Reactive Surfaces Ltd. LLP
`Ex. 1039 (Ray Attachment E)
`Reactive Surfaces Ltd. LLP v. Toyota Motor Corp.
`IPR2016-01914
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`Journal of Physics D: Applied Physics
`
`Some observations on fingerprint deposits
`
`To cite this article: G L Thomas and T E Reynoldson 1975 J. Phys. D: Appl. Phys. 8 724
`
`View the article online for updates and enhancements.
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`J. Phys. D: Appl. Phys , Vol. 8, 1975. Printed in Great Britain. 0 1975
`
`Some observations on fingerprint deposits
`
`G L Thomas and T E Reynoldson
`Police Scientific Development Branch, Home Office, London, SWlP 2AW
`
`Received 11 December 1974, in final form 7 February 1975
`
`Abstract. Measurements of refractive indices and droplet profiles of fingerprint deposits
`on flat solid surfaces have been made using transmitted light interference microscopy.
`Changes in the profiles with time and relative humidity have been determined. From
`these measurements inferences are made on the structure of the droplets. The absence
`of significant variation in contact angle for fingerprints on substrates of different surface
`energy is explained in terms of a contaminant film over the area of finger ridge contact.
`The thickness of this film corresponds closely to that expected for a monolayer of
`contamination.
`
`1. Introduction
`Little is known about the physical nature of fingerprints and few physical techniques for
`developing fingerprints have emerged since the widely used powdering method (Moenssens
`1971). The major aim of this work is to obtain a better understanding of fingerprints
`and how they differ from their surroundings. Results on the electrical properties of
`fingerprints have been reported elsewhere (Scruton and Blott 1973, Thomas 1975) and
`progress in this field has been recently reviewed (Thomas 1973).
`Fingerprints on reflecting surfaces can often be seen by the naked eye. To understand
`their interaction with the substrates on which they are placed magnification is necessary.
`In general any substance which can be transferred tactually by the papillary ridges can
`form a fingerprint, The primary component of fingerprints is eccrine sweat from the
`glands on the finger ridges. This sweat is usually accompanied by sweat that originates
`from finger contact with other parts of the body. Sebum, the secretion of the sebaceous
`glands, is the other major constituent, and aprocrine sweat is also present but to a lesser
`degree. Pure eccrine sweat is an aqueous solution of inorganic and organic (non-lipid)
`compounds (Kuno 1956), whereas sebum is mainly composed of lipids (Heinz and van
`der Velden van der Ende 1973).
`Because fingerprints are transparent, use is made of a dark ground, phase contrast
`or interference technique. Quantitative interference microscopy has been used to deter-
`mine a refractive index distribution of finger deposits, the thickness variation, and also
`distributions of contact angles of fingerprints on high and low energy surfaces. The
`existence of a thin film of contamination that covers the whole area of ridge contact
`accounts for our observations.
`
`2. Preliminary observations
`Interference microscopy shows that (U) the thickness of the deposit varies widely within
`the region contaminated by the finger (figure 1, plate); (b) that the ridges are delineated by
`
`724
`
`Reactive Surfaces Ltd. LLP
`Ex. 1039 (Ray Attachment E)
`Reactive Surfaces Ltd. LLP v. Toyota Motor Corp.
`IPR2016-01914
`
`

`

`Some observations on fingerprint deposits
`
`725
`
`independent islands of material (Bridges 1942) ranging in diameter from about 1-50 pm;
`(c) that the maximum thickness of these structures ranges from that corresponding to
`the minimum phase change detectable, about 10 nm, to approximately 2 pm; (a) that
`there is a wide range of angle of contact between the finger deposit and the substrate,
`as can be seen from the variation in width of the interference fringes near the perimeter
`of the droplets.
`Usually the region between these deposits appears uncontaminated but occasionally
`a continuous connecting film is apparent. An example of this is shown in figure 2 (plate)
`in which the continuous layer is approximately 30 nm thick. The thickness of this film may
`be increased to some 5-10 pm by deliberately coating the fingers with excessive sebum.
`Sometimes sodium or potassium chloride crystals are observed in finger deposits.
`These appear soon after the deposition of the prints when the water has evaporated.
`
`3. Refractive index measurements
`As can be seen from figure 1 the distance between the droplets is often smaller than their
`diameters. For this reason we chose to use the interphakot method (Beyer 1967), rather
`than the more conventional shearing technique, for measuring phase changes.
`Refractive indices of the droplets have been measured by comparing phase shifts in
`air with those obtained using an aqueous solution of barium mercuric iodide as the
`embedding medium. The sensitive purple was used as a colour index so the measured
`values refer to A=550 nm. The distribution of refractive index values is shown in
`histogram form in figure 3. The experimental error is k 6 x
`(sD).
`
`Refractive index
`Figure 3. Refractive index distribution of fingerprint deposits of less than a day old
`for A= 551 nm.
`
`The refractive indices of most of the long chain fatty material found in finger deposits
`fall within our measured distribution. It should be noted that the refractive index of
`water lies well outside this range, indicating that water is not present in finger deposits
`to the extent that it is in sweat collected from the fingers after encouraging excessive
`perspiration (Kuno 1956).
`
`4. Droplet profiles
`4.1. Changes with time and relative humidity
`We have studied the microscopic changes that occur in finger deposits with time and
`relative humidity. Fingerprints were stored in various environments for up to 3 months
`t The interference measurements were carried out using a Carl Zeiss (Jena) Amplival Interphako micro-
`scope.
`
`

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`726
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`G L Thomas and T E Reynoldson
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`Figure 4. A cross section across a typical droplet of finger deposit and its temporal
`variation.
`
`(see captions to figure 5). We noted that as aging proceeded the structures became quite
`irregular in topography and this was accompanied by an increase in viscosity of the
`deposit. A constant refractive index of 1.47 was used to calculate the thickness from the
`observed phase shifts and the variation of a typical profile is shown in figure 4. In figure 5
`we show the variation of the mean of the maximum height of the droplets as a function
`of time and storage environment.
`Presumably the large initial changes in thickness were due to evaporation of the
`more volatile constituents. The inhomogeneity of chemical composition of the droplets
`was apparent from the variation of drying rate within the droplets themselves (see for
`example figure 4). The lack of systematic dependence of drying rate with relative humid-
`ity indicates that the droplets have a low water content, at least near their surface. This
`is consistent with their measured refractive indices (see $3).
`
`4.2. Contact angles
`Our droplet profiles automatically give contact angles. We have extended these measure-
`ments to the observation of finger deposits on silicone polished glass (fingerprints are
`difficult to develop on silicone polished surfaces), Perspex and cellulose acetate. The
`
`

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`Some observations on fingerprint deposits
`
`727
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`distributions of measured contact angles are shown in figure 6 for the surfaces that we
`have examined. The error in the determination of 8, is +_ 10% (sD). This includes the
`error due to the location of the edge of the droplet (& 0.5 pm). It is apparent that the
`distribution of contact angles is more or less independent of the surface energy of the
`surface on to which the fingerprints have been placed. We noted that for about a third
`of the measurements the contact angles were zero and a definite point of inflexion in the
`droplet profile could be seen (see for example figure 4). Again this was independent of
`the surface energy. No significant time dependence in contact angles was observed over
`a 7 day period.
`
`14%
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`6%
`Lob
`36'h
`
`95%
`
`0
`0. I
`
`I
`
`100
`
`1
`1000
`
`IO
`Days
`Figure 5. The variation of the mean of the maximum thickness of fingerprint droplets
`as a function of time and storage environment. One fingerprint on a clean microscope
`slide from each of four donors was stored in each of 6 environments for 3 months.
`The environments were: an open laboratory ; 4 constant relative humidity cabinets
`(14%. 36%, 65% and 95% RH); outdoors with protection from direct rainfall in central
`London. Measurements were made on three droplets from each fingerprint.
`
`The close similarity of contact angle distribution for the surfaces examined indicates
`that the contact angles are a characteristic, not of the clean surface on which the deposit
`has been placed, but of a layer of contamination produced by the retracting liquid as the
`droplets are formed. It is of course well known that oriented monolayers can be formed
`by retraction (Chapman and Tabor 1957). An area of liquid contact which is much larger
`than the area covered by the droplets has recently been observed (Scruton et a1 1975).
`Since the droplets are presumably formed after the rupture of a liquid film the contact
`angles we have measured are largely receding angles. We might expect hysteresis in the
`contact angles due to the presence of the inferred contamination layer (Fowkes and
`Harkins 1940, Dettre and Johnson 1965). The observation of a concave meniscus
`indicates the presence of solid matter in the finger deposit, or at least material that be-
`comes solid as the temperature of the deposit reaches that of the substrate. Some solid
`
`

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`728
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`G L Thomas and T E Reynoldson
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`20
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`Polished glass
`-a
`30
`35
`25
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`20;
`10-
`0
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`Cellulose acetate
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`5
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`IO
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`15
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`20
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`25
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`30 -i5
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`Perspex
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`0 5
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`IO
`20
`25
`15
`Contact angle
`(deg.)
`Figure 6. Contact angle distributions for fingerprint deposits on various surfaces.
`
`30
`
`35
`
`material is clearly visible in figures 1 and 2. Our contact-angle measurements are in sub-
`stantial agreement with those reported in the accompanying paper (Scruton et a1 1975).
`
`5. Further observations on the contamination layer
`From our contact-angle measurements we are able to infer that when a finger touches a
`clean solid surface a layer of contamination is produced over the whole area of finger
`contact. In this section we make some attempt to establish the composition and thickness
`of this film using metal evaporation under vacuum. Our preliminary findings have been
`reported elsewhere (Thomas 1974). Confirmation of this effect has been provided by
`scanning electron microscopy (Scruton et a1 1975).
`It is well known that fingerprints inhibit the condensation of metal films under
`vacuum, and some detailed studies have been made of this phenomenon (Hambley 1972,
`Kent and Thomas in preparation). The inhibition may be observed by first condensing
`a thin film of gold (about 0.2 nm) on a fingerprinted surface and subsequently evaporating
`cadmium. The cadmium condenses on the gold in the spaces between the ridges but the
`whole area of finger contact, including the region between the droplets, inhibits conden-
`sation (figure 7, plate).
`It has been found that the amount of fatty material required to inhibit condensation
`of the cadmium is less than the minimum resolvable phase difference (10 nm) on our
`interference microscope. Thin films of fatty acids can easily be prepared by vacuum
`deposition (Baker 1971). We have used this method to produce films of stearic, palmitic
`
`

`

`Some observations on fingerprint deposits
`
`729
`
`and oleic acid on clean microscope slides. The thickness of these films was monitored
`using a quartz crystal monitor. Gold, 0.2 nm thick, was evaporated on to these films,
`followed by 4 nm of cadmium. We found that 3 nm of fatty acid, which corresponds
`closely to the thickness of an oriented monolayer, was sufficient to cause total inhibition
`of the cadmium. Inhibition could also be produced by similar thicknesses of other
`lipid components of fingerprints. We presume that the same inhibition mechanism is
`responsible for preventing cadmium condensation on the fingerprints and on the fatty
`acid films.
`When a ‘natural’ fingerprint was treated by AujCd deposition the ridge area was
`often quite patchy, indicating that the superficial density of deposited cadmium varied
`to some extent within the ridge area. In order to study this further, fingerprints with a
`high concentration of eccrine sweat were produced in the following way. A subject’s
`hand was washed in acetone and he then wore a polythene bag over his hand to encourage
`eccrine sweat production. Fingerprints were then deposited before the subject touched
`any other part of his body. Sebum rich fingerprints were then produced by touching of
`the side of the nose with an acetone cleaned finger before deposition. Both eccrine sweat
`rich and sebum rich marks were developed by Au/Cd deposition (see figure 7). It was clear
`the eccrine sweat rich deposits were significantly less efficient at inhibiting cadmium
`condensation. We believe that this is simply due to the differences in chemical composition
`of eccrine sweat and sebum. The lipid constituents of sebum do not occur in significant
`concentrations in eccrine sweat. We conclude then that the composition of the adsorbed
`monolayer of contamination depends on the relative amounts of sebum and eccrine
`sweat on the fingers. The surface energy of the adsorbed layer will thus vary within the
`film, and hence a large range of contact angles (84.2) is to be expected.
`
`Acknowledgments
`
`The authors wish to acknowledge Mr G Phillips, Director of the Police Scientific Develop-
`ment Branch of the Home Office for his interest in this work and for permission to
`publish this paper. They also thank Mr T Kent and Dr A M Knowles of the Home
`Office and Drs B Blott, B Scruton and B Robbins of Southampton University for many
`useful discussions.
`
`References
`
`Baker M A 1971 Thin Solid Films 8 R13-5
`Beyer H 1967 J. R. Microscop. Soc. 87 171-5
`Bridges B C 1942 Practical Engineering (New York: Funk and Wagnalls)
`Chapman JA and Tabor D 1957 Proc. R. Soc. A 242 96-107
`Dettre R H and Johnson R E 1965 J. Chem. Phys. 69 1507-14
`Fowkes F M and Harkins W D 1940 J. Am. Chem. Soc. 62 3377-86
`Hambley D S 1972 PhD Thesis, University of London
`Heinz K L and van der Velden van der Ende 1913 Cosmetics and Perfumery 88 41-6
`Kuno Y 1956 Human Perspiration (Springfield, Ill. : Thomas)
`Moenssens A A 1971 Fingerprint Techniques (New York: Chilton)
`Scruton B and Blott B H 1913 J. Phys. E. Sei. Instrum. 6 472-4
`Scruton B, Robbins B and Blott BH 1975 J. Phys. D: Appl. Phys. 8 714-23
`Thomas G L 1973 Criminologist 8 21-38
`- 1974 Thin Solid Films 24 S52-4
`- 1975 J. Forens. Sci. Soc. 15 accepted for publication
`
`

`

`J. Phys. D: Appl. Phys., Vol. 8, 1 9 7 5 - 4 L Tlroriios orid T E Reynok/.sorz (see pp 724-9)
`
`Figure 1. Interference micrograph (interphako) of part of a fingerprint ridge (magnifica-
`tion x -150,reductionby x+). A=551 nm.
`
`

`

`J. Phys. D: Appl. Phys., Vol. 8, 1975-G L Tllo~ims ord T E Reytiolrlsott (see pp 724-9)
`
`Figure 2. Interference micrograph (shearing) of edge of a fingerprint ridge using white
`light fringes. A continuous film of contamination about 30 nm thick is visible (light
`band adjacent to upper edge of the ridge) as well as a number of droplets around solid
`
`centres (magnification x - 500, reduction by x ?).
`
`

`

`J. Phys. D: Appl. Phys., Vol. 8, 1975-G L T/iot~ms orid T E Rcyriolrlsori (see pp 724-9)
`
`Figure 7. Fingcrprint ridges on glass bcl'orc (Iclt) and alter (right) being developed by
`Au/Cd deposition (magnification x -30, reduction by x +). Upper, natural fingerprint;
`middle, eccrine sweat rich fingerprint; lower, sebum-rich fingerprint.
`
`