Measuring the Thickness of Thin, Flowing, Liquid Films HERBERT H. BECK, Hanovia Chemical and Manufacturing Company, Newark, N. J., AND K. G. WECKEL, Department of Dairy Industry, University of Wisconsin, Madison, Wis.
T
consecutive points can be detected. However, manipulation of the micrometer causes interference with the normal smooth flow of the film, and undoubtedly some leverage is exerted a t contact with the metal surface. These difficulties produce errors which are additive, and probably constant. With suitable recognition of such error the micrometer method might nevertheless serve as a guide in appraising the reliability of other methods. To overcome the disadvantages of these methods, a new procedure which was found more suitable for measuring the thickness of flowing liquid films was developed.
H E commercial application of ultraviolet radiation in photochemical processes has created new scientific problems. The shallow penetrating power of these rays usually requires that the substance t o be treated be distributed in a thin layer. When the substance is a liquid, it is exposed in a thin flowing film. The commercial irradiation of milk to increase its vitamin D potency is an example of such a process, Because of the relatively small amount of activatable substance in milk and the shallow penetrating power of the active radiation, the effectiveness of the process may depend upon the characteristics of the film, and these should be understood.
Description of Method The method utilizes the well-established principle t h a t some light is reflected from any surface, and that the angles of incidence and reflection are equal (2). Parallel forward movement of the surface causes a proportional change in t h e position of the reflected beam. The position of the beam of light reflected from the surface will be likewise affected b y introducing an interjacent flowing film of liquid. The application of the principle is illustrated in Figure 1. A very narrow beam of light, AB, is projected onto a metal surface at B at an angle of incidence of 45' whence the angle of the reflection, BD, will also be 45'. If now a uniform film of liquid is made to flow down the metal surface, the beam is reflected from the surface of the liquid at some point, C. Since the angle of incidence isxnchanged, the reflection, CE, from the liquid surface is parallel to the reflection, BD, previously obtained from the metal surface. The distance between the two reflected beams can be measured, and is equal to the distance BC. The distance BC i; the hypotenuse of a right triangle whose acute angles are 45 . One side of this triangle is perpendicular to the metal surface at B and is equal in length to the thickness of the film. The latter dimension can, therefore, be accurately computed by multiplying the measured distance, BC, by the cosine 45' or 0.707. If an angle of incidence other than 46" is used, the calculations of the film thickness become more complicated. If the angle of incidence of the beam varies 1' from the desired angle of 45' an error of about 2 per cent is made.
FIGURE 1. DIAGRAM ILLUSTRATING MEASURING THICKNESS OF FILMBY LIGHT BEAM
I n a film of known width and capacity, the thickness of any cross section perpendicular to the direction of flow is dependent upon the average velocity of the fluid. When the average velocity of flow within a given distance has been measured, the average thickness of the film within that distance can be calculated. Similarly, when the thickness of a uniform cross section of film has been determined, the velocity of the fluid through the cross section can be calculated. It is possible, therefore, by measurement of the thickness of cross sections a t succeeding short intervals to determine any variations in velocity. Recently, in studies with milk, two methods have beeh employed for measuring and determining film velocity and/or thickness. A method utilized by Supplee and Dorcas (3) is one wherein the flow is measured with the aid of aggregates of finely divided pigment placed in the film. Application of this method revealed, however, that particles a t different depths within the film do not advance a t a uniform velocity. Compact groups of particles or drops of suspended colored material placed in a flowing liquid are elongated and dispersed to many times their original dimension by the film in the direction of flow. It was difficult to establish which particles advanced a t speeds representative of the average forward velocity of the film. For this reason the method does not lend itself to measurement of acceleration of velocity of a film through consecutive short distances of flow. The second method ( I ) , employing a screw type of micrometer suitably mounted in a tripod, was also investigated. With this instrument the thickness of the film is the distance measured between contact of the micrometer point with the surface of the liquid film, and the metal surface supporting the film. The advantage of this method is its flexibility which permits measurements to be made a t any point on a flowing film; variations in film thickness which may occur between
Arrangement of Apparatus The instruments necessary for providing the light beam are relatively simple (Figure 2). Any light system that will project a sharply defined narrow (about 0.05 mm. wide}
FIGURE 2. ARRANGEMENT OF APPARATUS
parallel beam easily visible under the conditions of general lighting is adequate. A satisfactory beam may be obtained by projection of light through two parallel slits. Sharp definition of the edges of the beam on the reflecting surface 258
JULY 15, 1936
ANALYTICAL EDITION
259
may be made by use of appropriate focusing condensers. A common reference point in the intercepted beam can be established by insertion of a fine cross hair in the slit nearest the reflecting surface.
A micrometer microscope (Gaertner) comparator is a convenient instrument for making film thickness determinations. The essential arts of the instrument are a low-power microscope (2 cm. Ecal length) containing cross hairs for accurate sighting, a lateral slide to carry the microscope, and a micrometer screw provided with scale and vernier calibrated t o 0.005 mm. The projector and microscope should be mounted with their center lines in the same plane, and with the line bisecting the angle of their intersection Tpendicular to the surface upon which measurements are to e made. When the surface is in a true vertical position, the center lines of the projector and microscope are in a horizontal plane and form an angle of 90". This relation should be maintained for all measurements. The instruments, therefore, may be mounted in a fixed position on a common frame or base plate to avoid the necessity for repeated .alignment. The angle of incidence of the projected beam of light may be conveniently adjusted with the aid of a 45" triangle. 'The beam can be aligned t o parallel the hypotenuse edge of the triangle appropriately held against the surface upon which measurements are to be made. Accuracy of the adjustment may be verified 'by proper use of mechanics squares. The construction of the comparator used in these studies is such that the microscope is perpendicular to the lateral slide plate. Consequently, a comparator of this type may also be .conveniently aligned with the aid of a 45" triangle. A photograph of the assembly of the instruments is given in Figure 3. It is essential, for accurate measurements, that the film be .established upon a rigid and truly plane surface. For this, a 0.63-cm. (0.25-inch) thick steel channel, planed smooth on its flat surface, was used. The end of the planed steel was further machined to provide a perfectly straightedge over which milk, from an attached trough, could be evenly distributed. A straightedge type of distributor (spillway type) was chosen because it permits free and natural formation of a film with small consequent effect on its initial velocity. Further t o facilitate the study ,of the influence of gravity on the thickness of a film, a quiet smooth approach of the liquid to the distributor edge was obtained by use of a deep trough. The supply of milk to the trough was maintained at a predetermined rate by means of a float control. Controlled flowing films deliver a definite quantity of liquid per unit time; consequently they have a definite capacity per unit width. Film capacity may be expressed in any convenient standard, such as pounds per lineal foot per hour. Physical characteristics of a film, such as velocity and thickness, adjust themselves to, and are dependent upon, the film capacity.
-
TABLEI. THICKNESS OF FILMS OF MILK Thickness of Film
c
Light beam method
Calculated from weight of film adhering on a metal strip
Mm.
Mm.
Mm.
0.29 0.42 0.52
0.26 0.39 0.49 0.0246
0.0238
Flow Rate
Micrometer
Lb./ft./hr. 100 300 600 Nearly zero
....
.... .
.
1
.
A convenient procedure in determining the thickness of a film with the instrument assembly when properly aligned is by first establishing the desired film capacity over the flow surface area. By means of a short baffle placed on the distributor edge, the flow may be temporarily diverted from a narrow vertical area which includes the point at which a measurement is to be made, and where the narrow beam of light is being reflected. It is necessary to clean the area a t hhe point of measurement by means of a dry cloth to assure reflection from the metal surface. The relative position of a Zine shadow in the reflected beam of light (shadow caused by cross hair inserted in the projector slit) can be determined by reading from the operative vernier and scale when the microscope is properly aligned. The flow may then be promptly restored by removal of the diverting baffle, and the changed position of the beam, now reflected from the surface of the
FIGURE 3. ASSEMBLY OF APPARATUS
film, read from the vernier scale. The difference of the two readings in millimeters is the distance DE or BC indicated in Figure 1, which when multiplied by cosine 45' gives the thickness of the film a t the point of measurement. The stability of the apparatus during the interval of measurement may be determined by repeating the initial measurement when the flow of the fluid is again diverted by the baffle. A number of measurements were made a t the same points on several films by both the light beam and micrometer methods. The individual results by each method were averaged for the comparison presented in Table I. It will be observed that the measurements made with the light beam are less than those made with the micrometer. A further comparison was made by measuring and weighing a film adhering to a strip of metal and calculating the thickness from the weight of the film. These results are also presented in Table I.
Conclusions A new method, employing reflected beams of light to determine the thickness of flowing films, is described. This method is sufficiently sensitive to detect variations in the thickness of a film, such as those caused by changes in the velocity, within successive short intervals.
Acknowledgment The authors are indebted to H. C. Jackson of the Department of Dairy Industry and L. R. Ingersol of the Department of Physics of the University of Wisconsin, and the Wisconsin Alumni Research Foundation, for their considerate assistance and aid in providing facilities and apparatus for this work. Literature Cited (1) Anderson, W.T., and Larson, C. J., unpublished studies, Hanovia Chemical and Manufacturing Co., Newark, N. J., 1934. (2) Duff, A. W., "Text Book of Physics," p. 557, Philadelphia, P. Blakiston's Son & Co., 1932. (3) Supplee, G. C., and Dorcas, M. J., J. Dairy Sci., 17,433 (1934). RECEIWDApril 2, 1936.
Preparation of Diphenylamine Indicator Solution HAROLDM. STATE Frick Chemical Laboratory, Princeton, N. J.
T
HE usual preparation of diphenylamine indicator by shaking the amine with concentrated sulfuric acid is slow and time-consuming. If the amine is first melted to a clear liquid (m. p. = 52.9" C.) and the required amount of concentrated sulfuric acid then added, complete solution may be effected by 15 to 30 seconds' shaking. RECEIVDD May 7, 1936
