Structures of Molecularly Thin Fluid Films Evaluated by Friction Measurements Using Surface Forces Apparatus

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When fluid molecules are confined in a narrow gap between two smooth surfaces, their dynamic properties are completely different from those in the bulk. The molecular motions are highly restricted, and the system shows "solid-like" responses when sheared slowly. In this paper, we describe friction measurements of molecularly confined films of a variety of fluids (from simple liquids to polymer melts) using a surface forces apparatus (SFA). Because the optical technique of SFA (fringes of equal chromatic order) allows us to perform accurate measurements of the true contact area and fluid film thickness during sliding, we can evaluate the dynamic structure of confined fluids. The following topics are described in this paper: i) molecular solidification mechanism of fluids due to confinement; ii) structural change of confined fluids in "stop-start" measurements; and iii) layering transition of a confined polymer melt during sliding.