Manufacturing Bits: Dec. 27

Coffee ring effect; molecular microscopes; motion detectors.


Coffee ring effect
In physics, the “coffee ring effect” has been the subject of study for years.

This phenomenon is a simple concept. A liquid or droplet hits a surface and dries. The particles in the droplet are suspended. And ultimately, it leaves a ring-like pattern. The phenomenon is named for the formation of a ring-like deposit when coffee or other liquid resides on a surface.

Previously, the ring was believed to have formed, because of the dynamics of the fluid flow inside the drop. “One paradigm for why this occurs is as a consequence of the solutes being transported towards the pinned contact line by the flow inside the drop, which is induced by surface evaporation,” according to researchers at the University of Nevada at Reno.

Now, the University of Nevada at Reno has a new theory about the coffee ring effect. “It is based on the bulk flow within the drop transporting particles to the interface where they are captured by the receding free surface and subsequently transported along the interface until they are deposited near the contact line,” according to researchers.

In other words, researchers examined the top layer of the droplet that is in contact with the air. This, in turn, plays a role in the deposition of the particles on a surface, thereby causing the coffee ring effect.

“When the drop evaporates, the free surface collapses and traps the suspended particles,” said Hassan Masoud, assistant professor in the Department of Mechanical Engineering at the University of Nevada at Reno, on the university’s Web site. “Our theory shows that eventually all the particles are captured by the free surface and stay there for the rest of their trip towards the edge of the drop.”

The findings could have some major ramifications in many fields. “Understanding and manipulating the dynamics of particle deposition during evaporation of colloidal drops can be used in DNA sequencing, painting, ink jet printing and fabricating ordered micro/nano-structures,” Masoud said. “And now we understand it better than ever before. Our discovery builds on a large body of work; we took an extra step though, modeling the interaction of suspended particles with the free surface of the drop. We believe our findings are going to fundamentally change the common perception on the mechanism responsible for the so-called ‘coffee-ring’ phenomenon.”

Molecular microscopes
The Max Planck Institute (MPI) for Biophysical Chemistry has developed a new fluorescence microscope that is more than 100 times sharper than conventional microscopy.

The fluorescence microscope, called Minflux, has achieved approximately 1nm precision and resolved molecules only 6nm apart. A fluorescence microscope is an optical microscope that uses fluorescence to obtain an image, according to Wikipedia.

The new technology enables researchers to investigate how life functions at the molecular level. With the technology, researchers tracked single fluorescent proteins and demonstrated “30S ribosomal subunits in living Escherichia coli.”

Minflux combines a trio of fluorescence microscopic techniques–stimulated emission depletion (STED) microscopy; photoactivated localization microscopy (PALM); and stochastic optical reconstruction microscopy (STORM).

STED microscopy enables super-resolution images. It makes use of a doughnut-shaped laser beam, which turns off a molecular fluorescence at a fixed location in the sample, according to MPI. “The advantage is that the doughnut beam defines exactly at which point in space the corresponding glowing molecule is located,” according to MPI. “The disadvantage is that in practice the laser beam is not strong enough to confine the emission to a single molecule at the doughnut center.”

Both PALM and STORM, meanwhile, are widefield fluorescence microscopic techniques. They obtain images with resolutions beyond the diffraction limit. They also switch on and off at random locations at the single molecule level.

“The advantage here is that one is already working at the single-molecule level, but a downside is that one does not know the exact molecule positions in space,” according to MPI. “The positions have to be found out by collecting as many fluorescence photons as possible on a camera; more than 50,000 detected photons are needed to attain a resolution of less than 10 nanometers. In practice, one therefore cannot routinely achieve molecular (one nanometer) resolution.”

Minflux, however, combines the strengths of these three techniques. Like PALM/STORM, Minflux switches individual molecules randomly. Like STED, the exact positions are determined with a doughnut-shaped laser beam. But unlike STED, the doughnut beam excites the fluorescence.

With Minflux microscopy, researchers can separate molecules optically. (Source: MPI)

With Minflux microscopy, researchers can separate molecules optically. (Source: MPI)

“We have routinely achieved resolutions of a nanometer with Minflux, which is the diameter of individual molecules–the ultimate limit of what is possible in fluorescence microscopy,” said Stefan Hell, director at the MPI for Biophysical Chemistry. “I am convinced that Minflux microscopes have the potential to become one of the most fundamental tools of cell biology. With this concept it will be possible to map cells in molecular detail and to observe the rapid processes in their interior in real time. This could revolutionize our knowledge of the molecular processes occurring in living cells.”

Tiny motion detectors
The National Institute of Standards and Technology (NIST) has developed a new plasmonic gap resonator.

These optical micrographs provide a top-down view of several plasmonic gap resonators. (Source: NIST)

These optical micrographs provide a top-down view of several plasmonic gap resonators. (Source: NIST)

The device can measure the motion of particles traversing distances shorter than the diameter of a hydrogen atom. The device could be used to sense trace amounts of hazardous biological or chemical agents. It could be used to perfect the movement of robots and to devise more accurate airbags. It could also detect weak sound waves traveling through thin films.

The tiny resonator consists of two layers of gold separated by an air gap. In practice, a laser beam strikes the resonator. “The top gold layer is embedded in an array of tiny cantilevers, (which are) vibrating devices resembling a miniature diving board,” according to NIST. “When a cantilever moves, it changes the width of the air gap, which, in turn, changes the intensity of the laser light reflected from the resonator. The modulation of the light reveals the displacement of the tiny cantilever.”

With the device, NIST has demonstrated “sensitive and spatially localized optical transduction of mechanical motion with a noise floor of 6fm·Hz−1/2, representing a 1.5 orders of magnitude improvement over existing localized plasmomechanical systems.”

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Indifferent_observer says:

Well, many new things are just well-forgotten old ones. Also, if one takes an old pair of shoes, puts some good amount of grease, polish and shine them well, the shoes start looking as new. But they actually are not.
This remark is about the coffee rings, liquid droplet drying etc. and the associated fluid dynamics and instabilities, Marangoni circulation and circulation flow driven motion of the floating and non-floating particles and precipitation species. It is all well-studied, well-discussed, presented many times at the surface cleaning forums and is considered as a pretty well-known phenomenon. And I am wondering what the unknowns exactly the University of Nevada at Reno have discovered, which allowed them to build a really new theory about the coffee ring effect and associated matters. It would be great if they will present it for one of the relevant LI groups some time in the nearby future.
Mark, this is Yehiel. Thanks for bringing this to our attention.

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