Observations of Changes in Surface Chemistry of Metallic FLSP Surfaces as a Function of Wettability

Graham Kaufman Author

04/01/2021 Added

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Poster presentation of surface analysis of superhydrophilic and hydrophobic silver (Ag) FLSP.

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[00:00:00.480]Hello. My name is Graham Kaufman.
[00:00:02.670]I'm a graduate student here at the university of Nebraska in the electrical
[00:00:05.670]engineering department.
[00:00:06.420]And today I'll be presenting my research on observations of changes in surface
[00:00:09.420]chemistry and metallic FLSP surfaces as a function of wettability.
[00:00:19.130]Here's some motivation behind the work: Metal surfaces processed with femtosecond
[00:00:22.970]laser surface processing or FLSP have a wide variety of applications.
[00:00:27.200]One application is the creation of superhydrophilic or hydrophobic surfaces on
[00:00:30.950]metals.
[00:00:31.700]Creating a superhydrophobic surface on an electrical power line can limit the
[00:00:34.820]ice build up during winter storms, preventing the cables from snapping,
[00:00:37.850]potentially saving millions of dollars annually.
[00:00:39.230]Metal FLSP surfaces are intrinsically superhydrophilic. However,
[00:00:44.060]when exposed to air, the wettability of the surfaces transitions from
[00:00:47.060]superhydrophilic to superhydrophobic.
[00:00:49.130]We are interested in investigating what changes in surface chemistry result
[00:00:52.580]in these shifts in wettability over time. Here, we focus on FLSP applied to
[00:00:56.330]aluminum and silver.
[00:00:57.640]Silver is investigated because the wettability properties change within hours of
[00:01:01.100]exposure to air versus more than a month with aluminum.
[00:01:04.340]Our goal is to use surface analysis techniques to study what changes in surface
[00:01:07.490]chemistry of a metal FLSP surface as
[00:01:09.830]the wettability transitions from superhydrophilic to hydrophobic due to
[00:01:13.010]exposure to air.
[00:01:20.270]A little background on what FLSP does and how it works.
[00:01:23.390]FLSP creates dual-hierarchal structuring on the micro- and nano-scale on metals in
[00:01:27.500]particular here,
[00:01:28.730]intense electric fields from a focused ultra short laser pulse ablate
[00:01:31.940]the metallic surface, leading to material removal, surface melts,
[00:01:35.300]and redeposition of material. And this leads to the self-organized structuring.
[00:01:39.080]We can see in figure (a) here. This is on silver. These
[00:01:43.970]micro-scale structures here are about 30 microns across and on the orders of
[00:01:47.840]about 50 microns tall. And the bottom right here,
[00:01:50.750]we can see the nano structuring,
[00:01:52.380]which is a result of the redeposition of material.
[00:01:58.730]Here's the experimental setup we use here. The laser we use in
[00:02:02.390]this work is a 6 mJ
[00:02:03.770]coherent Astrella laser with the center wavelength of 800 nm,
[00:02:07.100]a repetition rate of 1000 Hz and a pulse duration of 35 fs.
[00:02:11.750]In the case of silver [FLSP], we use a technique called dual pulse,
[00:02:15.020]which takes the incident beam and splits it into two different paths based on
[00:02:18.110]horizontal and vertical polarization The two different paths traveled
[00:02:21.830]different path lengths, nd since the speed of light is constant and air,
[00:02:25.700]these two different path links arrive at the sample at a slightly different
[00:02:29.270]time. In this case, we typically use less than a nanosecond of pulse delay.
[00:02:34.520]Dual pulse takes advantage of the time dependent light matter interaction,
[00:02:37.910]processes of FLSP.
[00:02:46.580]The procedure for this experiment is as follows: four samples of both silver and
[00:02:50.270]aluminum are individually cleaned in a 20 minute Sonic bath of acetone, then
[00:02:54.440]ethanol, and finally,
[00:02:55.430]distilled water. FLSP is applied to these samples using the following parameters:
[00:02:59.980]for the silver, Our pulse delay is 120 picoseconds. And for aluminum,
[00:03:03.460]there is no pulse delay since we're just using single pulse FLSP.
[00:03:06.760]The laser spot radius,
[00:03:07.900]the raster offset, the power, and the velocity of the raster all lead to two main
[00:03:11.920]parameters of the laser processing: the peak fluence,
[00:03:14.560]which is the energy per centimeter squared, and then the pulse count,
[00:03:18.520]which is how many times an individual spot on the surface is irradiated with the
[00:03:21.760]laser. Varying peak fluennce and pulse count gives different size and shape
[00:03:25.960]structures. For both the silver and the aluminum immediately after processing.
[00:03:30.100]and before the samples have had a chance to transition,
[00:03:32.590]One sample is placed inside the ultra high vacuum chamber for analysis and the
[00:03:36.880]contact angle is measured on the other sample.
[00:03:39.250]Two additional samples are used for hydrophobic experiments.
[00:03:42.400]These are set under a clean upside down Petri dish in open air,
[00:03:45.310]and we wait for those to transition.
[00:03:47.410]XPS is performed on a first superhydrophilic sample.
[00:03:50.440]And after the exposure to air,
[00:03:51.550]the contact angle is measured on the one of the remaining two samples.
[00:03:55.450]Once the sample's wettability has transitioned to hydrophobic,
[00:03:57.940]The other sample,
[00:03:58.510]which is also hydrophobic, is placed in the ultra high vacuum system
[00:04:01.750]And XPS is performed. Here, we have the results of the contact angle measurements
[00:04:05.800]The unprocessed silver has a contact angle of 80 degrees,
[00:04:08.650]and the unprocessed aluminum has a contact angle of 71 degrees.
[00:04:12.100]The super hydrophilic samples,
[00:04:13.300]which we can see in the left image are completely a wetting.
[00:04:16.330]So you almost can't see the water at all on the surface. We haven't had
[00:04:21.190]chance for the aluminum to fully transition from superhydrophilic,
[00:04:24.370]the hydrophobic,
[00:04:25.000]So we don't have contact angle measurements for hydrophobic aluminum,
[00:04:29.050]but we do for hydrophobic silver that can be seen on the right image there where
[00:04:32.020]the contact angle was 141.1 degrees.
[00:04:38.130]Here's the XPS data for the aluminum FLSP.
[00:04:42.120]I won't spend too much time talking about this because we have nothing to
[00:04:44.670]compare it against because the sample hasn't fully transitioned to hydrophobic
[00:04:47.370]So this is the superhydrophilic aluminum XPS data.
[00:04:53.130]Here,
[00:04:53.280]We have XPS data for the superhydrophilic and hydrophobic silver FLSP samples.
[00:04:57.300]The superhydrophilic
[00:04:59.040]data will be in the left column and a hydrophobic data will be in the right.
[00:05:02.070]Images (a) and (b) are the survey scans for both these samples and atomic percent
[00:05:05.670]compositions are calculated from the area underneath the peaks.
[00:05:10.080]Comparing the superhydrophilic's carbon concentration to the hydrophobic,
[00:05:12.810]We can see that they're both about 34%.
[00:05:15.600]both oxygen concentrations are about 28 or 29%.
[00:05:19.050]And these silver concentration makes up the rest,
[00:05:20.730]So it's about 37% on both of these.
[00:05:23.370]We initially thought that the transition of wettability may have been due to
[00:05:26.730]hydrocarbon attachment from hydrocarbons in the atmosphere.
[00:05:29.850]But if that were the case,
[00:05:30.660]we'd expect to see a higher percent composition of carbon in the hydrophobic
[00:05:34.260]surface compared to the hydrophilic surface. This is not what we see.
[00:05:40.230]Next, we look at the carbon 1S
[00:05:42.300]transition for the superhydrophilic and superhydrophobic. In the
[00:05:46.230]superhydrophilic image, (c),
[00:05:48.180]we see that we have a curve that's relatively consistent with what we'd expect
[00:05:51.240]for adventitious carbon. Adventitious
[00:05:54.330]Carbon is just a layer of carbon that settles from the atmosphere on most samples.
[00:05:58.510]On the right,
[00:05:58.970]we see that we kind of have that same shape over on the right
[00:06:01.910]side from the 285 to the 287 region.
[00:06:04.400]But we also have this new component at 288.8.
[00:06:07.280]We believe that this new component can be attributed to the fact that the
[00:06:10.010]surface chemistry changes as you transition from superhydrophilic to
[00:06:13.040]superhydrophobic or to hydrophobic. If we look at the O 1s transitions, again,
[00:06:17.600]we can see that we have a larger difference in the shapes of the curves.
[00:06:21.080]So we have the same three components fitting, both the image (e),
[00:06:24.410]which is the super hydrophilic and image and (f), which is the hydrophobic samples.
[00:06:28.160]However, the ratio of intensity for these peaks is substantially different.
[00:06:31.850]On the left. We have the peak
[00:06:33.710]at about 533 and the peak at about 530
[00:06:36.860]making a majority of the sample.
[00:06:38.540]But as we transition from superhydrophilic to hydrophobic over in image (f), we
[00:06:42.590]can see that the peak at 531.3 eV dominates the spectrum.
[00:06:46.580]We believe that this also has to do with the fact that
[00:06:49.610]the carbon in image (d) - so the addition of carbon from super hydrophilic to
[00:06:54.200]hydrophobic - most likely contains oxygen as well.
[00:06:59.390]The conclusions from this work: as metallic FLSP surfaces are left in an open
[00:07:03.650]lab environment,
[00:07:04.340]there wettability transitions from superhydrophilic to hydrophobic,
[00:07:07.070]and eventually superhydrophobic. XPS survey scans show little change in the
[00:07:11.060]atomic percent composition of the carbon and oxygen on the surface of the silver
[00:07:14.120]FLSP as the wettability transitions, however,
[00:07:17.120]high resolution scans of the carbon 1s and oxygen, 1S transitions
[00:07:21.170]for the silver FLSP show significant changes in the species present as the
[00:07:24.470]samples wettability transitions.
[00:07:24.830]The hydrophobic silver FLSP sample contains a new carbon
[00:07:29.720]species at a binding energy of 288.8 eV,
[00:07:32.330]often associated with a carbon-oxygen double bond.
[00:07:35.960]And the hydrophobic FLSP surface
[00:07:38.660]oxygen content is dominated by the species found at 531.3 eV.
[00:07:43.820]I'd like to thank the Nebraska center for energy science research (NCESR) for funding
[00:07:46.820]this work. The laser processing and surface characterization was performed in
[00:07:50.390]the center for electro-optics and functionalized surfaces (CEFS),
[00:07:53.270]And the SEM imaging was performed in the Nebraska nanoscale facility,
[00:07:57.380]the nano engineering research core facility, or the NERcF facility.
[00:08:01.520]My name is Graham Kaufman,
[00:08:02.540]and I'd like to thank you for watching my presentation.

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Observations of Changes in Surface Chemistry of Metallic FLSP Surfaces as a Function of Wettability

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