Experimental Group

Experimental Group

Lucas Lamb

Andrea Roeser

Chen Shu

Figure 1: Setup Diagram

Setup/Procedure (See Figure 1) -

Four equally sized posts (G) are screwed into the optical table (F) at an appropriate distance to support our Hele-Shaw cell.
A white wax paper background (below H) is placed on top of the four postsand a small hole is cut out in the middle of the paper. This will be used to provide color contrast for the camera recording the moving fluids in the Hele-Shaw cell.
A small amount of glycerol (B) is poured (roughly 1-inch in diameter) in the center of a plastic panel with the hole in the middle (H). The hole has been taped to not allow the glycerol to seep out and spacer strips (between I and H) (.32 mm width) are placed on opposite sides of the panel.
The second panel (I) is smoothly placed on top of the first panel with the glycerol so that the fluid flattens and spreads out with no air bubbles.
Four small plastic clamps are placed around the constructed cell to hold the fluid in place, with the spacers still providing some separation between the panels.
The cell is placed on top of the wax paper, resting on the four posts, hole-side down.
A ruler is placed on two posts close to the cell to reference a scale of measurement.
A syringe and tubing (E) is clasped on a stand nearby to the cell and 1.2 mL of green-dyed water is drawn into the syringe.
The needle at the end of the syringe’s tube is placed through the hole in the wax paper, and then securely placed in the already-made hole in the bottom plate of the cell.
After a camera is setup on the stand directly above the cell (A) to record the fluids’ motion (we were using an iPhone), varying masses (D) are placed on the syringe’s platform to apply different constant pressures for injecting the water (C) into the glycerol in the cell.
Videos and images are taken for different masses. See Parameters below for specifics.
The process is repeated for various other spacing sizes and for the Non-Newtonian fluid (polyethylene oxide, PEO) too.

Setup JPG

Figure 2: Setup from Lab

Parameters –

Volume of injected water remains constant (1.2 mL)
Cell spacing is kept constant at first (0.32 mm), varying the mass placed on the syringe (500 g, 1000 g, 1500 g, 2000 g)
After these various injection pressures, the cell spacing is changed to 0.52 mm and 0.83 mm respectively.
After these parameters have been accounted for, the fluid is changed from Newtonian to Non-Newtonian and the process is repeated.

NOTES: The masses for the Non-Newtonian were 125 g, 250 g, 500 g, and 750 g. Also, though the injected volume remained constant, the masses were varied so the flow rate for the injected water also varied. See the data table included in the “2015” folder on Google Drive for the flow rates and a full breakdown of the varied parameters, as well as all of the experimental videos for each trial. There were issues with calculating most flow rates; see “Experimental Error” below for a full description.

Results–

Discussion –

By increasing the mass for both Newtonian (Figures 3-5) and Non-Newtonian (Figures 6-8), the number of fingers extending off of each primary branch decreases, essentially “smoothing out” the branches. This is much clearer with the Non-Newtonian images, particular with the 0.83mm spacing in Figure 8. Also, when changing the fluid from Newtonian to Non-Newtonian, the space between the main branches increases, or in other words the number of main branches from the center decreases. This can be seen by noting the blank space within the images. The other groups’ projects will go into more mathematical detail regarding how these variables change the images.

Experimental Error -

There are many ways errors can occur from this experiment. When placing the masses on the top of the syringe, the syringe has a tendency to tip a little so the person putting the weight on might not put the full force of the mass onto the syringe the whole time while attempting to steady the mass. The syringe used also might have small air bubbles that could cause the amount of water injected to vary for each experiment, and the markings on the syringe are not that precise, so that might cause more discrepancies. Another problem that has occurred is that if the syringe is secured too tightly to the stand, the water will not be pushed into the cell. This might cause the massesto not have the correct effect – they are exerting enough pressure to overcome the pressure of the glycerol, but because the syringe has extra forces acting on it, the experiment will not work correctly. The needle also has a tendency to become clogged with glycerol or PEO, so it is imperative to clean it out after every experiment to ensure only water is in the needle, tube, and syringe. It also seems like either the cell itself is not perfectly level (due to the table not being level), or somehow the spacers are not all the same width because when the experiment is over, the water tends to go to one edge and drain out, meaning that there might be an extra component of gravity that is acting on the cell while the experiment is happening. There may have been a potential issue with the last day’s trials of PEO, as the fluid wasn’t heated and stirred before use like the previous days. No differences were clearly seen but it is still worth noting.

One major source of error is with the calculated flow rates. Besides the final three trials taken on 5/4/15 (when the problem was realized), the videos were all recorded using an iPhone’s slow motion feature for better quality. Using video editing software, the time for the water to be injected into the cell was measured by seeing how long it took for the water to stop spreading in the cell. However, because of the slow motion feature, the timing did not correctly transfer to the video editing software. This means that almost all of the times in the Parameters’ Spreadsheet were noted to be longer than they should be. This makes the flow rates in the spreadsheet smaller than they should be as well. While corrections will be estimated in an attempt to fix this problem, those looking at this data in the future should note the ‘-‘ signs next to all the trials with error in their flow rates and ‘*’ signs next to the few correctly calculated ones.

Continuation –

Working off of last year’s project that used the optical table as the background for the videos, we made an improvement to the experiment by creating a white wax-paper-covered background to increase contrast for our video recordings.

We learned after a few failed trials that the size of the hole in the plastic plates is very important. After trying to create more plastic plates for experimental efficiency, we found out that the hole in the middle of the new plates was too large and the needle was moving around during the trial. That movement made the injected fluid very inconsistent and caused some issues with some trials. Also, as mentioned in the Experimental Error section, it is best not to use the slow motion feature on the iPhone video camera, unless the user is fully aware of how to break down its timing. The slow motion created error in the calculations for the fluid flow rates, so it would be useful for future experimentalists to be aware of this issue.

All videos and data can be found in the shared Google Drive in the “2015” folder.