Tensiometer for free standing smectic films

We present the design of a machine which allows us to measure the surface 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 measure the surface tension over a wide range of temperatures with minimal 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 involving the interaction between the film and the meniscus around the frame 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 the movable side is opposed by a feedback system to stabilize the position 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 by generating electric current in a coil which applies force to a permanent magnet attached to the pendulum arm. The feedback control includes 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.