Modeling of Mechanical Buckling of Elastomeric Films for Adaptive Materials

Cierra Foster Author

07/28/2020 Added

151 Plays

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Generating models of wrinkled elastomers using Finite Element Analysis in ABAQUS.

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[00:00:00.030]Hello, my name Cierra Foster,
[00:00:01.560]and I'm doing my research on the modeling
[00:00:03.300]of mechanical buckling of elastometric films
[00:00:05.600]for adaptive materials.
[00:00:08.870]Understanding the influence of surface chemical
[00:00:11.030]and mechanical properties made through the oxidation process
[00:00:13.980]on generating wrinkles is crucial
[00:00:16.020]in designing dynamic, adaptive materials.
[00:00:19.140]Figure one depicts an elastomeric substrate
[00:00:21.580]that is hydrophobic due
[00:00:22.810]to the engineered, extruding, triangular surfaces.
[00:00:26.290]When the elastomeric substrate gets strained,
[00:00:28.560]the surface topography smooths out,
[00:00:30.420]allowing the water droplet to wet the strained surface.
[00:00:34.380]We modeled the wrinkle formation on elastomeric films,
[00:00:37.610]such as polydimethylsiloxane,
[00:00:39.780]using finite element analysis simulations in Abaqus.
[00:00:43.010]Understanding these wrinkles will further assist
[00:00:45.150]in the designing of dynamic, adaptive materials.
[00:00:49.690]By changing the surface chemistry
[00:00:51.220]and mechanical properties of the model,
[00:00:53.170]you can tune the characteristics of the model.
[00:00:55.460]Figure two portrays the cycle of fully testing a model
[00:00:58.410]from designing and simulating the model computationally
[00:01:01.270]to experimentally testing and comparing the results
[00:01:03.940]and then revising and furthering the design of the model.
[00:01:09.770]The first step in creating these wrinkles is
[00:01:12.010]to thermally pre-strain the bilayer to 20%
[00:01:14.470]by heating the substrate.
[00:01:15.940]Then we fabricate the oxidized film
[00:01:17.980]by using plasma or ultraviolet ozone oxidation,
[00:01:20.720]creating silica.
[00:01:22.460]This hard silica layer is called the film,
[00:01:25.120]and the rest of the material is known as substrate.
[00:01:27.760]By cooling down the elastomer to zero degrees Celsius,
[00:01:30.440]it releases the strain and wrinkles then form due
[00:01:33.170]to the differences in Young's modulus,
[00:01:34.950]which is the measure of stiffness in a solid material.
[00:01:37.730]Figure three depicts the general process
[00:01:40.220]of creating these wrinkles.
[00:01:42.940]This slide shows the mechanical wrinkling procedure.
[00:01:45.360]Figure four shows the process of generating PDMS
[00:01:48.160]and the cross-linking structure of PDMS
[00:01:50.140]when the base and the catalyst are mixed together.
[00:01:53.151]SYLGARD 184 is the base,
[00:01:54.990]and the curing agent has a catalyst in it.
[00:01:57.210]The more curing agent added to the mixture,
[00:01:59.100]the more cross-linking occurs due
[00:02:00.710]to free-radical polymerization.
[00:02:03.200]The more cross-linking, the stiffer the material,
[00:02:05.600]which in turn creates a higher Young's modulus.
[00:02:08.820]Figure five shows the PDMS substrate
[00:02:11.200]after the curing of the mixture
[00:02:13.300]and the chemical structure of the PDMS.
[00:02:15.930]An important part about this elastomer
[00:02:18.090]is the silicone-to-methyl bond
[00:02:19.550]that allows oxidation to occur, creating silica.
[00:02:23.210]Figure six shows ultraviolet ozone light used
[00:02:25.650]to oxidize the top of the pre-strained PDMS.
[00:02:28.650]The UVO light creates ozone, and that oxidizes the film
[00:02:32.280]by creating a hard silica layer.
[00:02:34.740]The longer the PDMS is exposed to UVO,
[00:02:38.370]the thicker the hard, glassy layer is on top.
[00:02:40.800]Figure seven shows fully formed wrinkles
[00:02:42.900]after PDMS has gone through this process.
[00:02:48.200]Table one describes the physical properties assigned
[00:02:50.540]to the film and substrate.
[00:02:52.090]The film's properties are noted with the prime symbol,
[00:02:55.010]and the non-prime symbols represent the substrate.
[00:02:57.790]The Young's modulus of the substrate is less than the film
[00:03:00.450]by one order of magnitude.
[00:03:02.330]The Poisson's ratio,
[00:03:03.320]which describes the expansion or contraction of a material,
[00:03:06.550]for the substrate is larger than the film.
[00:03:08.870]This means the substrate is softer
[00:03:10.830]and more pliable than the film is.
[00:03:12.990]The expansion coefficient describes how much a layer moves
[00:03:16.200]when there is an increase of temperature.
[00:03:18.340]The density and specific key are only needed
[00:03:20.490]for the substrate, as they describe how much heat
[00:03:22.660]is required to raise the temperature one degree.
[00:03:27.840]Figure 8A displays the releasing of the strain
[00:03:30.210]on the substrate to the critical buckling point.
[00:03:32.980]The critical buckling stress
[00:03:34.160]for the simulation is 1.042 megapascals.
[00:03:40.210]Figure 8B shows the critical buckling strain
[00:03:42.360]of the simulation.
[00:03:43.550]The critical strain is the point
[00:03:45.190]where the formation of wrinkles start to occur
[00:03:47.440]and can be found using this equation.
[00:03:51.980]8C shows the complete relaxation of the substrate
[00:03:54.850]compressing the film and creating fully formed wrinkles.
[00:03:57.780]The compression force of this film is 24.198 kilopascals.
[00:04:04.090]These tables display the measured
[00:04:05.590]and calculated wrinkling profiles
[00:04:07.190]based off these equations on the slide
[00:04:09.520]and the finite element analysis simulation.
[00:04:12.330]It is important to note that the strain is measured
[00:04:14.510]as the release of pre-strain.
[00:04:16.800]Figure nine shows the effect of mechanical strain
[00:04:18.890]on wrinkle amplitude.
[00:04:20.470]As the strain increases,
[00:04:21.730]amplitude of the waves increase as well.
[00:04:24.480]Figure 10 shows the effect of mechanical strain
[00:04:27.340]on wrinkle wavelength.
[00:04:29.430]As the strain increases,
[00:04:30.620]the wavelength of the waves decreases.
[00:04:33.450]For both figures nine and 10,
[00:04:35.350]the red squares are for the mathematically calculated values
[00:04:38.950]and the black diamonds are for the simulated values.
[00:04:42.010]These wrinkle formations can be modeled comparably
[00:04:45.020]to the sine wave function.
[00:04:48.270]The ability to accurately model the physical properties
[00:04:51.100]of these wrinkles and understand the influence
[00:04:53.700]of surface chemistry and mechanics will allow us
[00:04:55.890]to design materials with dynamic, adaptive functionality,
[00:04:58.200]such as mechano-activated surfaces, self-cleaning apparel,
[00:05:01.450]mechano-switchable lensing technologies, and more.
[00:05:04.140]Future work includes simulating
[00:05:05.730]an embedded stress-relieving structure
[00:05:07.360]into a hard, pre-strained substrate
[00:05:09.330]and straining a hard substrate
[00:05:10.640]with an embedded soft well in the middle.
[00:05:14.540]This is my poster.
[00:05:15.450]Thank you for listening.
[00:05:16.670]I'd like to give a special thanks
[00:05:18.030]to my PI, Dr. Morin, Dr. Griep,
[00:05:21.080]my mentor, Ali,
[00:05:22.430]the University of Nebraska-Lincoln,
[00:05:24.300]and the National Science Foundation
[00:05:25.690]Research Experience for Undergraduates.

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Modeling of Mechanical Buckling of Elastomeric Films for Adaptive Materials

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