica surface across liquids are presented. In the nonpolar liquid octamethylcyclotetrasiloxane oscillatory solvation forces are found, the range of which is shorter than between two mica surfaces. In aqueous electrolyte solutions long‐range double‐layer forces are present. At smaller separations the interaction is attractive in conductivity water but steeply repulsive in 10−4 M sodium carbonate.

Forces between bilayers containing charged glycolipids

Abstract Measurements of the forces between supported bilayers containing the ganglioside GM1 (monosialoganglioside) are reported. The interaction between pure GM1 bilayers bathed in 1 mM sodium chloride is well described by DLVO theory at large separations. At 100 m M sodium chloride a weak attractive minimum due to a van der Waals force is measured outside the range of the double layer interaction. When GM1 is mixed with dioleoylphosphatidylcholine (DOPC) the force is again well fitted by double layer theory at large separations. A hard wall in the force is encountered at 4.5 nm which is the separation at which the hydration force from DOPC is expected. The interactions between GM1 layers mixed with dipalmitoylphosphotidylcholine (DPPC) shows a hard wall in the force curve at a much larger separation (8–9.3 nm). The force in all cases remained repulsive at small separations although DLVO theory predicts an attraction due to van der Waals forces. It is proposed that loss of this attraction is due to a combination of hydration and steric repulsion and as a result GM1 alone cannot promote adhesion between bilayers.

Force feedback surface force apparatus: Principles of operation

The principles of operation of a force feedback surface force apparatus are described. A magnetic force transducer and a bimorph displacement sensor are connected in a servo loop. The dynamic behavior of the system is analyzed and the effect of eddy currents on the frequency response of the magnetic field is discussed. The apparatus has several advantages over more traditional force measuring techniques. Cantilever instability associated with the measurement of attractive forces is reduced and the apparatus may be used for hydrodynamic measurements. A resonance technique for measuring the spring constant is also described.