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Surface deformation of thin coatings caused by evaporative convection
Surface Deformation of Thin Coatings Caused by Evaporative Convection I. Macroscopic Surface Replication J. N. ANAND AND R. Z. BALWINSKI Plastics Fundamental Research Laboratory, The Dow Chemical Company, Midland, Michigan ~86~0
Received Febuary 27, 1969 Study and replication of microscopically rough surfaces is important for mathematical and experimental analyses of the autohesion mechanism. Saran-coated surfaces have a polygonal cellular texture for which interference microscopy is most suited. A method for surface reproduction by line scanning the labelled fringe topograph has been developed. Assembly of the profiles obtained at consecutive crosssections yields a life size replica.
INTRODUCTION A theory of autohesion comprising contact establishment as the first and bond formation as the second stage has been developed (1). This theory shows that the rate of contact is determined by the shape of surfaces at the interface. A mathematical description of surfaces is therefore, needed for analyzing the mechanism of autohesion. Besides, a macroscopic reproduction of rough surfaces facilitates experiment on a large scale for better understanding of the phenomena occurring on a microscopic scale. The simplest type of surfaces that can be analyzed mathematically and replicated consists of repeated simple shapes such as segments of spheres, cylinders, solid or hollow, etc. Such surfaces are usually termed as "cellular". An example of such a surface is provided by the texture developed by a hotcast Saran coating. The polygonal cellular texture is caused by unbalanced surface tension forces created by nonuniform cooling of the film and evaporation of the solvent at the surface. A study of this mechanism forms the subject matter of the next two papers (2, 3). Journal of Colloid and Interface Science,
Interference microscopy is the most suited technique for these surfaces (4). It yields topographs where all points at the same level are marked by a fringe. The fringe order determines the level of points, and the wavelength of light determines the level difference between consecutive fringes. The profile of a cross section is obtained by scanning a line across the micrograph. The fringes are labelled in order. The intersection of line with the fringe determines coordinates of the points in the plane of paper while the levels are obtained from the fringe order. The plots are put on a substrate, the profiles cut out, and the surface reproduced by stacking these together in proper order. The mismatching is evened out by a filler material. MICROSCOPIC
STUDIES
Coatings were made on microscope slides from a 20% by weight solution of Saran. The solvent consisted of a mixture of 65 % methyl ethyl ketone (iViEK) and 35% toluene. The solution temperature at casting was 72°C and after casting the samples were heated in a 120°C oven to drive off the
Vol. 31, No. 2, October 1969 196
FiG. 1. Diffuse light transmission photomicrographs of the surface of Saran coatings showing the regular cellular s t r u c t u r e : (a) Coati~g thickness = 1 rail ~ 25.4 ~; (b) coating thickness = 20 mil 508 p.
,/\, B
A
~A--.~
C
!
~
Jh------
PARTIALLYSILVERED
MIRROR OR A COMBINATION OF PRISIMS.
SOURCE -'-V~ i -'V----GREENFILTERED APERTURE
MERCURYVAPOR LAMP.
I
J
SILVERED SIDES FACING
EACHOTHER
FIG. 2. Schematic drawing of the set-up for interference microscopy used.
FIG. 3. Interference microscope topograph of the surface of thin coating taken with monochromatic green light, wavelength = 5500 A. 198
SURFACE DEFORMATION CAUSED BY EVAPORATIVE CONVECTION. I
199
FIG. 4 Interference microscope topograph of the surface of thick coating taken with monochromatic green light, wavelength = 5500 A. excess solvent. The cellular structure develops in a short time and does not change with heat treatment. Figure 1 shows oblique diffuse light transmission micrographs of thin- and thickcoated samples. A detailed mechanism of the surface texture formation and the factors determining the cell size and shape are
discussed in next two papers. These figures are very illusive since depending upon the angle of observation of the viewer the cells appear as hills (convex upwards) or valleys (concave upwards). Individuals usually see both structures which alternate with time. T h e concavity of these cells is established
Journal of Colloid and Interface Science, Vol. 31, No. 2, October 1969
200
ANAND AND BALWINSKI
later in this section by means of interference microscopy. A reflecting-type of microscope was employed to produce interference patterns. Figure 2 shows a schematic of the arrangement. Monochromatic light is essential for producing sharp fringes. A mercury vapor lamp in conjunction with a green light filter gave light of wavelength, ;~ = 5500 A. The surface of the sample and a thin optical flat were coated simultaneously by
A '~
--~B
A
evaporting aluminum in a vacuum chamber to give the same reflecting properties to both. This results in high contra~t fringes. The thickness of coating was such that the optical fiat was still partially transmitting. F[onochromatic light from the source is reflected from terM-internal-reflecting split prisms and falls on a combination of the optical flat glass and the surface. The latter are put together such that their reflecting sides face each other with the Saran surface facing the light. Thus, the light is partially reflected and transmitted by the optical flat. The transmitted portion is then reflected back from the aluminum-coated Saran surface and interferes with the light t h a t was originally reflected from the optical flat. The condition for interference is d = nX/2
1
t
'tl,
"
t '
FIG, 5. Schematic drawing for comparison of ~fringes between a point source and an extended ;source for which the fringes broaden in the direction of increasing depth.
where d is the depth or half the path difference and n is an integer. For n odd or even the fringes are dark or light, respectively. Thus, the path difference between two consecutive fringes is X. Figures 3 and 4 are interference topegraphs of the two surfaces. The surface of the thick-coated film is such that its inter-
20
I0
Z O a:O
f
J
_
¢2--
-I0
20 -~O
L 50
IOO
150
200 250 MICRON
300
350
400
450
FIG. 6. Profile of the cross-section at Line XX in Fig. 4, plotted by the digital computer. Journal of Colloid and Interface Science, Vol. 31, No. 2, October 1969
SURFACE DEFORMATION CAUSED BY EVAPORATIVE CONVECTION. I ference topograph lends itself better to macroscopic replication. Theoretically, sharp fringes will be produeed if the light source is point and the lens used is perfect. However, deviation from these do always exist. For extended sources and lens imperfections, angles of incidence vary from the normal over a range. This produces broadening of the fringes and is illustrated schematically in Fig. 5. A de-
FIG. 7. Schematic of the surface reproduction from cross-sectional profiles.
201
tailed discussion of this effect by Brosswell is given in Ref. (4). From broadening of the fringes in Fig. 4, it may be concluded that the cells are deeper at the center and hence, are concave upwards. PROFILE GENERATION Starting from a zero-order fringe as an arbitrary reference point, all the fringes are labelled sequentially. The label numbers increase as the level increases and vice versa. Parallel scanning lines are drawn across the topograph at regular intervals. The ~ i d t h of the interval is determined by the nature of the surface and precision required. One such line is dra~m as X X in Fig. 4. The profile of the cross-section at any line may be obtained by plotting the levels of fringes vs. the coordinates of their points of intersection with the above line. The levels are obtained, as indicated earlier, by multiplying the fringe order by half the wavelength o which in the present case is 5500/2 = 2750 A. Figure 6 is profile of the cross-section at line X X in Fig. 4. The plots are put on a cardboard substrafe whose thickness is equal to the width
FIG. 8. Photograph of the final replica of the surface of Fig. 4. Journal of Colloid and Interface Science, VoL 31, N o . 2, O c t o b e r 1969
202
ANAND AND BALWINSKI
of the interval of scanning. The profiles are then cut out. B y stacking these cut-out substrates together, as shown in Fig. 7 a macroscopic replica is produced. The mismatching between adjacent profiles is evened out b y a filler material. Figure 8 is a photograph of the actual replica. Reading of the coordinates of the points of intersection of the scanning line with the fringes and determining the fringe order is a very tedious task to do manually. Use has been made of the hybrid analog-digital computer to simplify this task. The same arrangement is employed to obtain the output in the form of a paper tape which in turn is translated to another tape that dlives an automatic three-dimensional milling machine. This enables generation of the surface in an automated manner and hence facilitates the operation considerably.
CONCLUSIONS Interference microscopy can be successfully employed for study and replication of microscopic surfaces. Its resolution is of the order of 50-100 A. However, there are regions where the fringes broaden so much that it is not possible to resolve them. o
This is a good technique to establish if the cells are concave or convex. This is sometimes hard to do by using ordinary microscopes, by using diffuse light or b y shadowing by vapor depositing at an angle. The surface may be cut in a metal by a milling machine for further replication of a polymer surface in a compression molding machine. Thus, this affords a good and accurate method of surface generation which is necessary for autohesive experiments on a macroscopic scale. The average cell size is found to be I00 t~ for the thin coating and 400 # for the thick coating. The corresponding cell depths are i.I and 8.25 ~, respectively. REFERENCES 1. ANAND, J. N. AND K•RAM, H. J., J. Adhesion 1, 16 (1969). 2. ANAND, J. N., J. Colloid Interface Sci. 43rd National Colloid Symposium (Preprints) 115121 (1969). 3. ANAND, J. N. AND KARAM, H. J., J. Colloid Interface Sci. 43rd National Colloid Symposlum (Preprints) 127-134 (1969). 4. TO~ANSKI, S., "An Introduction to Inter* ferometry," Longmans, London, 1955.
Journa~ of Colloid and Interface Science, Vol. 31, No. 2, October 1969
Received Febuary 27, 1969 Study and replication of microscopically rough surfaces is important for mathematical and experimental analyses of the autohesion mechanism. Saran-coated surfaces have a polygonal cellular texture for which interference microscopy is most suited. A method for surface reproduction by line scanning the labelled fringe topograph has been developed. Assembly of the profiles obtained at consecutive crosssections yields a life size replica.
INTRODUCTION A theory of autohesion comprising contact establishment as the first and bond formation as the second stage has been developed (1). This theory shows that the rate of contact is determined by the shape of surfaces at the interface. A mathematical description of surfaces is therefore, needed for analyzing the mechanism of autohesion. Besides, a macroscopic reproduction of rough surfaces facilitates experiment on a large scale for better understanding of the phenomena occurring on a microscopic scale. The simplest type of surfaces that can be analyzed mathematically and replicated consists of repeated simple shapes such as segments of spheres, cylinders, solid or hollow, etc. Such surfaces are usually termed as "cellular". An example of such a surface is provided by the texture developed by a hotcast Saran coating. The polygonal cellular texture is caused by unbalanced surface tension forces created by nonuniform cooling of the film and evaporation of the solvent at the surface. A study of this mechanism forms the subject matter of the next two papers (2, 3). Journal of Colloid and Interface Science,
Interference microscopy is the most suited technique for these surfaces (4). It yields topographs where all points at the same level are marked by a fringe. The fringe order determines the level of points, and the wavelength of light determines the level difference between consecutive fringes. The profile of a cross section is obtained by scanning a line across the micrograph. The fringes are labelled in order. The intersection of line with the fringe determines coordinates of the points in the plane of paper while the levels are obtained from the fringe order. The plots are put on a substrate, the profiles cut out, and the surface reproduced by stacking these together in proper order. The mismatching is evened out by a filler material. MICROSCOPIC
STUDIES
Coatings were made on microscope slides from a 20% by weight solution of Saran. The solvent consisted of a mixture of 65 % methyl ethyl ketone (iViEK) and 35% toluene. The solution temperature at casting was 72°C and after casting the samples were heated in a 120°C oven to drive off the
Vol. 31, No. 2, October 1969 196
FiG. 1. Diffuse light transmission photomicrographs of the surface of Saran coatings showing the regular cellular s t r u c t u r e : (a) Coati~g thickness = 1 rail ~ 25.4 ~; (b) coating thickness = 20 mil 508 p.
,/\, B
A
~A--.~
C
!
~
Jh------
PARTIALLYSILVERED
MIRROR OR A COMBINATION OF PRISIMS.
SOURCE -'-V~ i -'V----GREENFILTERED APERTURE
MERCURYVAPOR LAMP.
I
J
SILVERED SIDES FACING
EACHOTHER
FIG. 2. Schematic drawing of the set-up for interference microscopy used.
FIG. 3. Interference microscope topograph of the surface of thin coating taken with monochromatic green light, wavelength = 5500 A. 198
SURFACE DEFORMATION CAUSED BY EVAPORATIVE CONVECTION. I
199
FIG. 4 Interference microscope topograph of the surface of thick coating taken with monochromatic green light, wavelength = 5500 A. excess solvent. The cellular structure develops in a short time and does not change with heat treatment. Figure 1 shows oblique diffuse light transmission micrographs of thin- and thickcoated samples. A detailed mechanism of the surface texture formation and the factors determining the cell size and shape are
discussed in next two papers. These figures are very illusive since depending upon the angle of observation of the viewer the cells appear as hills (convex upwards) or valleys (concave upwards). Individuals usually see both structures which alternate with time. T h e concavity of these cells is established
Journal of Colloid and Interface Science, Vol. 31, No. 2, October 1969
200
ANAND AND BALWINSKI
later in this section by means of interference microscopy. A reflecting-type of microscope was employed to produce interference patterns. Figure 2 shows a schematic of the arrangement. Monochromatic light is essential for producing sharp fringes. A mercury vapor lamp in conjunction with a green light filter gave light of wavelength, ;~ = 5500 A. The surface of the sample and a thin optical flat were coated simultaneously by
A '~
--~B
A
evaporting aluminum in a vacuum chamber to give the same reflecting properties to both. This results in high contra~t fringes. The thickness of coating was such that the optical fiat was still partially transmitting. F[onochromatic light from the source is reflected from terM-internal-reflecting split prisms and falls on a combination of the optical flat glass and the surface. The latter are put together such that their reflecting sides face each other with the Saran surface facing the light. Thus, the light is partially reflected and transmitted by the optical flat. The transmitted portion is then reflected back from the aluminum-coated Saran surface and interferes with the light t h a t was originally reflected from the optical flat. The condition for interference is d = nX/2
1
t
'tl,
"
t '
FIG, 5. Schematic drawing for comparison of ~fringes between a point source and an extended ;source for which the fringes broaden in the direction of increasing depth.
where d is the depth or half the path difference and n is an integer. For n odd or even the fringes are dark or light, respectively. Thus, the path difference between two consecutive fringes is X. Figures 3 and 4 are interference topegraphs of the two surfaces. The surface of the thick-coated film is such that its inter-
20
I0
Z O a:O
f
J
_
¢2--
-I0
20 -~O
L 50
IOO
150
200 250 MICRON
300
350
400
450
FIG. 6. Profile of the cross-section at Line XX in Fig. 4, plotted by the digital computer. Journal of Colloid and Interface Science, Vol. 31, No. 2, October 1969
SURFACE DEFORMATION CAUSED BY EVAPORATIVE CONVECTION. I ference topograph lends itself better to macroscopic replication. Theoretically, sharp fringes will be produeed if the light source is point and the lens used is perfect. However, deviation from these do always exist. For extended sources and lens imperfections, angles of incidence vary from the normal over a range. This produces broadening of the fringes and is illustrated schematically in Fig. 5. A de-
FIG. 7. Schematic of the surface reproduction from cross-sectional profiles.
201
tailed discussion of this effect by Brosswell is given in Ref. (4). From broadening of the fringes in Fig. 4, it may be concluded that the cells are deeper at the center and hence, are concave upwards. PROFILE GENERATION Starting from a zero-order fringe as an arbitrary reference point, all the fringes are labelled sequentially. The label numbers increase as the level increases and vice versa. Parallel scanning lines are drawn across the topograph at regular intervals. The ~ i d t h of the interval is determined by the nature of the surface and precision required. One such line is dra~m as X X in Fig. 4. The profile of the cross-section at any line may be obtained by plotting the levels of fringes vs. the coordinates of their points of intersection with the above line. The levels are obtained, as indicated earlier, by multiplying the fringe order by half the wavelength o which in the present case is 5500/2 = 2750 A. Figure 6 is profile of the cross-section at line X X in Fig. 4. The plots are put on a cardboard substrafe whose thickness is equal to the width
FIG. 8. Photograph of the final replica of the surface of Fig. 4. Journal of Colloid and Interface Science, VoL 31, N o . 2, O c t o b e r 1969
202
ANAND AND BALWINSKI
of the interval of scanning. The profiles are then cut out. B y stacking these cut-out substrates together, as shown in Fig. 7 a macroscopic replica is produced. The mismatching between adjacent profiles is evened out b y a filler material. Figure 8 is a photograph of the actual replica. Reading of the coordinates of the points of intersection of the scanning line with the fringes and determining the fringe order is a very tedious task to do manually. Use has been made of the hybrid analog-digital computer to simplify this task. The same arrangement is employed to obtain the output in the form of a paper tape which in turn is translated to another tape that dlives an automatic three-dimensional milling machine. This enables generation of the surface in an automated manner and hence facilitates the operation considerably.
CONCLUSIONS Interference microscopy can be successfully employed for study and replication of microscopic surfaces. Its resolution is of the order of 50-100 A. However, there are regions where the fringes broaden so much that it is not possible to resolve them. o
This is a good technique to establish if the cells are concave or convex. This is sometimes hard to do by using ordinary microscopes, by using diffuse light or b y shadowing by vapor depositing at an angle. The surface may be cut in a metal by a milling machine for further replication of a polymer surface in a compression molding machine. Thus, this affords a good and accurate method of surface generation which is necessary for autohesive experiments on a macroscopic scale. The average cell size is found to be I00 t~ for the thin coating and 400 # for the thick coating. The corresponding cell depths are i.I and 8.25 ~, respectively. REFERENCES 1. ANAND, J. N. AND K•RAM, H. J., J. Adhesion 1, 16 (1969). 2. ANAND, J. N., J. Colloid Interface Sci. 43rd National Colloid Symposium (Preprints) 115121 (1969). 3. ANAND, J. N. AND KARAM, H. J., J. Colloid Interface Sci. 43rd National Colloid Symposlum (Preprints) 127-134 (1969). 4. TO~ANSKI, S., "An Introduction to Inter* ferometry," Longmans, London, 1955.
Journa~ of Colloid and Interface Science, Vol. 31, No. 2, October 1969