TENSIOMETER FOR FREE STANDING SMECTIC FILMS

We present the design of a machine which allows us to measure the surf ace tension of a free standing smectic film with an accuracy of 0.1 mN /m absolute and 0.01 mN/m relative. Our design makes it possible to me asure the surface tension over a wide range of temperatures with minim al drift in the detector itself. The response time of our detector can be as small as 0.01 s which allows us to study dynamic processes invo lving the interaction between the film and the meniscus around the fra me that supports the film. Our design is based on a rigid frame with a movable side suspended as a pendulum. The surface tension force on th e movable side is opposed by a feedback system to stabilize the positi on of the movable side. The stabilizing force then is a measure of the surface tension. An optical system detects deflection of the movable side due to the surface tension force, and applies a restoring force b y generating electric current in a coil which applies force to a perma nent magnet attached to the pendulum arm. The feedback control include s an integrator stage to achieve finite force with negligible error in position. The device can be calibrated with a simple lever and weight scheme to simulate a surface tension force. The design elements that determine the stability and accuracy of the system are discussed. (C) 1996 American Institute of Physics.