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Dynamics and mechanism of the physical developer process for visualization of latent fingerprints on paper

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View the page of Jodie Coulston

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Jodie Coulston provides a detailed mechanistic study of the PD process, focused on the nucleation and growth dynamics of silver particles on fingermarks deposited on a paper surface, from macroscopic (whole fingermark) and microscopic (particle level) perspectives.

Image courtesy of interviewee. January 17, 2024

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This video video discusses the process and chemistry behind physical development (PD), a widely used technique for visualizing latent fingerprint marks on porous surfaces, particularly paper. PD relies on a redox reaction to reveal finger marks that are otherwise invisible to the naked eye. The technique is recognized for its effectiveness but has been hindered by limited research into its underlying chemistry and the recent ban of a key surfactant (Sym N) due to environmental concerns.

The study aims to explore the surface chemistry and dynamics of silver growth in the PD process. It describes the steps involved in developing fingerprints, including pre-washing with maleic acid to neutralize alkaline fillers in paper, followed by immersion in a working solution consisting of surfactants and silver nitrate. The resulting silver deposits are examined both macroscopically and microscopically.

Key findings include that sebaceous (water-insoluble) and water-soluble components from finger marks are both essential for optimal development of prints. It was observed that the particle sizes of silver deposits in the working solution are about 880 nanometers on average, and the process allows rapid, selective deposition of silver on latent marks.

Importantly, the study concluded that silver particles grow primarily on the surface rather than in solution, indicating that the nucleation for silver deposition is instantaneous. Compositional analysis confirmed that the developed marks are solely silver, without iron contamination, which plays a role only in the initial reaction during the formation of the working solution.

The research opens avenues for further studies on how different factors, such as paper type and donor variability, might affect the PD process, highlighting the complexity of fingerprint residue and the need for deeper understanding in forensic applications.

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X-TIMESTAMP-MAP:LOCAL=00:00:00.000,MPEGTS=900000

00:00.455 --> 00:05.095
Physical developer is based on a redox reaction.

00:05.635 --> 00:10.055
It is essentially a finger mark development method,

00:10.785 --> 00:11.855
which allows us

00:11.955 --> 00:16.255
to visualize latent finger marks on paper

00:16.855 --> 00:20.215
surfaces, latent finger marks, basically meaning

00:20.215 --> 00:22.895
that they are not visible to the naked eye,

00:23.405 --> 00:26.935
therefore we have to visualize them first in order

00:26.995 --> 00:29.575
to use them for identification purposes.

00:30.155 --> 00:32.415
As we know, physical developer is used

00:32.415 --> 00:35.575
to develop latent finger marks on porous surfaces,

00:35.595 --> 00:39.175
and these will predominantly be paper surfaces.

00:40.165 --> 00:43.775
This could include any paper surfaces such

00:43.835 --> 00:46.815
as financial documentation, currency,

00:47.355 --> 00:49.655
or any other correspondence.

00:50.765 --> 00:53.335
It's a widely used technique,

00:53.675 --> 00:56.055
not only in the UK but globally.

00:56.845 --> 01:00.895
However, there is surprisingly little research into the

01:00.905 --> 01:02.615
underlying chemistry of the process.

01:03.965 --> 01:08.185
We know that it is a very effective technique,

01:08.365 --> 01:11.865
but the operational success can vary in terms

01:11.865 --> 01:14.345
of producing high quality finger marks.

01:15.155 --> 01:18.965
This could be attributed to the practical demands

01:18.965 --> 01:22.365
of the process because it is quite labor intensive.

01:23.315 --> 01:26.045
Coupled with that limited understanding of

01:26.045 --> 01:30.405
that underlying chemistry, it is a chemical process

01:31.065 --> 01:36.045
and one of the crucial reagents, a surfactant named Sym N

01:36.745 --> 01:40.205
was banned due to its environmental toxicity.

01:41.265 --> 01:43.285
In order to replace that surfactant,

01:43.425 --> 01:46.285
the underlying chemistry has to first be understood.

01:47.345 --> 01:49.605
In addition to this, currently in the uk,

01:49.915 --> 01:52.125
it's the only technique recommended

01:52.665 --> 01:56.085
on those porous surfaces which may have previously

01:56.155 --> 01:57.245
been wetted.

01:58.065 --> 02:00.475
Without a suitable surfactant replacement,

02:00.585 --> 02:02.595
practitioners could lose that ability

02:02.895 --> 02:06.035
to develop any latent marks of wetted paper

02:06.065 --> 02:07.755
that comes in as evidence.

02:08.785 --> 02:12.315
Alternative methods have been studied, which includes oil,

02:12.495 --> 02:14.195
red, O and Nile red,

02:14.575 --> 02:17.995
but these haven't been shown to actually outperform PD

02:18.535 --> 02:21.195
and the effectiveness actually decreases with time.

02:21.745 --> 02:25.355
Whereas research has shown that PD is still effective

02:25.355 --> 02:28.115
for Marx, which have been aged for up to 90 years.

02:29.295 --> 02:31.515
So it's a combination of all these factors

02:31.625 --> 02:35.275
that have motivated this study, which has been addressed

02:35.415 --> 02:40.355
by exploring the solution to surface chemistry dynamics

02:40.355 --> 02:41.435
of the silver growth

02:42.095 --> 02:45.195
and compositional analysis of those developed marks.

02:45.995 --> 02:49.675
Physical developer is based on a redox solution,

02:50.175 --> 02:54.115
and in that working solution you have a fine balance

02:54.935 --> 02:58.635
of Ferris ions, which reduce silver ions

02:58.855 --> 03:00.205
to elemental silver,

03:01.025 --> 03:04.725
and in turn, those ferrous ions are oxidized.

03:05.675 --> 03:08.455
The elemental silver itself is highly

03:09.055 --> 03:10.575
unstable in the solution,

03:10.995 --> 03:13.175
and if you don't have that chemical control,

03:13.725 --> 03:15.175
they would just aggregate,

03:15.305 --> 03:18.335
which would form much larger silver particles,

03:18.665 --> 03:20.895
which would just crash out of the solution

03:21.035 --> 03:22.295
and then it cannot be used.

03:23.505 --> 03:26.805
The silver particles in the solution are looking for

03:27.325 --> 03:30.285
nucleation sites to deposit and grow.

03:31.185 --> 03:34.325
So we have a conflict between needing a solution

03:34.595 --> 03:36.685
that has a long shelf life,

03:37.465 --> 03:40.565
but also when we want to use it, it will rapidly

03:41.075 --> 03:42.085
deposit silver.

03:43.025 --> 03:46.485
And this is where we have the addition of both cationic

03:46.505 --> 03:49.845
and non ionic surfactant, which is the symphonic N,

03:50.135 --> 03:53.085
which is added to stabilize the working solution.

03:54.005 --> 03:55.295
When this is added

03:55.515 --> 03:58.415
and the process is as carefully controlled as possible

03:58.415 --> 03:59.695
with the concentrations

03:59.955 --> 04:03.575
and correct with storage, you can have a stable solution

04:03.575 --> 04:04.655
of up to five days.

04:05.675 --> 04:09.575
And once you introduce that sample to the working solution,

04:10.035 --> 04:13.255
you will get spatially selective deposition of silver

04:14.235 --> 04:16.495
on only the finger mark residue.

04:17.275 --> 04:19.655
But this will only be achieved through

04:19.655 --> 04:21.415
that meticulous preparation

04:21.675 --> 04:25.935
and very clean scratch free glassware, hence those really

04:26.565 --> 04:28.935
high practical demands of the process.

04:29.875 --> 04:32.255
So the physical developer itself consists

04:32.415 --> 04:33.695
of three different steps.

04:34.515 --> 04:39.055
The first step being an immersion in a maleic acid pre-wash,

04:39.425 --> 04:42.775
which allows you to neutralize any of those common

04:43.895 --> 04:46.695
alkaline fillers that are added to paper because

04:46.695 --> 04:48.935
otherwise the silver would just target those.

04:50.155 --> 04:52.695
You then have immersion in the working solution

04:52.915 --> 04:54.855
and the working solution is made up

04:54.855 --> 04:56.455
of three different solutions.

04:57.075 --> 05:00.015
The redox solution, that's detergent,

05:00.065 --> 05:01.535
which are your surfactants

05:01.835 --> 05:03.895
and silver nitrate, which is your source

05:04.075 --> 05:08.055
of those silver iron, it's your sample is immersed in

05:08.055 --> 05:11.055
that working solution for up to 15 minutes.

05:12.275 --> 05:15.175
And after about 15 minutes it's taken out

05:16.175 --> 05:19.655
immersed in water up to three times to remove any of

05:19.655 --> 05:21.215
that excess solution

05:21.795 --> 05:23.495
and then it's air dried for you

05:23.495 --> 05:25.495
to view your developed marks.

05:26.205 --> 05:29.335
Firstly, we looked at different sweat types.

05:29.795 --> 05:32.215
So finger mark residue itself

05:32.885 --> 05:34.855
comprises both sebaceous,

05:34.985 --> 05:37.575
which are water insoluble components

05:38.595 --> 05:41.935
and rine, which are water soluble components.

05:43.055 --> 05:46.595
So what we did is we broke this down into sebaceous rine

05:46.735 --> 05:49.995
and what we consider more natural marks.

05:50.585 --> 05:53.955
Natural finger marks are used to reflect a little bit more

05:53.955 --> 05:56.635
of a realistic scenario where the

05:56.645 --> 05:58.795
Donor will wash their hands, proceed

05:58.795 --> 06:01.355
with their usual daily tasks for about 30 minutes,

06:01.655 --> 06:03.235
and then they'll deposit the marks.

06:03.975 --> 06:06.515
We used a single donor throughout the study,

06:07.015 --> 06:09.235
and even though this is not reflective

06:09.295 --> 06:11.635
of the wider variability in terms

06:11.655 --> 06:13.315
of finger mark composition,

06:14.175 --> 06:17.275
we were more interested in the mechanism of pd,

06:17.975 --> 06:21.315
not the performance, and looking at these different sweat

06:21.315 --> 06:24.515
types, we found that RY marks didn't

06:25.065 --> 06:27.835
give any ridge development in either dry

06:27.935 --> 06:31.515
or wet conditions, which is not a surprising result

06:31.745 --> 06:35.715
because the PD process involves repeated exposure

06:36.015 --> 06:37.075
to aqueous medium.

06:37.535 --> 06:39.475
So any of that water-soluble content

06:39.655 --> 06:40.755
is just gonna wash away.

06:41.975 --> 06:45.715
The sebaceous marks did reveal finger mark development in

06:45.715 --> 06:50.095
both wet and dry conditions, and then the natural marks,

06:50.655 --> 06:53.855
although the result was much more dotted in appearance,

06:54.355 --> 06:56.655
the results in cells were similar in both

06:56.875 --> 06:57.975
wet and dry conditions.

06:58.515 --> 07:00.815
So this meant that both sebaceous

07:00.815 --> 07:04.495
and equine material are required for optimum development

07:04.495 --> 07:07.655
with PD regardless of any exposure to water.

07:09.055 --> 07:11.915
We also looked at some different paper types

07:12.535 --> 07:13.595
and to the naked eye,

07:14.015 --> 07:17.435
the development on a macroscopic level, which is looking at

07:17.435 --> 07:21.515
that whole mark was consistent across all of the samples

07:21.735 --> 07:22.875
for both wet and dry.

07:23.815 --> 07:26.955
And when we viewed these developed marks microscopically,

07:27.465 --> 07:29.835
what we saw were silver deposits

07:29.835 --> 07:34.715
that were narrowly dispersed and fairly uniform in size.

07:35.855 --> 07:38.555
The main observation from this was

07:38.555 --> 07:40.635
that the smaller silver particles

07:41.395 --> 07:44.355
resulted in much better ridge development across the whole

07:44.385 --> 07:49.155
mark, whereas when we had larger silver particle particles,

07:49.785 --> 07:52.675
this was seen with more partial development.

07:53.755 --> 07:57.735
The paper type itself did not appear to impact the mechanism

07:57.795 --> 08:01.175
of pd, and the diameter ranged from about six

08:01.755 --> 08:03.775
to 20 micrometers.

08:05.145 --> 08:08.845
We next moved on to looking to understanding that solution

08:08.845 --> 08:10.085
to surface chemistry.

08:10.855 --> 08:13.555
And one of the interesting things we found is that the size

08:13.555 --> 08:16.835
of the silver particles in the actual working solution

08:17.655 --> 08:19.675
had never actually been reported.

08:20.775 --> 08:24.435
And so we did some dynamic like scattering to determine

08:24.435 --> 08:27.035
what this was because our question was

08:27.895 --> 08:31.915
if we have developed marks with particle sizes of up

08:31.915 --> 08:36.355
to 20 micrometers, are the silver particles growing to

08:36.355 --> 08:39.275
that size in the solution and then depositing,

08:39.895 --> 08:41.155
or are they depositing?

08:41.215 --> 08:45.075
And then growing dynamic like scattering

08:45.915 --> 08:48.955
revealed the particle diameter to be an average

08:49.655 --> 08:53.835
of about 880 nanometers, which is roughly

08:54.615 --> 08:57.285
one micrometer in the solution.

08:58.345 --> 09:00.005
So then we had to look at the growth

09:00.025 --> 09:01.685
of the silver on the surface.

09:02.545 --> 09:05.805
So we conducted an experiment where we took a finger mark

09:06.315 --> 09:11.045
from 30 seconds to 15 minutes, developed this with PD

09:11.705 --> 09:15.725
and used microscopic analysis to determine the size

09:15.725 --> 09:17.005
of the silver particles.

09:18.145 --> 09:20.485
And what we could find from the naked eye was

09:20.485 --> 09:22.805
that you could see the silver on the paper

09:23.355 --> 09:24.605
from about two minutes.

09:25.345 --> 09:29.245
So this already suggested pretty quick initial deposition.

09:30.385 --> 09:31.685
So this confirmed to us

09:31.685 --> 09:34.925
that the surfactants are maintaining the stability

09:34.985 --> 09:37.685
of the solution when you don't have any

09:37.685 --> 09:39.285
of those nucleation sites.

09:39.945 --> 09:43.325
But as soon as you introduce the sample, you get

09:43.325 --> 09:44.765
that selective deposition.

09:45.835 --> 09:49.375
And after about 30 seconds, we could actually see

09:50.165 --> 09:54.455
microscopic particles which were about two micrometers in

09:54.525 --> 09:58.495
size, which is roughly double what they are in the solution.

09:58.875 --> 10:02.135
So this further emphasized that you were pretty much getting

10:02.965 --> 10:07.175
instantaneous nucleation from the

10:07.605 --> 10:09.575
experiments presented in the study.

10:10.065 --> 10:11.975
There was no evidence to suggest

10:12.365 --> 10:15.735
that any growth was happening in the solution itself

10:17.465 --> 10:20.965
to explore and understand the dynamics of

10:20.965 --> 10:24.685
that growth a little bit further, we plotted the time

10:24.705 --> 10:27.325
of the development against the diameter

10:27.385 --> 10:30.165
of the silver particles in a log graph,

10:30.985 --> 10:34.325
and this gave us a slope value of about 9.53.

10:35.355 --> 10:38.165
What this is suggestive of is diffusion growth.

10:38.225 --> 10:40.405
So it's happening on the surface.

10:41.545 --> 10:43.045
And this was further proved

10:43.105 --> 10:45.325
by looking at the population density.

10:46.655 --> 10:47.945
This determined that

10:48.065 --> 10:52.065
that particle density was actually independent of time.

10:52.965 --> 10:56.345
So this means that that nucleation is instantaneous

10:56.845 --> 11:00.385
and any observations thereafter are a result

11:00.385 --> 11:02.305
of growth on the silver surface.

11:03.005 --> 11:04.425
So what this means is as soon

11:04.425 --> 11:08.785
as you have those nucleation sites, they just get bigger

11:09.525 --> 11:13.025
as opposed to more silver depositing in different areas.

11:14.445 --> 11:17.105
And finally, the last experiment looked at the

11:17.105 --> 11:18.625
compositional analysis.

11:19.285 --> 11:22.705
We used scanning electron microscopy coupled

11:22.705 --> 11:27.025
with energy dispersive X-ray analysis,

11:27.165 --> 11:29.665
to determine the composition of the developed marks.

11:30.245 --> 11:34.185
And it is quite obvious you would expect them to be silver,

11:35.315 --> 11:37.015
but what we were looking at was whether

11:37.015 --> 11:38.615
or not anything else was actually there.

11:39.365 --> 11:41.825
And it did confirm that yes, those

11:42.465 --> 11:44.345
particles are in fact silver,

11:45.045 --> 11:49.785
but there is also no iron present, which concluded

11:50.015 --> 11:51.865
that the iron itself

11:52.525 --> 11:55.045
is solely involved in the redux reaction

11:55.045 --> 11:58.445
during the working solution formation as opposed

11:58.545 --> 12:01.045
to the subsequent growth on the surface.

12:01.995 --> 12:06.205
From a qualitative perspective, both EQU

12:06.225 --> 12:09.565
and sebaceous material are required in order to give you

12:10.285 --> 12:11.965
optimal PD performance.

12:12.305 --> 12:14.885
So as high quality finger marks as you can get,

12:15.905 --> 12:19.325
the paper type doesn't have an impact on the actual PD

12:19.475 --> 12:23.765
mechanism and exposure to the water is not going to impact

12:23.835 --> 12:25.005
that silver deposition.

12:26.275 --> 12:28.805
Understanding what exactly

12:29.965 --> 12:32.405
physical developer itself is targeting

12:33.105 --> 12:34.245
hasn't been answered here,

12:34.305 --> 12:37.525
and that requires much more experimentation

12:37.835 --> 12:40.285
because finger mark residue is complex.

12:40.595 --> 12:43.445
It's not just one sebaceous one component.

12:43.945 --> 12:46.565
And research in Australia has looked at this a little bit,

12:46.985 --> 12:49.405
but it requires a lot more further understanding.

12:49.915 --> 12:52.635
Further, we achieved the objective

12:52.735 --> 12:54.475
of determining the particle size

12:54.495 --> 12:58.355
and solution at an average of 818 nanometers,

12:59.055 --> 13:02.955
and the dynamics of the PD process were revealed such

13:02.955 --> 13:06.875
that the deposition of silver is almost instantaneous

13:07.975 --> 13:11.435
and it is these nucleation sites which provide a

13:11.435 --> 13:15.315
preferential site for subsequent silver deposition and

13:15.315 --> 13:16.835
therefore growth on the surface.

13:18.265 --> 13:21.435
Diffusion growth is suggested due

13:21.435 --> 13:24.355
to an increase in the silver size with the square root

13:24.355 --> 13:28.435
of development time and compositional analysis has revealed

13:28.435 --> 13:33.155
that the deposits are in fact silver with no iron present.

13:34.595 --> 13:38.245
Through those growth experiments, it was evident at the time

13:38.355 --> 13:42.125
that some of the marks did not appear to improve

13:42.585 --> 13:44.925
before the 15 minute development time,

13:45.625 --> 13:47.765
and there were visual differences between

13:47.875 --> 13:51.205
what you would view macroscopically compared

13:51.425 --> 13:53.045
to microscopically.

13:53.625 --> 13:57.485
So this is an area that could benefit from further research.

13:58.545 --> 14:02.285
We did use a single donor as indicated, so it would be

14:02.805 --> 14:04.405
interesting to consider whether

14:04.625 --> 14:09.125
or not donor characteristics do have an impact on the

14:09.125 --> 14:12.495
mechanism of PD and replacement.

14:12.525 --> 14:16.055
Surfactants for that Sym N have been studied

14:16.675 --> 14:18.935
in collaboration with the home office in the uk

14:19.555 --> 14:23.335
and this has been cited in a recent publication within this

14:23.335 --> 14:23.615
study.

00:00 - Physical developer is based on a redox reaction. It is essentially a finger mark development method, which allows us to visualize latent finger marks on paper surfaces, latent finger marks, basically meaning that they are not visible to the naked eye,

00:23 - therefore we have to visualize them first in order to use them for identification purposes. As we know, physical developer is used to develop latent finger marks on porous surfaces, and these will predominantly be paper surfaces. This could include any paper surfaces such as financial documentation, currency, or any other correspondence. It's a widely used technique, not only in the UK but globally.

00:56 - However, there is surprisingly little research into the underlying chemistry of the process. We know that it is a very effective technique, but the operational success can vary in terms of producing high quality finger marks. This could be attributed to the practical demands of the process because it is quite labor intensive. Coupled with that limited understanding of that underlying chemistry, it is a chemical process and one of the crucial reagents, a surfactant named Sym N

01:36 - was banned due to its environmental toxicity. In order to replace that surfactant, the underlying chemistry has to first be understood. In addition to this, currently in the uk, it's the only technique recommended on those porous surfaces which may have previously been wetted. Without a suitable surfactant replacement, practitioners could lose that ability to develop any latent marks of wetted paper

02:06 - that comes in as evidence. Alternative methods have been studied, which includes oil, red, O and Nile red, but these haven't been shown to actually outperform PD and the effectiveness actually decreases with time. Whereas research has shown that PD is still effective for Marx, which have been aged for up to 90 years. So it's a combination of all these factors that have motivated this study, which has been addressed by exploring the solution to surface chemistry dynamics

02:40 - of the silver growth and compositional analysis of those developed marks. Physical developer is based on a redox solution, and in that working solution you have a fine balance of Ferris ions, which reduce silver ions to elemental silver, and in turn, those ferrous ions are oxidized. The elemental silver itself is highly unstable in the solution, and if you don't have that chemical control,

03:13 - they would just aggregate, which would form much larger silver particles, which would just crash out of the solution and then it cannot be used. The silver particles in the solution are looking for nucleation sites to deposit and grow. So we have a conflict between needing a solution that has a long shelf life, but also when we want to use it, it will rapidly deposit silver.

03:43 - And this is where we have the addition of both cationic and non ionic surfactant, which is the symphonic N, which is added to stabilize the working solution. When this is added and the process is as carefully controlled as possible with the concentrations and correct with storage, you can have a stable solution of up to five days. And once you introduce that sample to the working solution, you will get spatially selective deposition of silver

04:14 - on only the finger mark residue. But this will only be achieved through that meticulous preparation and very clean scratch free glassware, hence those really high practical demands of the process. So the physical developer itself consists of three different steps. The first step being an immersion in a maleic acid pre-wash, which allows you to neutralize any of those common alkaline fillers that are added to paper because

04:46 - otherwise the silver would just target those. You then have immersion in the working solution and the working solution is made up of three different solutions. The redox solution, that's detergent, which are your surfactants and silver nitrate, which is your source of those silver iron, it's your sample is immersed in that working solution for up to 15 minutes. And after about 15 minutes it's taken out

05:16 - immersed in water up to three times to remove any of that excess solution and then it's air dried for you to view your developed marks. Firstly, we looked at different sweat types. So finger mark residue itself comprises both sebaceous, which are water insoluble components and rine, which are water soluble components. So what we did is we broke this down into sebaceous rine

05:46 - and what we consider more natural marks. Natural finger marks are used to reflect a little bit more of a realistic scenario where the Donor will wash their hands, proceed with their usual daily tasks for about 30 minutes, and then they'll deposit the marks. We used a single donor throughout the study, and even though this is not reflective of the wider variability in terms of finger mark composition,

06:14 - we were more interested in the mechanism of pd, not the performance, and looking at these different sweat types, we found that RY marks didn't give any ridge development in either dry or wet conditions, which is not a surprising result because the PD process involves repeated exposure to aqueous medium. So any of that water-soluble content is just gonna wash away. The sebaceous marks did reveal finger mark development in

06:45 - both wet and dry conditions, and then the natural marks, although the result was much more dotted in appearance, the results in cells were similar in both wet and dry conditions. So this meant that both sebaceous and equine material are required for optimum development with PD regardless of any exposure to water. We also looked at some different paper types and to the naked eye, the development on a macroscopic level, which is looking at

07:17 - that whole mark was consistent across all of the samples for both wet and dry. And when we viewed these developed marks microscopically, what we saw were silver deposits that were narrowly dispersed and fairly uniform in size. The main observation from this was that the smaller silver particles resulted in much better ridge development across the whole mark, whereas when we had larger silver particle particles, this was seen with more partial development.

07:53 - The paper type itself did not appear to impact the mechanism of pd, and the diameter ranged from about six to 20 micrometers. We next moved on to looking to understanding that solution to surface chemistry. And one of the interesting things we found is that the size of the silver particles in the actual working solution had never actually been reported. And so we did some dynamic like scattering to determine what this was because our question was

08:27 - if we have developed marks with particle sizes of up to 20 micrometers, are the silver particles growing to that size in the solution and then depositing, or are they depositing? And then growing dynamic like scattering revealed the particle diameter to be an average of about 880 nanometers, which is roughly one micrometer in the solution. So then we had to look at the growth of the silver on the surface.

09:02 - So we conducted an experiment where we took a finger mark from 30 seconds to 15 minutes, developed this with PD and used microscopic analysis to determine the size of the silver particles. And what we could find from the naked eye was that you could see the silver on the paper from about two minutes. So this already suggested pretty quick initial deposition. So this confirmed to us that the surfactants are maintaining the stability

09:34 - of the solution when you don't have any of those nucleation sites. But as soon as you introduce the sample, you get that selective deposition. And after about 30 seconds, we could actually see microscopic particles which were about two micrometers in size, which is roughly double what they are in the solution. So this further emphasized that you were pretty much getting instantaneous nucleation from the experiments presented in the study.

10:10 - There was no evidence to suggest that any growth was happening in the solution itself to explore and understand the dynamics of that growth a little bit further, we plotted the time of the development against the diameter of the silver particles in a log graph, and this gave us a slope value of about 9.53. What this is suggestive of is diffusion growth. So it's happening on the surface. And this was further proved

10:43 - by looking at the population density. This determined that that particle density was actually independent of time. So this means that that nucleation is instantaneous and any observations thereafter are a result of growth on the silver surface. So what this means is as soon as you have those nucleation sites, they just get bigger as opposed to more silver depositing in different areas. And finally, the last experiment looked at the

11:17 - compositional analysis. We used scanning electron microscopy coupled with energy dispersive X-ray analysis, to determine the composition of the developed marks. And it is quite obvious you would expect them to be silver, but what we were looking at was whether or not anything else was actually there. And it did confirm that yes, those particles are in fact silver, but there is also no iron present, which concluded

11:50 - that the iron itself is solely involved in the redux reaction during the working solution formation as opposed to the subsequent growth on the surface. From a qualitative perspective, both EQU and sebaceous material are required in order to give you optimal PD performance. So as high quality finger marks as you can get, the paper type doesn't have an impact on the actual PD mechanism and exposure to the water is not going to impact

12:23 - that silver deposition. Understanding what exactly physical developer itself is targeting hasn't been answered here, and that requires much more experimentation because finger mark residue is complex. It's not just one sebaceous one component. And research in Australia has looked at this a little bit, but it requires a lot more further understanding. Further, we achieved the objective

12:52 - of determining the particle size and solution at an average of 818 nanometers, and the dynamics of the PD process were revealed such that the deposition of silver is almost instantaneous and it is these nucleation sites which provide a preferential site for subsequent silver deposition and therefore growth on the surface. Diffusion growth is suggested due to an increase in the silver size with the square root of development time and compositional analysis has revealed

13:28 - that the deposits are in fact silver with no iron present. Through those growth experiments, it was evident at the time that some of the marks did not appear to improve before the 15 minute development time, and there were visual differences between what you would view macroscopically compared to microscopically. So this is an area that could benefit from further research. We did use a single donor as indicated, so it would be interesting to consider whether

14:04 - or not donor characteristics do have an impact on the mechanism of PD and replacement. Surfactants for that Sym N have been studied in collaboration with the home office in the uk and this has been cited in a recent publication within this study.

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