Caltech

Introduction to Microfluidics

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Hi again,

The question that most desperately needs answering at this point seems to be: why is a Caltech applied physics major doing microfluidics research at Stanford this summer?  The answer to that question will be the topic of this post as a segue into my current work here at Stanford (which will be described later).  It is a little heavy on background and technical details, so bear with me, and feel free to ask for clarification in the comments.

During my sophomore year (over a year ago), I decided I wanted to do a SURF (Summer Undergraduate Research Fellowship) abroad.  One of my favorite features of SURF program at Caltech is that it allows us to work at pretty much any institution we want, so long as we can find a mentor interested in working with us.  For the most part, SURFs away from Caltech are arranged like any other SURF; prior arrangements with a mentor are required to receive the fellowship.  However, Caltech also offers a few special foreign exchange SURFs: to Iceland, Hong Kong, India, and Singapore, plus France, where students apply to the program, and find a mentor once accepted to study abroad.  I applied to the program for both Iceland and Hong Kong, and was awarded the SURF for Hong Kong.  (I was the only student to apply for the Hong Kong program, so I recommend that anybody interested should definitely apply when the time comes!)  I found a professor, Weijia Wen, with whom I wanted to work, and he accepted me into his lab for the summer.

Picture: me holding two firedragon fruits at HKUST

Professor Wen’s “Smart Materials Lab” specializes in both microfluidics (the study of fluid flow in very small channels) and electrorheological (ER) fluid (a type of liquid that changes its viscosity depending on the voltage applied to it).  I had thought I wanted to work with ER fluid from a chemical perspective, but I discovered that he was doing something even better in his lab; he and his grad students were working on ER as a precision control mechanism for microfluidic devices.

Microfluidics is a relatively new field that has developed greatly in the last ten years.  The point of microfluidic research is to create functional Labs on Chips, functional biological or chemical laboratories that fit in the palm of your hand, and require only nanoliters of chemicals to run.  Already, devices exist that cut the time required to perform PCR (polymerase chain reaction, a process to replicate DNA) by 66%.  Yet these chips are inefficient, hard to control, and difficult to use, due to the fundamental problem of microfluidics: it’s hard to control fluids in 100 micrometer (and smaller) channels!  The rules that govern the movement of fluids at large scales just don’t apply in the microfluidic realm; whereas on human scales, the inertia of the fluid is a driving force for fluidic movement (think of the wake of a motorboat or the turbulence of a river), at small scales, the viscous force dominates.  For a good description of the difference between the macro and the micro fluidic worlds, read Purcell’s lecture “Life at Low Reynolds Number”

http://jila.colorado.edu/perkinsgroup/Purcell_life_at_low_reynolds_number.pdf

(Purcell is near and dear to the heart of many Caltech students; his textbook is required for the Physics 1b and 1c analytical track, and is one of the best Electricity and Magnetism “introductory” textbooks out there.)  In short, if you want to control a microfluidic device, you have to come up with completely new control mechanisms to do so.  The techniques worked out for macrofluidics for making transistors, diodes, inductors, etc. will fail in microfluidics; very little that works at large scales still applies when you make a device small.

There are a handful of good solutions to this problem that are currently under development, and two major ways of thinking about microfluidic flows.  In general, flows can be continuous streams of fluids, or can utilize discrete droplets (“digital” microfluidics).  As an example, think of water droplets suspended in oil as they travel down a channel.  This view is clearly ripe for thinking of droplets as 1’s, and their absence as 0’s, thus creating a microfluidic complement for electronics circuits.  The biggest difference between electronic and microfluidic circuits, though, is that the droplets in microfluidics are useful for something other than serving as a bit of information; these droplets are inherently useful, given that they can contain chemical, biological, electrical, and optical information within a single bit.  Although this is a useful feature, it doesn’t mean that microfluidics will be replacing electronic circuits anytime soon (or ever); your computer will still be electricity-based for a while.  So while digital microfluidics won’t turn your computer into a chemical laboratory, it CAN turn a chemical laboratory into a simple computer.  At least in theory…and that’s where I come in

Picture of a digital microfluidic device, from Manu Prakash’s paper in Science Magazine 

My work deals with control mechanisms for digital microfluidics.  My lab mates in Hong Kong had already figured out how to make highly precise droplets (1’s and 0’s) using electrorheological fluid.  In the following picture, they use the ER mechanism and a computer to spell out HKUST with electrorheological fluid:

 

The question I decided to solve was whether we could use the 1’s and 0’s generated by their techniques to perform logical operations.  I started out by asking: can I take a signal of droplets and generate its inverse (the NOT function)? The answer turns out to be: “very yes;” not only can we make NOT gates, but also a “Universal Logic Gate” as well, a device that can perform all 16 two-input logic functions with a little reprogramming.  I figured out that last one with about a week left in Hong Kong.  However, by the time I got around to building it, my time at HKUST had run out (an inconvenient typhoon on my last day didn’t help), and I couldn’t test my final product.  However, the mathematics work, and so did all previous designs that I managed to build correctly (I had about a 5% success rate in building my microfluidic chips).

Dr. Wen's Smart Materials Lab

This is where my research was left for almost a year.  Over the course of my junior year, I occasionally sketched out new ideas, but I only had vague ideas about how I was to proceed once I finished testing my universal logic gate.  Even then, my logic gate is not that useful by itself (although it has dramatic potential).  The gate itself is like a switch that responds to a droplet that activates in response to a droplet.  What I really needed to do was figure out how to make the switch DO something; to use an electronics analogy, I HAD a switch attached to a small test bulb (to tell me whether the switch was on, or off), whereas I WANTED a switch mounted to the wall and controlling my fancy chandelier.  And to do that, I knew I’d have to overcome some rather formidable technical barriers, barriers that are the focus of my summer research.  To put it colorfully, I had 99 problems, but a Switch ain’t one.

Then, at the start of Spring Term, my work started to pick up pace.  I had known for some time that Dr. Quake’s group was one of the few places where I could do anything useful with my project, and a helpful contact in the Caltech Technology Transfer Office, Fred Farina, put me in touch with Dr. Quake, who then accepted me into his group for the summer.  Then I took Shuki Bruck’s fabulous IST 4 menu class (Menu classes are required for all Caltech students, as a way of examining areas of science we ordinarily wouldn’t explore as part of our major), where I began to study the formalities of computer logic.  I’m quite lucky in this regard; the CS (Computer Science) majors figured out most of the theory needed to effectively utilize simple computer logic almost 50 years ago.  I know, in general at least, what I’m supposed to be building as an end product.  I just have to figure out how to get there.  All it will take is building the logic gates, getting them to work effectively, physically connecting all the tubes and electronics into a device with channels smaller than 100 microns across and 10 microns high, and then extensively testing these products.  Should be a piece of cake, right?  At least the hard math is done…

This is where my research was when I arrived at Stanford three weeks ago.  I’ve made considerable theoretical progress since then (lots of pen and pencil sketches in a couple notebooks), but I haven’t yet had a chance to start building anything.  My photolithography masks just came in this afternoon, so I should be able to start working in the clean room tomorrow (yay bunny suits!)  If anybody is interested, I’ll talk about some of my specific problems in a later post.  As always, I welcome your comments!

-Casey

Welcome

Good morning, I'm Casey Glick, and I'll be the host of this admissions blog for the rest of the summer.  Since you're stuck with me for a while, why don't I introduce myself properly, using the time honored grade-school technique of applying the “wh” questions?  Who am I?  As stated previously, I'm Casey Glick.  I am something of a combination of scientist, student, and artist in everything I do.  As a scientist, I question and analyze just about everything I see, hear, and do.  For this trait, my father calls me "the chemistry" because I play with everything inquisitively (I earned the nickname in jest, as I tended to break things when I was younger...) For example, when I was assisting with my family's remodel earlier this summer, I determined that by heating those cans of compressed air (for cleaning hard-to-reach places) in warm water, one can "recharge" them when they get so cold they no longer work effectively.  As student, I like learning whatever I can, whenever.  This caused me to compete in Academic Decathlon in High School, and at Caltech has led to me taking just a few (I can see my friends rolling their eyes at that understatement) more units than I should.  As an artist, I try to make whatever I do fun and beautiful.  In science, this leads to fun and interesting ideas for crazy things to do next, and in "life," I enjoy dancing (preferably ballroom and swing), and singing Opera, much to the dismay of my roommates and neighbors.

Perhaps more pertinent to you reading the blog is the next question: what am I?  I'm a rising senior at Caltech who is double-majoring in Applied Physics and Geochemistry and minoring in History.  Rising senior means I will be taking my fourth year of classes starting in the fall, and the double-major and minor means I do more work than is probably good for me.  I study geology because I love the subject.  I have collected rocks and gems since I was little, and taking courses in geology here at Caltech (which has one of the best geology programs in the world) has only expanded my excitement.  I'm the kind of person who tends to look down when I'm hiking, and at road-cuts when I'm in a car.  I promise, I don't do this when I'm actually driving, but this trait is familiar to geologists, and has led to many a joke about the quality of our driving.  With history, it's the same story for me.  I take history classes at Caltech because they are exciting and because I am something of a history buff.  The primary classes I take are in medieval history because I love Warren Brown's (he's our medievalist on campus) teaching style, where he hands you a primary source like Beowulf and asks us to do history directly from that, rather than from reading a stuffy textbook with all the fun taken away.  I study applied physics for a different reason.  I do love the subject, yes, but in a different way.  The applied physics/physics curriculum at Caltech is hard for me because much of it is so theoretical and analytical.  The actual applied physics, though, is a different story.  The key word here is "applied," which means that I often don't have to rigorously understand everything I'm doing, so long as I have a good gut intuition about what I'm doing.  I work in a field called microfluidics, which you will become quite familiar with if you continue reading my blog, and it is work that has taken me from Hong Kong (where I studied last summer) to Stanford (where I currently am).  And that brings me to:

Where?  Stanford University in Palo Alto, California, outside the Lyman Graduate Residences, in a car that is entirely too cold for my taste.  But it's the only way I can be sure I won't get a ticket for parking without a permit (I promise, I'll explain more in a moment).  Over the summer, I'll be working in the group of Stephen Quake, a top researcher in microfluidics, and a professor who formerly "professed" at Caltech.  I won't meet Dr. Quake until early August, however, because he's decided to spend much of June and all of July in Europe.  I'll meet his group later today.

When?  Far too early...Okay, here's the story.  I drove down to Stanford from Chico yesterday afternoon.  I got something of a later start than I intended, and because I didn't know the way, took three decently-sized detours finding my way to the school, and another few once at the campus.  This means I arrived after the parking kiosk closed in the evening, meaning I couldn't buy a permit.  The parking policy is enforced from 6 AM until 4 PM, so I was safe for the evening.  But (you guessed it) even though I could receive a ticket for parking without a permit at 6 AM, I can't get said permit until 8 am.  So, I woke up at 6:00 and moved my car to metered parking, which was entirely too expensive AND ate some of my coins.  Hence, I've been sitting in this car for the past hour ensuring that I don't have to pay anything more than the dollar in coins I coughed up to satisfy the insatiable meter.  Other "whens" for the next few days include when I turn in my forms for work here (after I can legally park somewhere), when I meet my lab-mates (after that), and when I start taking voice lessons (this afternoon at 1:30).

Why?  Why am I here?  That's a fun question, one I'll be talking about for the duration of the summer.  Suffice it to say, I invented something really interesting last summer, and now I want to make it work properly.  Plus, it's Stanford, a school my uncle attended 30 years ago on a football scholarship (sadly, I have none of my male ancestors' superb football ability, even if I am fairly good at dancing, karate (I am a black belt), and hurdling in track.  Plus (even more plus) one of my best friends, Cynthia, just graduated Stanford, but will be working here over the summer and through next year, and this will give the two of us time to catch up without having to resort to telephone calls every once-in-a-while (I haven't seen her in person for over three years).

How?  Now that doesn't start with "Wh" does it?  Since you asked nicely, however: I drove here, I will use a bike to get around campus, and because a man at Caltech helped me get the job.  How I'll make my project work is another matter entirely, and I won't tell you now because I don't know myself.

But it's about time to start being able to park legally.  A bit of business before I go.  I'll be providing two updates (and comments) weekly.  One will be about the work I'm doing here, and I promise I'll give more than a brief spoiler about the science.  The other post will be about the fun things I'm doing that aren't work related.  This is summer, after all, and I've earned the right to enjoy myself from time-to-time.

-Casey

Late Update: I put my car into temporary parking for the day and then received a community permit that will last me until September.  So, all is calm on the western front.