Project goals

• investigate a method to implant a speckle pattern plane into the middle of a volume of poly(acrylamide) hydrogel
• modify the linear tribometer to be able to deform the hydrogel while taking pictures of the deformation of the plane
• process the DIC and identify
           • surface deformation and strain at contact interface 
​           • deformation and strain directly under the indenting probe

Project Results

This project has been presented as a part of Jack Famiglielli's honors thesis and the complete write up can be downloaded by clicking the following link.
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honors_thesis_jack_famiglietti.pdf
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An overview of the results are shown below

Here we see a 6.45 mm diameter glass probe indenting a crosslinked PEG-Acrylate hydrogel. The surface deformation can be visibly seen and measured very close to the surface in contact. The speckle pattern is created and printed using black paint crosslinked in PEG-Acrylate using a previously defined method (see Method of DIC in YSF). The PEG-Acrylate is mixed with a low polymer content granular carbomer and a photo-initiator to form a liquid like solid that allows a speckle pattern to be 3D printed with no change in position. After the speckle pattern is printed, the entire volume is photocroslinked using UV light. 

The experimental setup utilizes a custom built micro-newton indenter alligned with a grayscale ccd camera. This setup is also capable of sliding motion since the DIC pattern is fixed with respect to the ccd camera. 

​Magnitude of displacement

​DIC analysis of the indentation reveals that the magnitude of displacement can be seen extending throughout the entire image. The scale ranges between 1 and 10μm of displacement

​Strain in vertical direction

​Shear strain

​The DIC analysis also allows for the measurement full field strain and shear distributions 

March 2018

3/12/18
PEG A to DA ratio varied for several samples to determine best clarity and change in modulus with change in PEG A to DA. 

DA/(A+DA) ratios used: 0.05, 0.025, 0.0125. Each sample swollen in DI water to determine volume changes. Lowest DA concentrations swelled the most and were the softest samples. Edges of samples swell drastically, compared to the bulk, upon first glances. 

"Negative" printing technique thought of. This is where a PEG+Carbomer solution is in a container, and non-polymerizing material (plain carbomer) printed in a cylinder around a horizontal speckle pattern. Idea being that when the  entire container is UV cured, the cylindrical plug within the plain carbomer barrier should be able to be removed as a single piece. Hopefully with good dimensional stability.

Many prints were done in this time period to improve the plane printing techniques, and ultimately, decent samples were made in this time. 
The samples with a printed vertical speckle plane should be let to sit for 30 minutes after printing to allow for the gel/polymers to diffuse through the plane. If this is not done, the plane will form a seam in the cured gel, making it not homogeneous. 

3/23/18
First time running the optirod microtribometer myself. This was a good learning experience, however, due to set-up issues (camera not straight, not the best lighting, plastic container) the images came out with a significant skew toward one side. This was also due to shadows, and not taking a reference image with the probe in the middle (to try to keep the same shadows in the reference image as would be in the deformed images). 

Downselecting code was written to attempt to pick the images that all aligned together without doing so by hand. Some success here with a method that used a consistent step size to go from picture to the next picture (forward and backward). Next method tried was a correlation method that was supposed to find the image that best correlated to the initial reference. That is, the images that also had the probe in the center of the frame, moving one direction or the other. This correlation technique had promise, but was not fine tuned quickly enough to be used. Ultimately, picking images by hand proved the most efficient when taking into account the time taken to write codes and debug them. 

April 2018

4/9/18
Hand selected image stacks were analyzed with DIC using Ncorr on matlab. 

4/11/18
Code was written to analyze the MAT files from the DIC. The method was to stack the magnitude plots of displacement on top of one another so that the probe tips aligned and take an average along the stack to get an averaged magnitude plot of the  deformation for a given speed, force, and direction of sliding. 

4/23/18 through end of April
Samples were made and tests were run on the microtribometer for many speeds and forces. These tests shed light on several issues to be addressed:
-there were still shadows that were affecting the images: need more even lighting
-The surface of the material, really is not that flat: checking normal force fluctuations along the surface gives a place to run more tests. That is, the most flat region of the material is where tests should be run. 
     -The inclination of the surface in one direction versus the other would cause an increase in normal force, which would give an artificial result showing an asymmetry toward one side
     -Solution: make a box with a lid  to polymerize the sample

All the while, working on my presentation for the SEM graduate symposium in Huntsville Alabama. 

May 2018

5/7/18
Began making a new experimental set up for the microtribometer. This includes a new glass box for the sample, a new metal base plate to mount onto the tribometer stage and a new camera mounting method on to the plate. The camera's 1/4-20 tapped hole on the bottom of it was used to mount the camera to the plate, and the plate was designed so that the sample and camera were mounted on the same plane. 

The sample container was made out of glass for better optical clarity. This box was made using UV cured optical glue, with a lid (also glass) that could be placed on top of the unpolymerized gel sample to ensure a flat surface on the final product. Edges on the mounting plate were used to locate the sample in the same spot every time, relative to the camera. The camera and sample container were stationary with respect to each other. This is important so that the camera is always imaging the same region of the speckle pattern for the DIC analysis.

5/11/18
Another sample container was made, out of plastic, that was 30% smaller (in each direction) than the original glass box. This was done so that a smaller sample could be printed and swollen,  and still fit inside the glass optical container. This was done after realizing that samples printed in the glass box, then swollen for 24 hours, would not fit back into the box without major amounts of the sample being cut away.

Furthermore, it was realized early on that the samples become hazy  (opaque) when swollen. This was remedied by degassing the printing materials in a vacuum chamber and then sonicating everything. Any micro air  bubbles within the material creates a light scattering interface. These interfaces, even if too small to be seen by eye, create a general haziness in the material when swollen. By removing the air in this method, the haziness in the swollen sample was greatly reduced. It was now seen that the samples do not become hazy, to the point of not being usable anymore, after a much longer period of time.

5/19/18 - 5/25/18
Presented my work at STLE (Society of Tribologists and Lubrication Engineers) in Minneapolis, Minnesota​​. More tests were run in the preceding days to the conference, and the DIC images were presented as preliminary data. 

5/28/18-5/30/18
Upon return from STLE, work was done on a swelling experiment  with this PEG+Carbomer gel. The idea was to print a horizontal plane into a thin sheet of the gel, cure it, and then punch out a circular section so that the circular plug has a speckle pattern through it. The sample was then to be placed in water, suspended, and imaged for a long period of time. The images were then used for DIC to determine swelling rates at different places in the gel. The desired result was to determine if there was any difference in swelling rates through the material. That is, if there is a higher rate of swelling at the edges of the sample or not. It was noted by a colleague at STLE that this data would be useful. 

The test was done with the sample suspended on cheese cloth, submerged in water. This proved to be a problem because  the cheese cloth was visible through the sample in the images. This means that as the sample swelled, the cheese cloth background did not, and hence there is a distortion in the DIC images of the swelling. It appears as if the sample is not swelling symmetrically around its center point, and this was due to the cheese cloth background. To fix this problem, a white fully swollen HEMA gel could be used as a semipermeable backing to suspend the sample that would not create the lines in the background like the cloth did. However, this test is to be done at a later point in time.  

Project progress (As of 11/3/17)

9/18/17
Tested various methods of swelling dehydrated carbomer to get a vertical plane. 
 • Tried a fame method
      • frame made of rope stiffened with wire
      • Fail :
             • the carbomer does not hold its own weight 
             • without speckles, the carbomer is not visible 

     • Ideas to try
              • hanging methods... toothpick, air bubbles, monofilament.... etc
                     • somewhat successful since it doesnt break, however it is difficult to hold vertical (maybe try this with weighted end)
​              • hanging methods... toothpick, air bubbles, monofilament.... etc
 

9/20/17
Used pieces of line with varying densities as possible items to hang the dehydrated carbomer sheet from. 
      • Allowed sections of different lines to become saturated with water to see whether they float or not
      • With a piece of floating line in the water, draped a piece of carbomer sheet over it
              • The sheet did hang off of the line without breaking under its own weight
              • The saturated sheet did not hang vertically on its own, it does need something dense on its bottom end to hold it up right 
      • Hydrated sheet in water, placed a piece of sinking line on top of it
              • once saturated, the line did sink and brought the sheet with it, making a vertical plane on its way down. 
                   • Concerned that the rate of the line sinking will tear a sheet that is held at the  other end

Put strands of sinking and floating line in pairs within carbomer gel on a piece of glass and let sit to dry over the weekend.
Plan to cut out a section of the sheet that has both a floating line and sinking line in it and hydrate them to see if the sheet will be held vertical and without sinking. 

10/2/17
Successfully placed a plane of hydrated carbomer with black speckles  horizontally within the volume of water in a beaker using a frame made from a paperclip. (see picture on left)
      • The frame was bent in such a way so that it could be hung from the edge of the beaker and hold the plane horizontal.
      • The dried carbomer sheet was wrapped around the frame while dry. The carbomer concentration was 0.07%. 
      • The 0.07% concentration carbomer sheet seems to be flexible enough to bend around the frame without having brittle fracture
      • The 0.07% concentration carbomer is able to stick to itself when wrapped around the frame
      • Problems/things to address:
              • The oped edge of the sheet (the edge not held by the frame) needs to be larger in order to create enough area for the surface probe
                 to probe material  with unchanged material properties
              • The open edge of the sheet needs to be held  up against the edge of the container 

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10/13/17
Met with research group to discuss new ideas:
      • Dissolvable plastic gels on which to place a speckle pattern
      • In hydrated clear carbomer, use fine graphite powder to place a grid pattern in the gel. This would hopefully very slightly increase the stiffness of the               gel when dried. The powder should expand with the gel when placed in water, so the grid method of quantifying deformation could be used.
      • A thin, long strip of dried carbomer could be used. The long strip would expand mostly in the lengthwise direction.  The strip could then be used at the              surface of the bulk gel using a mechanism to take the slack out of the strip that comes with the expansion.
      • A sort of "cotton candy" like material or similar that dissolves in water and does not add any significant stiffness to the matrix could be painted with a            speckle pattern 
      • Leave the dried carbomer sheet on the glass sheet that it dried on and use the flat surface of the glass to keep it planar and place it in the matrix while            on the glass sheet. Once saturated, the carbomer should slip off the sheet. ​​

10/25/17
• 0.1% carbomer placed on glass, and 100nm APS activated carbon, mixed with other 0.1% carbomer was injected by hand into a grid pattern in the gel.
• The gel was allowed to dry completely. The wet and dried carbomer samples  with carbon are shown.
• Upon placement in the water, the sheets saturate like we expect and the carbon strips expand as well. This is good news, as the carbon may be able to be used as a scaffolding. The carbon also provides a significant amount of stiffness to the areas where it is injected.
• The carbon also tends to make the carbomer sheet sink once fully saturated.
• Using paper clips formed into two concentric "U shaped" frames, the carbon infused carbomer sheet was able to be gently pushed into a vertical position and held between the frames.
          • This is a partial success, but the geometry still poses a problem
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• Idea: use carbon as a high density region, and another substance as a low density region and create a sheet of carbomer that will float vertically on its own due to buoyancy forces.
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10/30/17
• idea to use carbon particles mixed with carbomer as sinker and low density powder as float, dry both in carbomer and cut  out sheet.
• Tried styrofoam. Doesnt blend well, can't get small enough pieces, hard to remove water
• Tried mixing other foams, that were sanded or blended into fine pieces and mixed with carbomer. They did not work.
• Fumed Silica: fiberglass thickener. Makes the carbomer very thick, hard to suck up into pipette. Dries hard and does not bind with the carbomer. 
• mysterious brown powder: floats very well, mixes with carbomer decently. It raises the PH of the carbomer and turns it back to liquid. Doesnt expand.
• Other foams, unknown variety, same issues. 
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11/3/17
• Idea to use PDMS that has been highly agitated to add air bubbles into it, laid down on the glass within the carbomer to be used as the "float"
• Carbon strips used as the sinker still
• Upon drying, the PDMS had lost all of the air bubbles that were in it, and was still a sticky mess that did not bond with the carbomer. fail.
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