Memo
Date: February 17, 2015
To: Inst. Trott and GTA Mohini Dutt
From: Joshua Epperson, Brooke Ott, Luisa Parish, and Ben Weisman
Subject: Detection Circuit Lab___________________________________________________
Introduction
During the microfluid lab, two different experimentation methods were performed to
look at fluid flow and capillary forces. The capillary forces and fluid flow was examined in
order to determine the movement of liquids in a chip. While another section of the procedure
examined the capillary forces of water in the capillary tube. In the following memorandum, the
results of the experimentation are presented, then the results are analyzed and discussed, and
finally the NTM Questions are answered.
Results
A sample calculation for pressure change is shown below assuming a liquid density of 998.2
kg/m3, gravitational acceleration of 9.81 m/s2, and a change in height of water of 0.0050 m.
∆𝑃 = −𝜌𝑔∆ℎ
(1)
∆𝑃 = 𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
𝜌 = 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑞𝑢𝑖𝑑 = 998.2 𝑘𝑔/𝑚3
𝑔 = 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑑𝑢𝑒 𝑡𝑜 𝑔𝑟𝑎𝑣𝑖𝑡𝑦 = 9.81 𝑚/𝑠 2
∆ℎ = 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 ℎ𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑙𝑖𝑞𝑢𝑖𝑑 = −.0050 𝑚
Figure 1, shown below, illustrates how pressure changes by varying the diameter of a capillary
tube. Using the equation of the trend line created by the data points, the change in pressure of a
capillary tube with a 200 micron diameter was extrapolated and calculated to be 33 kg/m-s2.
Change in Pressure (kg/m-s2)
Change in Pressure vs. Diameter
100
90
80
70
60
50
40
30
20
10
0
y = 17615x + 29.523
0
0.0005
0.001
0.0015
0.002
Diameter of Tube (m)
Figure 1: Change in Pressure vs. Diameter for Capillary Tubes
Discussion
0.0025
This paragraph discusses the effect of channel size on the flow in the channel. If the
same amount of pressure is being pushed into the channel from the syringe, then the flow would
be faster in a smaller channel compared to a wider or deeper channel. This occurs because the
same amount of volume in going through a smaller channel, but in order to get that same amount
of liquid to the detection well, for example, the channel that is smaller has to have a quicker
flow. This shows that check valves should stop liquid flowing back up other staging wells
because the liquid in the channels flow faster than in the check valve because the check valves
have more volume and the flow of liquid in them is slower.
The result of the lab show that the channel with the 90 degree angle with a check valve
worked the best. However all of the channels met at one point before going into the detection
well with was an issue with back flow into the other staging wells. If the channel ended in the
detection well then the back flow into the other channels may have occurred once the detection
well was filled but since the channels met up at one point the other channels got filled before the
detection was. This happened with all of the staging wells regardless of the slight suction at the
end of the waste well. The channel that had the straight channel path moved very quickly and
the liquid had to be put in slower compared to the curved or 90 degree angle channels.
This paragraph is going to discuss ways to improve the procedure of the lab was well as
the chip design. To improve the chip the channels should end at the detection well and not meet
up together before going into the detection well. In Lab 1 the wells that worked the best were the
ones with a slice taken out in the staging wells. This should be incorporated into the chip design
to make it perform better. To help stop the back flow into the other channels if it was completely
possible to plug the staging wells that were not being used this would help the fluid from not
entering the channels leading up to the staging well and the staging well itself. Another way to
create this would be just to have one staging well so there are no other places for the fluids to go.
Also in theory if there was more than one check valve this would help stop the liquid from going
up the channels into the other staging wells. Also with the slight suction at the waste well and
the injection of the fluid at the staging well both should be a consistent rate. However, if the
suction was too quick the fluid would not fill the detection well completely and it would go
directly into the waste well. If there was a way to have both flows be equal then that would not
only fill the detection well but also stop the back flow.
NTM Questions
In detergents, surfactants are used to help dissolve substances that are organically
hydrophobic. Surfactants assemble into micelles that bond to the hydrophobic tails of the
molecules, thus allowing the hydrophilic heads to interact with the water molecules. Once the
bonds between the hydrophobic tails are broken by the micelles the water molecules are able to
separate the hydrophobic molecules and thus the oily substance is dissolved in the water. When
the triglyceride is placed in pure water the hydrophobic tails maintain their bonds and
triglyceride molecules are much less soluble.
For this problem the number of molecules must first be calculated. This is done by
dividing the total surface area by the area covered by one fatty acid molecule—yielding 1e13
molecules needed. Next, the number of grams must be found by dividing the molecules needed
by the amount in 1 gram. This gives 5e-9 grams. Finally, the density is divided by this number
to find the volume in mL. Therefore, 6.25 nL are needed, or 6.25e-6 mL.
Conclusion
In conclusion, this lab helped look at fluid flow on a small scale. The pressure was
calculate of the capillary tubes in order to determine there was more pressure in a smaller
capillary tube. This means that the fluid is going to flow at a higher pressure as the channels in
the Lab-On-A-Chip get smaller. From testing the sample Lab-On-A-Chip, it was determined
that the channels should not connect before going into the detection well otherwise the fluid
flowed into other staging wells.
During the lab, several difficulties were encountered. During the cleaning procedure, it
was difficult to assemble the chip with proper alignments using tweezers. Another difficulty in
testing the fluid flow was created the suction in the waste well while inserting the fluid into the
staging wells. The fluid continually flowed back into the staging well before entering the
detections wells. This was a reoccurring problem for all of the staging wells.
Appendix:
Table A1: Capillary Testing Data
Capillary A: Inside Diameter = 2.26 mm
Trial 1
Δh: -0.0069 m ΔP: 68 kg/m-s2
Trial 2
Δh: -0.0091 m ΔP: 89 kg/m-s2
Trial 3
Δh: -0.0052 m ΔP: 51 kg/m-s2
Capillary B: Inside Diameter = 1.03 mm
Trial 1
Δh: -0.0031 m ΔP: 30 kg/m-s2
Trial 2
Δh: -0.0065m ΔP: 64 kg/m-s2
Trial 3
Δh: -0.0050 m ΔP: 49 kg/m-s2
Sample Calculations:
Change in
Pressure
∆𝑃 = −𝜌𝑔∆ℎ
∆𝑃 = −(998.2 𝑘𝑔/𝑚3 )(9.81𝑚
/𝑠 2 )(−0.0050𝑚)
∆𝑃 = 49 𝑘𝑔/(𝑚 ∗ 𝑠 2 )
Change in Pressure= (density)*(gravity)*(change in
height)
(1)