抄録
Contact angle of an oil droplet placed on a horizontal metal surface is used one of the characteristic values of such clock oil, lubricant fraction of mineral oil, rust preventive oil, etc. This value shows the adhesion wetting of test oil against metal surface. And it is necessary to standardize the dropping method, the drop volume and the time after deposit on toget its reproducibility. It is better to use a small wire immersed in test oil, draw out and trickle the drop let on the test surface, for better reproducibility. The volume of drop is about 0.004ml. The contact angle of oil is measured after 3 minutes by means of a goniometer eyepiece constructed by the author. When the metal surface is absorbed and coated by oleophobic molecular layers of such stearic acid or its derivatives etc., an oil drop on it shows a large contact angle of 40-50°. But the contact angle of oil drop for bare metal surface is less than 10°.
The difference between new polished surface by emery paper and its aged surface about 24 hours can be distinguished by the contact angle of distilled water drop, but it can not be distinguished by that of oil drop. Surface tention of mineral oil is scarcely changed by addition of 4-5% of the polar type rust preventive additives.
