Researchers use microfluidic devices for testing in bio labs and as micro-reactions cells for chemical sensing and fluid analysis. Microfluids often must be mixed but right now scientists lack a simple and reliable way to do it.
“Mixing liquids in tiny volumes,” says materials scientist Jim Martin (1112), “is surprisingly difficult.” When fluid is pushed down a big pipe, Jim explains, eddies are generated; as the eddies swirl, they create mixing. But if fluid is pushed down a small pipe, there are no eddies. And no eddies means no mixing — unless you subject the fluid to tremendous pressure, which isn’t usually easy or feasible.
Researchers in the laboratory and in industry have tried a wide range of approaches to create mixing in microfluids, with only “mixed” success.
“In small devices,” Jim says, “people have tried all kinds of pillars and mixing cells to initiate mixing, but these approaches don’t work well.”
Researchers need simpler and more reliable ways to mix in tiny places such as micrometer-sized channels.
Jim’s discovery of how to mix tiny liquid volumes arose from LDRD-funded research directed at improving the sensitivity of the chemical sensors developed in his lab. That project, “Field-Structured Composite Studies,” was a joint effort with Rod Williamson (now retired). While their LDRD project did not lead to the expected results, Jim and Rod were surprised by the wide variety of physical effects they discovered along the way, including magnetic mixing. These effects, Jim says, ended up being much more interesting and important than the original goal.
Since the project began, DOE’s Division of Material Science and Engineering, Office of Basic Energy Sciences, has now started a new project whose goal is to better understand the fundamental science of field-structured composites. So the program succeeded even as it failed, and eventually Jim and PhD student Doug Read developed better ways to increase sensor sensitivity.
In the new method of mixing, when you turn on a particular kind of magnetic field, the magnetic particles suspended in the fluid form into chains — like strings of pearls — that start swirling around; that’s what does the mixing. The particles are then removed magnetically, leaving a nice mixed-up liquid.
More technically, the new mixing method, which Jim calls vortex field mixing, subjects a suspension of microscopic, magnetizable particles to a magnetic field whose direction is constantly spinning in a motion similar to a spinning top as it is about to collapse on its side, but much faster. In this “vortex field” the particles assemble into countless microscopic chains that follow the field motion, stirring every nook and cranny of the fluid. The vortex field stirs the liquid vigorously, and surprising fluid effects are possible, such as a kind of washing machine agitation where the spinning direction alternates periodically.
Currently Jim, Lauren Rohwer (1715), and PhD student Kyle Solis (1112) work with the vortex field mixing, among other projects. Their experimental report, published in the July issue of Physical Review, has generated interest, including a Physical Review Focus article and a Research Highlight in the September MRS Bulletin.
This type of magnetic mixing with particles that assemble into micro-stir bars isn’t like the magnetic mixing you remember from high school chemistry class.
“In your high school chemistry class,” Jim says “when you mixed a beaker of water on a stir plate, underneath the plate was a permanent magnet spinning around to make the stir bar spin. If that hidden magnet suddenly became twice as strong, the magnetic field would double but you wouldn’t see any increase in the stirring at all.
“With our process,” he says, “if we make the magnetic field twice as strong, the stirring becomes four times as strong because the stronger field makes the particle chains longer.”
With conventional stir-bar mixing you can increase the mixing torque by increasing the speed of the stir bar instead. It’s easy to feel this effect by simply holding the beaker slightly above the stir plate. In vortex field mixing increasing the speed of the wobbling doesn’t help, because the chains simply break into smaller pieces and the mixing torque doesn’t change at all.
Vortex field mixing stirs just as effectively with magnetic nanoparticles as with traditional micrometer-scale powders. In fact, excellent mixing torques have been obtained using 100 nanometer particles. This means even the tiniest fluid volumes can be mixed, as well as the largest.
As strange as these effects are, they were initially predicted by Jim in a theory paper published in the January 2009 issue of Physical Review. This paper also explains why a simple rotating magnetic field doesn’t induce mixing and predicts the optimal wobbling angle of the magnetic field.
Vortex field mixing requires only the modest magnetic fields provided by simple wire coils that can be scaled to the size of the fluid cavity. After mixing, a researcher can trap the particles with a permanent magnet, decant the mixed liquid, and recycle the particles endlessly.
The impact of this new method of mixing is hard to predict, but its applicability to fluid volumes of all shapes and sizes suggests many applications will follow. But Jim’s lab has turned its attention to training magnetic suspensions to effortlessly conduct heat in any desired direction.
This work was supported by DOE’s Division of Materials Science and Engineering, Office of Basic Energy Sciences. -- Stephanie Holinka
By Mike Janes
The Department of Homeland Security’s Rapidly Deployable Chemical Detection System (RDCDS), in which Sandia has played a leading development role, has made a big splash these past few years, having been deployed to venues such as the Rose Bowl in Pasadena, McAfee Stadium in Oakland, and the Democratic National Convention in Denver. Designed and built by Sandia with contributions from Lawrence Livermore National Laboratory and Pacific Northwest National Laboratory, the RDCDS provides early warning of an attack from either chemical warfare agents or toxic industrial chemicals at high-profile special events.
The RDCDS is designed to provide broad, high-confidence coverage of more than 40 different chemicals using multiple overlapping detection technologies and live video.
But Nate Gleason (8125), who serves as Sandia’s RDCDS program manager in support of the DHS program, says detection is only one part of the overall protection puzzle. So this year, armed with a $400,000 increase in DHS funding, Sandia’s researchers are moving forward with a comprehensive chemical defense architecture that goes beyond a mere detection system.
Venue selection and response time
“A detection system in isolation is useless. If you don’t do something when the detectors alarm, then why bother detecting in first place?” Nate asks.
Sandia’s emphasis, however, isn’t merely focused on response plans. Nate says the work starts with venue selection and continues systematically.
“We need to start with a full threat and vulnerability assessment of the venue and we need to figure out the possible and likely attacks on that venue,” he says. “What exactly are we protecting against? What options do the venue operators currently have that can take those threats off the table? Those are the kinds of questions we’re asking, and only then can we determine what elements from the RDCDS we can provide that will enable venue operators to execute the proper responses.”
For example, Nate says, consider a typical football stadium packed with some 60,000 spectators. A detection system, all by itself, might adequately detect a dangerous chemical release. But if it hasn’t been determined previously just how long it will take to evacuate the stadium, the detectors could prove useless if not enough warning time for emergency personnel has been allowed to usher fans out of the venue in a safe and orderly fashion.
To address these and other issues, DHS has directed Nate and his colleagues to expand the RDCDS capability on its behalf by executing two pilot activities in the coming months.
Pasadena and New York City
For the next several months, the Sandia team will focus on the Jan. 1 Rose Bowl game in Pasadena. For that event, templates and checklists are being developed to assess the venue and the surrounding area. One asset the researchers will leverage, Nate says, is existing databases on chemical infrastructure in and around Pasadena, including chemical storage tanks, plants, or even shipment routes — all considered sources of risk. This, he says, will give the team a more thorough understanding of how terrorists might plan an attack and what resources they might have available to them.
The other upcoming pilot, due to take place in the spring, will occur at New York City’s Port Authority Bus Terminal (PABT), which sees some 250,000 passengers come through its turnstiles every day. Though DHS has deployed RDCDS to other indoor venues in the past, the PABT — the world’s busiest bus terminal — presents a unique set of challenges.
Unlike other indoor venues where RDCDS has been deployed, the PABT is a facility that needs to be protected 24/7, not just for a one-time event. Though seven of the eight RDCDS detectors are capable of continuous operation, Nate says one of them currently has only a 500-hour life span, good for no more than a couple of weeks. Sandia’s RDCDS team, consequently, will replace the eighth detector for the PABT pilot and will also analyze a number of new detection technologies. In addition, biodetectors will be included in the PABT pilot, as the ultimate goal of RDCDS is a full chemical, biological, and radiological detection capability.
Sandia’s specific RDCDS customer, the Department of Homeland Security’s Office of Health Affairs, has increased the program’s funding to roughly $1.6 million this year, signifying a new level of commitment and interest in the program.
“The program seemed to be stuck in neutral for a while, with no new detection hardware and no new initiatives,” says Nate. “The new program manager has a clear vision for the RDCDS, and he seems determined to make it really worthwhile.”
Sandia is gearing up for a local exercise that will test at least one important element of DHS’s Rapidly Deployable Chemical Detection System (RDCDS). In late October, Sandia will participate in Alameda County’s Urban Shield exercise, touted as “a real-life, tactical multidisciplinary training exercise.” With a multitude of regional agencies supporting the weekend event, the exercise will simulate a terrorist attack on a nearby nuclear facility.
Though RDCDS’s full capability will not be exercised, says Nate Gleason (8125), event organizers are interested in the system’s ability to quickly deploy a video monitoring system consisting of a full suite of cameras, video recording, and networking capabilities.
“The goals are to stream live video directly to exercise controllers and to record the proceedings for analysis later on,” says Nate. Though the RDCDS can conduct its video surveillance activities remotely, Sandia will likely have a staff member on site during the exercise in order to more effectively communicate with controllers. -- Mike Janes
By Bill Murphy
‘Standby, Safety. We’re making the move . . .’
The voice from the control room is steady, practiced, confident, unhurried. Outside, a small observatory-like dome begins to rotate, gliding smoothly and quietly on well-oiled gears.
“OK, we’re tracking, Safety. Are you guys good?”
Dave Denning (5737), surrounded by a suite of rack-mounted instruments and glowing LCD monitors, speaks into his headset mike, querying his team: Safety. Laser. Telescope.
One by one, they report back. “We’re good.”
Tonight, the group is firing a high-powered laser beam through the atmosphere, illuminating the optical sensors on a GPS Block II satellite in a semisynchronous orbit some 12,500 miles above the Earth.
As members of Sandia’s LAZAP (Laser Applications) team, this night they’re going through a process that has been played out thousands of times over the past 30-plus years.
Supporting the test ban treaty
Sandia’s LAZAP program was started in the mid-1970s to test and calibrate the optical sensor systems aboard America’s Vela reconnaissance satellites. The Vela constellation was launched over a period of several years in the 1960s and 1970s to detect atmospheric nuclear detonations. It was part of a suite of technologies designed to monitor compliance with the 1963 Limited Test Ban Treaty, which forbade signatory nations from conducting above-ground nuclear tests.
By sending out bright laser pulses, LAZAP helped Vela controllers confirm that their satellites were, indeed, detecting bright optical events. LAZAP’s laser flashes also enabled mission operators to periodically calibrate the Vela sensors in orbit.
Here’s how it worked: LAZAP operators found and tracked a Vela satellite. (In the early days, this process was done visually and manually using a powerful telescope; today the tracking is computer-controlled.)
Once the target Vela was confirmed, the LAZAP team fired a laser at the satellite, illuminating its optical sensors, which were optimized to pick up ultrabright, ultrafast flashes of light — the kinds of flashes that might characterize a nuclear blast.
Modern systems still need to be calibrated
Since the LAZAP laser was being fired from a known location with known coordinates, testers could calibrate the sensors by comparing that known location with where the Vela satellite “thought” it saw the event. If, for example, the Vela thought the pulse of laser light was coming from, say, the vicinity of Amarillo, when it actually originated in Albuquerque, controllers could build in a compensation factor. That kind of precision would be vital in pinpointing the location of a treaty violation.
The process was not unlike zeroing in the sights on a rifle, getting the crosshairs on the scope to correspond to where the bullet actually hits the target.
The Vela constellation was replaced in the 1980s by the Defense Support Program (DSP) satellites and in the 1990s by the Global Positioning System (GPS) constellation. Both the DSP satellites and the modern GPS satellites still carry optical sensors. And even though they are more advanced, capable, and sensitive than Vela-era sensors, they must still be tested and calibrated much the same way their Vela forebears were.
That’s why, on multiple dark, clear nights each year, the LAZAP team is still working late, tracking, locking on to, and laser-illuminating the latest generation of satellites that make up the space-borne component of the nation’s nuclear detonation (NUDET) detection system.
‘Ready to let the ruby out’
Tonight, the dome has stopped its rotation, its open slot aimed toward a point in the southeastern sky.
In his headset, Dave, who for the past several years has been the LAZAP project lead, is getting reports from his two visual aircraft observers and his radar controller. (The radar controller is working from a feed provided by a local FAA air traffic control center.) Visual and radar agree: The sky is clear. No aircraft are in the vicinity.
Dave glides effortlessly on his wheeled office chair from one side of the control station to the other, checking data on an LCD monitor. It’s a chart displaying so-called predictive avoidance data; that is, windows of time when LAZAP will not inadvertently illuminate nontargeted satellites. It’s the space-based equivalent of making sure no aircraft are nearby. A quick check confirms: The window is open.
With everyone good to go, Dave says into his headset: “Team, I’m going to start shooting the ruby [laser]. I’m going to let the ruby out now.” He counts down, “Five, four, three, two, one . . .”
A green light shoots from the dome
Outside, quick red flashes light up the slot in the dome. The Class 4 ruby laser is being pumped through a Cassegrain telescope beam director, which pinpoints the beam onto the target satellite.
Then, more dramatically, a green light shoots from the dome, its pulsating, concentrated beam slicing the sky and visible all the way to the very edge of the atmosphere. Dave explains, “Right now, we do all our calibration testing with the ruby laser, but there are some potential advantages to using the green laser, so we’re doing some testing with it tonight.”
While the test is going on, Victor Chavez (5737), the team’s laser guru, comes in to tell Dave the laser may be on the verge of failing. That’s something that happens periodically and something the team is ready to deal with. Tonight, though, the laser holds up and the data collection is good. Meanwhile, as the test proceeds, Gus Rodriguez (5737), who maintains the LAZAP electronics suite, is busy keeping the system up and running. Debra Yzquierdo-Trujillo (5737) is on the radar board, monitoring air traffic, while the visual safety team outside continues to scan the skies.
Placing the test on hold
Just as Dave is emphasizing that every member of the team can stop the laser firing at any time, he gets a call on his headset and an alert on the status board: One of the visual observers has put a hold on the test as an aircraft appears to be headed toward the quadrant of the sky where the laser is aimed. After a couple of minutes, the sky is clear again, Dave initiates another countdown. The test resumes and continues until the target GPS satellite moves below the horizon.
Later, Dave says the previous night’s tests gathered important data that will be useful in calibrating the spacecraft’s optical sensor. With a constellation of more than 30 GPS satellites and with a number of DSP satellites still being used for NUDET applications, the LAZAP team has a busy and demanding schedule.
Throughout its 35-year history, LAZAP’s primary customers have been the US Air Force NUDET Detection System Program, which owns the satellite platforms that carry NUDET sensor technologies and the NNSA Office of Nonproliferation Research and Development (NA-22) and its predecessors in the Department of Energy. And as space-based optical sensor technology evolves — as will be the case with the upcoming GPS Block III constellation — LAZAP will evolve to provide the necessary calibration and testing protocols.
And, says Dave, “As long as there are optical sensors on board American satellites, we’ll have an essential mission.” -- Bill Murphy