In Part I, we investigate the problem of the thickness of a flowing helium film. Past experiments have either confirmed Kontorovich's prediction that the flowing film is thinner than the stationary film, or have observed no difference (e.g, the experiment by Keller (6)). Goodstein and Saffman have proposed a theory that tries to reconcile the difference in observations as the result of slightly different experimental conditions between different experiments. Our experiment tried to test Goodstein-Saffman's theory by attempting to produce both thick and thinned flowing films in the same experimental setup but under slightly different conditions. We constructed a cell in which the temperature could be varied from about 0.5°K to 1.9°K and film thickness was measured by the capacitance technique. Film flow from an external bath into a stainless steel beaker was terminated in two ways - either in a velocity step function or in an oscillation. The film thickness in the first case showed a sharp jump. In the second case, film thickness oscillated at twice the frequency of the bath level oscillation. The same behaviour was observed from 0.5°K to 1.9°K. Our result therefore (i) does not confirm Keller's result, which motivated Goodstein-Saffman's theory, (ii) shows that for Goodstein-Saffman's theory to be correct, the τ1 in their theory would have to be much longer than one hour. The conclusion is that it is unlikely for any more stable state than the thinned film to exist. In Part II, we study the feasibility of verifying Lifshitz's theory of Van der Waals force by measuring the helium film thickness on a nickel surface form a height of about 1.5 mm to 0 mm. In this range, the retardation effect predicted by the theory would be more evident. The film thickness was measured at about 0.6°K by the capacitance technique on a horizontal parallel plate capacitor about 1.5 mm above the bath level. The level was raised in a virtual manner by applying a de bias across the capacitor. With a properly balanced cell, the virtual level could be changed in a continuous manner from about 1.5 mm to about 40 μm. Our measurement shows a film profile about 350 Å thicker than predicted by theory. We suggest that the discrepancy maybe ascribable to the roughness of the metallic surface, a quantitative description of which is lacking~ In view of this, a quantitative test of Lifshitz's theory will have to wait until a smooth metallic surface can be produced in a well-controlled manner. In Part III, we investigate theoretically the existence of instability of the helium film under a strong electric field. We found that such an instability does occur when the film thickness reaches about 4000 Å for our existing cell geometry. We offer experimental evidence that we may have observed such an in stability .
D. S. W. Kwoh (Thu,) studied this question.