Manufacturing Bits: Nov. 5

Nanoliter measurements; finding surface free energy.


Nanoliter measurements
The National Institute of Standards and Technology (NIST) has developed an optofluidic measurement system that can measure the flow of liquids at the nanoliter scale.

Targeted for the field of microfluidics, the system can measure the flow of liquids as small as 10 billionths of a liter per minute. A nanoliter (nL) is one billionth of a liter. A liter is 33.814 ounces.

At the rate of 10 billionths of a liter per minute, “it would take a liter bottle of water about 190 years to drain,” according to NIST. “A single drop of water contains 50,000 nanoliters.”

The technology could help advance the field of microfluidics, which deals with the control of fluids in tiny devices. Microfluidics consist of a device with tiny channels. The make use of complex pumps and external plumbing to transport a given fluid in a device. The ability to pump the fluids at the micro-scale level is just one of the challenges.

Microfluidic devices are used in cell sorting and detection systems, gene analysis, inkjet print heads, lab-on-a-chip units and point-of-care diagnostic tools. Some devices are used in the delivery of drugs for patients. For diseases, microfluidic devices dispense medicine at the nanoliter per minute rate into the bloodstream.

Therefore, the flow must be precise. The problem? Making precise measurements with current systems is limited. And as the flow approaches zero, the measurements can vary, according to NIST.

This is where NIST’s optofluidic measurement system fits in. The system uses a laser. In operation, the laser shines light on molecules in a liquid flowing through a microchannel in a microfluidic device.

“The interaction of the laser light with the molecules depends on the rate of flow of the liquid,” according to NIST. “If the fluid is flowing relatively rapidly through the microchannel, the laser simply causes the light-sensitive molecules to shine or fluoresce. But for liquids that flow more slowly and are therefore exposed to the laser light for a longer time, the story is more complex: After a certain amount of light hits the molecules, they burn out and no longer fluoresce. Thus, the slower the flow, the greater the number of light-sensitive molecules that are extinguished and the dimmer the fluorescence.”

The lowest rate of flow measured is 0.2 nL, or 200 trillionths of a liter per minute. It could control a rate of flow as small as 2 nL per minute with an uncertainty of just 5%.

Finding surface free energy
The University of Hawaii has developed a new maximum particle dispersion (MPD) method to determine the surface free energy (SFE) of nanoparticles.

MPD provides a way to quantitatively measure particle hydrophobicity. Hydrophobicity is the physical property of a molecule, which is repelled by water. SFE defines the forces at the interface between two different media.

Solid material surfaces are used in a multitude of industries. The precise characterization of these surfaces is critical, especially when they are subjected to other materials. For example, the surface and its wettability characteristics are important in processes like painting, according to Biolin Scientific.

Here’s another way to describe SFE: “Picture a single molecule in a drop of liquid. The molecule is surrounded by a homogeneous environment and will experience cohesive forces from adjacent molecules, causing the molecule to tend to stay in the bulk,” according to Nanoscience Instruments. “As we move toward the surface of the liquid where it is in contact with another phase, the molecule will experience cohesive forces toward the bulk but also some weaker adhesive forces toward the adjacent phase. The result is a net attraction into the bulk that tends to reduce the number of molecules at the surface and increase the intermolecular space between surface molecules. The increased separation requires energy, just like when stretching a spring, and this excess energy gives rise to surface tension and surface free energy (SFE).”

Surface free energy is calculated through contact angle measurements, according to Biolin Scientific. The problem? There is not a simple method to determine the surface free energy for particles.

In response, the University of Hawaii has found a new and different way to handle these tasks by using the maximum particle dispersion (MPD) method. MPD makes use of particle dispersion, settling/centrifugation and visible-light spectroscopy.

With the technology, researchers studied nine different particles. These are triethoxycaprylylsilane-coated zinc oxide nanoparticles, multiwalled carbon nanotubes, graphene nanoplatelets, molybdenum(IV) sulfide flakes, neodymium(III) oxide nanoparticles, two sizes of zeolites, poly(vinylpolypyrrolidone), and polystyrene microparticles, according to a recent issue of Analytical Chemistry.

The SFE of these micro- and nanoparticles covered the range from 21 to 36 mJ/m2. “The major advantage of this method resides in its simplicity,” said Yi Zuo, professor in the College of Engineering at the University of Hawaii at Mānoa. “For the first time, the scientific and industrial community will have access to an inexpensive and easy-to-use method for quantitatively determining the hydrophobicity of particles. Our method relies on a novel measuring principle and common laboratory procedures and equipment such as pipetting and visible-light spectroscopy.

“Our method can be used to quantify the hydrophobicity of nanoparticles, which is of crucial importance for the study of potential health risks and biomedical applications of nanomaterials,” Zuo said, “It may also find application in microbial science because the surface free energy of bacterial cells determines the cellular adhesion and proliferation in biofilms.”

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