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Firstly, in part b how is elution time connected to Qt. Is it (1/Qt) * V of reactor? Or something else.

Next in part c can you explain how the scaling laws would work. Are we altering length or diameter. I suppose we keep length constant and continue scaling with D in order to reduce elution time.

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Hi Haddy! could you please provide a model solution to Q1 bi of that paper please. Do we use q1+q2+q3=0 and solve for the temperature difference?

Thank you !!

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Hi Hadiza, in response to an earlier question, you said that the length was calculated to be 15.cm (for part ii). However, to calculate Re, should we not use 2w instead of w. If w is defined as the distance from the channel wall to the interface, then the diameter of the channel to be used in the calculation of Re should be 2w should it not? This would give a length of 0.079m.

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Good Luck in your exams!!

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As mentioned in the lecture recordings and relevant places, you will remember the formulas Qeo and Qp represents the expanded formula below. The highlighted formula is haagen poiseuille right?
I hope this answers your question.

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Only if the value falls too closely to the 2 boundaries

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Yes

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Please can you explain why it is that we neglect q3?

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Look at my explanation about the ranges on slide 6 and 7 in the response for "Slide 6 slip regime" above.
I did not select continuum, I said the continuum flow model can be used because if you look closely using the graph on slide 6 and 7, the value falls close to the border of both regimes (slip~0.01-1) and continuum (< 0.01).

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No, you cant use the continuum model for the slip regime except if the number falls too closely to the border of the regimes.

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Regarding the regime ranges, there is difference in the one on slide 6 and 7. Slide 7 has the graph and the picture at the bottom, only the Free Molecular Flow region has the same range for all 3. I will just advise you adapt one and use it to translate your result (different ranges from different sources). I will choose the graph if I have to select one.
For the class example, the whole calculation will change because I didn't convert the pressure from 1mTorr but rather used 1Torr.
For the regime choice as slip flow, most probably, I used the ranges from slide 6.
The kahoot question is correct. Why do you think it is wrong? Looking at the ranges from slide 6 or 7, continuum equation only apply at less than 0.001 or 0.01.

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Hi Haddy, thanks for your reply. Please can you explain why hagen-poiseuille still applies as in EOF we don't follow the hagen poiseuille flow rate equation? The flow rate of the EOF pump Q=Qeo+Qp ?

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Can you use the continuum NS model in the slip region?

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Can you clarify what exactly your question is here?

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Yes, use the formulas on side 8 of L5 to approach the question.
Remember, the EO pump is also in a channel, so Hagen poiseuille applies.

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You are right about potential growth of the particles and no need for quantitative proof. What the question is asking is what will happen base on an increase in the channel length.
Ans: This will allow interfacial reaction and the particle will continue to grow until it reaches the end of the channel.

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All questions without an answer is because Dr Shahid has answered them in the Q&A session

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Like I have mentioned, we always scale with respect to the scaling factor.
In my example, our scaling factor was radius and you mentioned the textbook's scaling factor is flowrate. Obviously, we will get different answers.

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- Yes, use Pi.r²
Qeo = 1.48x10³ nL/sec
Peo = 546.5 Pa

- base on the value of Qeo, we can use it for dosing and analytical applications

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All questions here were addressed during LOIL session

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Please can you explain how you would get the velocity of the cooling fluid to calculate Re without using the overall heat exchanger flux? You mentioned above that you wouldn't use h3 but please can you then explain how we would relate any of the information to the cooling fluid?

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Hi Haddy, please could you go through what method we would use to answer this question? I know people have asked similar questions above but I'm still not clear on the method. Would we be able to follow slide 8 lecture 5 when it asks for the scale up effects on h and L in an EOF pump? Even though in the lecture this was assuming just (Haagen-Poiseuille) pressure flow and in the exam questions we also have to consider EO flow?

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Hi - what would happen to average nanoparticle size if the channel length is increased? I don't believe I've seen an equation for particle size anywhere. I know Td > Tp so more mixing would occur so potentially larger molecules but I can't quantitatively prove this.

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I've read the above responses regarding q1, q2 and q3. Please could you clarify, in the past paper you are saying we should ignore the conductive heat flux (q3) and this was the same in your example problem from lecture 5. Could I just confirm, is it always conductive heat flux we should ignore, and convective we should include? Also is there a textbook chapter or any material we could refer to for more information explaining why? Thanks.

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Hi, you state that pressure drop scales with l^-2, however, the microfluidics textbook states that it scales with l^-4. Is this because you have derived it at constant velocity while they have derived it at constant flowrate? Thanks

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Please could you go through this question in the revision session or please could you help clarify on here?

In the above response to "hydraulic diameter" you state that to find the height we use the below:

"Next is to substitute for h in the equation for TD (i.e. TD=(h^2)/Dh)"

Which suggests that you're using the hydraulic diameter on the denominator, Dh? Where has this equation come from? As in the notes TD=(w^2)/D where D is the diffusion coefficient? Also in this question it already gives the width, so can the diffusion time also be related to the height?

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For the 1st part, we are required to calculate the cross-sectional area
of frit. Is that just pi(a^2)? Also can you give the numerical answer to this question?

Also, for part ii, what application can we use the pumps for? I can't seem to find the relevant information in the notes.

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In the exam papers, flux is being used in so many different ways. When I read a question and I help to answer it, I go with the context of the question and not generalize what was meant by flux as a parameter.
In your case, please state the question number and I can answer it correctly without assuming. I thought you were asking which is closer to flux (vel or Q) that is why I said it is better to use velocity.
The answer to 2017/18 Q1 c(i) is numbering up parallel channels to achieve 1000x higher flux.

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No, not this one.
Qtot=0 means Peo max. The profile you have here is EOF with no pressure gradient (written on the far right of your image).

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Qv means total volumetric flow while qv is volumetric flow in each channel.
which means Qv and qv are the same thing. only that when you sum up a number of certain number of qv it gives you a total Qv.
So! what do you mean by keeping one constant and changing the other?

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I have uploaded solutions to practice questions under the lecture folder

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You can find your answer in the Q & A recording on Moodle.

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It is a suggestion, you can design yours differently and justify it.

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Loading Effect is the dependence of etch rate on the quantity of material being etched

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This slide is just to help you know the concept and I am sure slide 23-25 of the lecture has an elaborate explanation to what plasma is including a video to explain further. watch it again, as nothing was skipped.
Plasma is a partially ionized gas composed of ions, electrons and other neutral species in ground and excited states.
To explain further, Sheath field is the cathode surface and this is where the substrate to be etched is placed.
The glow region of the plasma is the part that has a constant positive potential.

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I have uploaded them now

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Yes, it is

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EOF Mass Flux Scale Up Follow Up Question

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EOF Mass Flux Scale Up Follow Up Question

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Hi Haddy,

I have read through all the responses here regarding scaling the EOF mass flux and there seems to be a slight contradiction. In some responses you suggest that this can be achieved by numbering up, while in others you stated the mass flux refers to the fluid velocity. However, the velocity could not be increased by numbering up (instead we would need to reduce the diameter as u proportional to 1/d^2, or increase the electric field).

Please could you confirm: when we are asked to increase the mass flux of an EO, which of these approaches is correct. Thanks.

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Sketch the velocity profile in a single channel for Qtot=0.

What type of velocity profile are they looking for? Is it this one?

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I think we have to narrow our thinking to microsystems and use what was taught in this course.
Please I will keep repeating, stop importing your knowledge of macrosystems into microsystems. Some will work and some wouldn't. Just go by what was taught and agree this is how these systems work!

d) How to approach it is still the same as my explanation above and as we did during the lecture (See solution sheet on Moodle).
I have corrected this question in class as I believe what the question is sought is t₁ and not t₂ because you end up with a higher value than the inlet. (Pls, see column 4 here under Heat Transfer and my response to the spotted error of this question)

e) You are only asked to calculate Reynolds number and discuss the implication of your answer on the column design; why are we talking about overall heat transfer eqn?

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Your calculation is right. Just use this together with the instructions of the question to calculate your new residence time.

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Surely there are some occasions where the overall heat transfer coefficient needs to be taken into account? Please have a look at 2016-2017 and advise us on how we could solve part (d) without deltaTLM and part (e) without overall heat transfer eq.

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For minimum channel length, I get 15.8 cm. Surely this is too high for a microchannel? Not sure where I have gone wrong as the question is asking us to use the reaction rate coefficient provided, so our tau_P is fixed at 50 s.

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Because we are now looking at it in terms of volume and volume relationship to characteristic length is l ³.

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See column regarding this topic (EOF) and column for interaction to see how you can answer this question particularly and some other useful tips to scaling questions.

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It depends what the question is asking.

The formula you are choosing is correct, just try and continue to reason around your chosen formula and you will gradually have a grasp of how to approach the scaling questions.

Hint: They usually have short and straight forward answers.

Your answer to this question is above.

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Hi, in the 2017 / 16 the question asks how we can use scaling laws to reduce the elution time. Is the correct approach to assume that the flowrate remains constant and to reduce the channel diameter to increase the velocity. Since the residence time = L/u this should reduce the residence time I think? Thanks.

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- By pushing (flowing) fluid using voltage controls, you are using it as a pump.

- The back pressure is maximum when there is no flow (Q=0) L7.2 slide 16

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1) Refer to my response HAA_2017/18 Q1)bi above

q ₃ (conductive heat flux) is what you are leaving out. Pls,  understanding the entire question is key to knowing what to consider or ignore.

2) For your second question, i will take the former i.e. velocity

- Look at question regarding this problem above "HAA_EOF Pump and Scale-up"

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Yes, the answer to Q)ci is 1.6 sec

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Which paper do you need help with?

The questions covered in your IPT are also past questions and some of your LOILs too. And these were randomly picked in no particular order.

If you look at questions asked either in the lecture column or here in the interaction column, you will find answers to questions asked about past exam EOF.

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I am assuming this is the 2019/20 paper Q)di.
Pls, it will help me address questions more quickly if you point to the right place to look for your question and answer properly.

When you are asked such a question, it means you should calculate the minimum channel size (w=width) where you will get a good mixing/diffusion.

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-  A contact angle of >90 degrees means non wetting behaviour and this means  there wont be a reaction at this interface.
- The aqueous phase would concentrate in the centre of the channel while toluene would coat the channel walls. A smooth interface would no longer be achieved and this will require the redesign of this reaction.
- However, looking at the given conditions, it would still be possible to produce nanoparticles but with a large size distribution due to the nonhomogenous interface between the two phases.

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Yes, it is.

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- Yes, when the channel is rectangular, you are expected to present your diameter as hydraulic diameter.

- You are also right about equating Tp = 1000 TD  (It is already given in the question)
Then you calculate TD from your Tp (because you have all the parameters to calculate Tp )
Next is to substitute for h in the equation for TD (i.e. TD=(h^2)/Dh)

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To use the overall heat exchanger flux is addressing it as a macrosystem because in macro heat exchangers, we use the LMTD (Log mean temperature difference) because looking at it diagonally/cross-sectionally, the heat at every point won’t be the same-right?.
In micro channels (in this case micro heat exchanger), it is not the case because it is instantaneous heat transfer due to the length scale.

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Yes, I did mention that. I will upload them and email the class when I do.
Sorry I forgot!

I have uploaded them now.

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Q1) Assuming no other heat sources or sinks, or heat losses,
b(i)  Calculate the increase in temperature across the channel generated by the applied electric field, with the inlet fluid at room temperature.

q₃ is not relevant for solving this question. You only need q₁ and q₂

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I thought my exams will be the 2020 paper? Have you seen it already?? :D

I will go for the latter, number up and hence voltage will remain unaffected.

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1) I think the confusion about scaling laws is that you are over thinking it!
To scale-up in microsystems,  the approach is usually numbering up; except when you are asked to show how a parameter will be affected (which will be clearly asked if that is the case, therefore, you don't need to do any calculations).
So! for the answer to this exam question see "HAA_EOF Pump and Scale-up" above

2) 2017/16
First, every question should be approached based on what the question is asking. You can apply numbering up if the question warrants that.
In the case of Q.1c, the formulas you are using don't even relate to the question asked except at the end of option A where you stated the right formula and then you mixed it up again.

The right approach:
Q1c) Using scaling laws, discuss how you can reduce the elution time of analytes in electro-chromatography.

Note: t means "tau"

t=L/uEO  (Lec 4, slide 6)=L/uEOE   (Lec 7.2 slide 7) where u here is "miu"  viscosity.

In terms of scaling laws t~l ² and  t~(1/E)  where I is the characteristic length of the system. This means that elution time can be decreased using shorter columns and/or higher electric field .

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I just had a look at the 2017/18 paper.
In this kind of situation, it was just an application question asking you how to scale-up in order to provide 1000 times higher flux.

The simple answer is to number up! or write a sentence like "a 1000x higher flux can be obtained by having n = 1000 parallel channels"

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Yes, the formulas in this question can be found in lecture 4.

As for the reaction kinetics, k and e are constants added to your equation, while k is given to you, e is just a mathematical constant (and can be found on your calculator).

The questions are simple, the added constants are just there to test your understanding of the applications of these topics.

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You are getting things mixed up.
How are you able to see flow from a 2D diagram?
Explaining its use as a valve is what you just did in your write up above.
Flow starts when you set it to.

NOTE this point!
It is within your control how you want to use EOF and pressure-flow depending on what the application requires. Just a pump or a valve or for mixing etc.

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Hi Haddy, thanks for the above response but I know there is no actual valve. I asked how it works as a 'valve' as there is still EOF flow to the right? In the lectures you say it works as a valve and there is no flow when back pressure = EOF flow, but in the velocity profile, there is still flow going towards the right near the walls. So surely it can't completely shut off the flow when working as a 'valve'?

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I have uploaded the solutions to the IPT questions.

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I fail to understand what the actual question is here.
Can you point out what exactly your question is?
I tried to understand it but all I can say is I am confused by this statement " wouldn't having a lambda D larger than the channel". How can Lamda D be greater than the channel?

Pls, if there is a typo, can you send back a reworded question? or just clarify to me what the question is.

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The solution to IPT Questions can be found on Moodle

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Contact angle is looked at in 3 ways (slide 5, like you mentioned). If you want to relate this to the slip/no slip behavior as explained in Lecture 3, then that will depend on the material used for chip fabrication. The application of an electric field to a hydrophobic material, will change its contact angle from slip to a no slip.
Example, Teflon is hydrophobic (non wetting), this contact angle of greater than 90 degrees can be lowered by applying an electric field (electrowetting) to change its behavior and make it less hydrophobic and therefore allowing the fluid to slid on it.

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Everything you mentioned is absolutely correct!
Let me add; why the velocity profile is the same is because the surfaces have different charges, one being positive (slide 13) and the other -ve (slide 9) thereby influencing the flow direction (via charge distribution in the channel).

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So! there is no actual valve. It is just being manipulated in the way that it does the function of a valve (control flow).

Example, if you match up the flow side (voltage) and the back pressure, there will be a stand still. You can now lower either side to allow flow in your desired direction.

So!, it is within your control how you want to use EOF and pressure-flow depending on what the application requires. Just a pump or a valve or for mixing etc.

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I don't know how many times I will say this, I will only cover what is in the scope of this lecture and not other courses.
Please contact the lecturer responsible for TP2 to explain things to you if you need assistance with it :)
- Btw, which lecture is this? 7.1 or 7.2?
- Just to reiterate, there are no derivations in the exam!

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Done!

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Done already!

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1) Q2c, Ok thank you. You can convert and adjust the answer appropriately. I will note it too.

2) Yes - Qout~l
what makes it efficient for the wall thickness of 10um is the thinness of the column wall making it able to transfer more than the generated.

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Because the first one is relating deltaP/L to Q while the second part is relating it to the radius.

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I only have answers to my exam questions.
Sorry!

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Hi. I am still a bit confused about the relation Qv~l^3. from the equation we have that Q~l. why is Qv related to l to the power of 3 and not just to l? Thank you.

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Refer to Revision class (recording) and where I responded to answers to padlet questions.
If you want me to explain after listening to that, drop me a message here again and refer it as "7.2 Q2 again"

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The charge on the wall surface will influence the distribution of charge within the channel and hence directing the flow profile to the right terminal.

See my response to a question titled "Problem Part 7" in this column. It should help explain how these work with wall charges further.

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1) Your reason to why the velocity profile is to the anode is correct. It is because the wall channel is positively charged and hence the dominating/net charges within the channel will change and so will the direction of flow to the right terminal.
2) I am not sure I understand what you mean by the adjacent profile. Is it the pressure side? This notes will be adjusted properly for future because I can see assuming some things are straight forward actually isn't.
So! to answer your question, that will be the pressure profile if at all there is one applied in the opposite direction; but in this case, none is applied and hence no change to the flow side, while on slide 15, where pressure is applied, you can see a change on the flow side.
3) Let me know what slide you are talking about so I can respond properly.
But, if we are talking about velocity profile of the pressure side, I am sure you remember hagen poiseuille from your TP, for the flow side, when the flow is fully developed (see slide 9), then the bulk will be dragged by the diffuse layer, hence why there is movement there.

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1) Yes, you are right about Zeta x Epsilon value.

2) Qmax is right too but we are using different units-ok!

3) Your Pmax is more correct than mine. So! stick to yours.
Thank you for pointing this out.

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The monolayer is a very small percentage of the entire channel and the channel itself is quite small (microsystems are in micron to nanometers). So, it will be overcome by the drag.
You can find more explanation in the email I will send out tomorrow.

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Part 7 has 7.1 and 7.2, Lets try and be specific to make response faster for the future.
7.2 (Q2) has i,ii and iii. Which one do you want me to explain?

Pressure Flow profile:
Refer to slide 16 of Lecture 7.2,  Q=0 means maximum pressure which means the pressure side will have the parabolic shape. The EOF movement is dependent on the net charges of the channel.

The charge on the channel dictates the direction we will get EOF. So if the channel is -ve charge, then the net flow will be due to accumulation of +ve charges towards the cathode.

If the channel is +ve charge then the net flow will be due to accumulation of -ve charges towards the anode. So just by changing the electrode charge with same channel, you can control the direction of flow.

I hope this is clear. Let me know :)

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All questions were addressed during revision class.

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In 2017/18 1 b i) should we balance q1 + q2 + q3 or ignore convective effects as we have in other examples? How do we know when to use conduction or convection? In the same paper, when it says scale up the flux does it mean volumetric flux (velocity) or volumetric flowrate. Thanks.

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please could you provide us with a model answer to the questions regarding electroosmotic pump in one of the papers? They are much harder compared to the ones we covered in the LOILs and in the IPT
thank you

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Is the answer to part Ci) 1.6 s?

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Is the whole of part d part of our course?

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If you have an aqueous phase & toluene through a PTFE channel, and the aqueous phase has a contact angle of greater than 90 degrees, what happens to the two phases? Could there be reaction at the interface?

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In past paers, it often gives the width, depth and length of the channel for rectangular channels. Do we need to calculate an equivalent diameter? In the 2015-2016 paper, it asks us to find the height of the channel. To do this, do we need to equate tau_P=1000*tau_D and find the equivalent diameter, then say D=4A/P=2*w*h/(w+h) <-- the equation for hydraulic diameter?

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Y Junction Microreactor - Past Paper Questions

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Y Junction Microreactor - Past Paper Questions

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Please can you explain what the question means when it asks to calculate the minimum channel size which there are no transport resistances at the interface?

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In this example, please can you can explain why we can just equate the heat generated to the convective heat flux? As we also have the conductive heat flux into the hot channel from the cooling fluid so don't we have to use the overall heat exchanger flux?

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Please could you explain how this actually works as a pump? And how the max back pressure = the max pumping capability of the pump?

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Hi Hadi, am I right to think that wet/dry etching are not exactly fabrication methods when considered alone, as these are the methods involved in lithography/photolithography to etch away silicon/glass under the photoresist masking layer?

So a possible fabrication method would be e.g., photoresist + wet etching, instead of just wet etching?

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Not sure whether to take the conductive heat flux into account? How is this relevant?

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I think in the revision session you mentioned extra problem sheets, please could you upload these? Scaling law questions would be especially useful.

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Please can you explain how we should approach past paper questions that ask how we can increase the flux of an EOS with scaling laws. Could we have a similar example?

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In question 1 d i and ii is the equation for particle production in our course? If so where and what is e in the equaiton -k*tp=ln(1/e)

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For the scaling question, is the below correct, applying similar logic to lecture 5 slide 8,  or should we approach it more simply? - I don't think you can apply numbering up to this question.

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For the 2017/18 scaling is this working correct? My thoughts were volumetric flux = velocity, and the obvious way to increase velocity at constant flowrate is reduce the diameter. Could we have a similar example?