Tag Archives: City University of Hong Kong

Getting a more complete picture of aerosol particles at the nanoscale

What is in the air we breathe? In addition to the gases we learned about in school there are particles, not just the dust particles you can see, but micro- and nanoparticles too and scientists would like to know more about them.

An August 23, 2017 news item on Nanowerk features work which may help scientists in their quest,

They may be tiny and invisible, says Xiaoji Xu, but the aerosol particles suspended in gases play a role in cloud formation and environmental pollution and can be detrimental to human health.

Aerosol particles, which are found in haze, dust and vehicle exhaust, measure in the microns. One micron is one-millionth of a meter; a thin human hair is about 30 microns thick.

The particles, says Xu, are among the many materials whose chemical and mechanical properties cannot be fully measured until scientists develop a better method of studying materials at the microscale as well as the much smaller nanoscale (1 nm is one-billionth of a meter).

Xu, an assistant professor of chemistry, has developed such a method and utilized it to perform noninvasive chemical imaging of a variety of materials, as well as mechanical mapping with a spatial resolution of 10 nanometers.

The technique, called peak force infrared (PFIR) microscopy, combines spectroscopy and scanning probe microscopy. In addition to shedding light on aerosol particles, Xu says, PFIR will help scientists study micro- and nanoscale phenomena in a variety of inhomogeneous materials.

The lower portion of this image by Xiaoji Xu’s group shows the operational scheme of peak force infrared (PFIR) microscopy. The upper portion shows the topography of nanoscale PS-b-PMMA polymer islands on a gold substrate. (Image courtesy of Xiaoji Xu)

An August 22, 2017 Lehih University news release by Kurt Pfitzer (also on EurekAlert), which originated the news item, explains the research in more detail (Note: A link has been removed),

“Materials in nature are rarely homogeneous,” says Xu. “Functional polymer materials often consist of nanoscale domains that have specific tasks. Cellular membranes are embedded with proteins that are nanometers in size. Nanoscale defects of materials exist that affect their mechanical and chemical properties.

“PFIR microscopy represents a fundamental breakthrough that will enable multiple innovations in areas ranging from the study of aerosol particles to the investigation of heterogeneous and biological materials,” says Xu.

Xu and his group recently reported their results in an article titled “Nanoscale simultaneous chemical and mechanical imaging via peak force infrared microscopy.” The article was published in Science Advances, a journal of the American Association for the Advancement of Science, which also publishes Science magazine.

The article’s lead author is Le Wang, a Ph.D. student at Lehigh. Coauthors include Xu and Lehigh Ph.D. students Haomin Wang and Devon S. Jakob, as well as Martin Wagner of Bruker Nano in Santa Barbara, Calif., and Yong Yan of the New Jersey Institute of Technology.

“PFIR microscopy enables reliable chemical imaging, the collection of broadband spectra, and simultaneous mechanical mapping in one simple setup with a spatial resolution of ~10 nm,” the group wrote.

“We have investigated three types of representative materials, namely, soft polymers, perovskite crystals and boron nitride nanotubes, all of which provide a strong PFIR resonance for unambiguous nanochemical identification. Many other materials should be suited as well for the multimodal characterization that PFIR microscopy has to offer.

“In summary, PFIR microscopy will provide a powerful analytical tool for explorations at the nanoscale across wide disciplines.”

Xu and Le Wang also published a recent article about the use of PFIR to study aerosols. Titled “Nanoscale spectroscopic and mechanical characterization of individual aerosol particles using peak force infrared microscopy,” the article appeared in an “Emerging Investigators” issue of Chemical Communications, a journal of the Royal Society of Chemistry. Xu was featured as one of the emerging investigators in the issue. The article was coauthored with researchers from the University of Macau and the City University of Hong Kong, both in China.

PFIR simultaneously obtains chemical and mechanical information, says Xu. It enables researchers to analyze a material at various places, and to determine its chemical compositions and mechanical properties at each of these places, at the nanoscale.

“A material is not often homogeneous,” says Xu. “Its mechanical properties can vary from one region to another. Biological systems such as cell walls are inhomogeneous, and so are materials with defects. The features of a cell wall measure about 100 nanometers in size, placing them well within range of PFIR and its capabilities.”

PFIR has several advantages over scanning near-field optical microscopy (SNOM), the current method of measuring material properties, says Xu. First, PFIR obtains a fuller infrared spectrum and a sharper image—6-nm spatial resolution—of a wider variety of materials than does SNOM. SNOM works well with inorganic materials, but does not obtain as strong an infrared signal as the Lehigh technique does from softer materials such as polymers or biological materials.

“Our technique is more robust,” says Xu. “It works better with soft materials, chemical as well as biological.”

The second advantage of PFIR is that it can perform what Xu calls point spectroscopy.

“If there is something of interest chemically on a surface,” Xu says, “I put an AFM [atomic force microscopy] probe to that location to measure the peak-force infrared response.

“It is very difficult to obtain these spectra with current scattering-type scanning near-field optical microscopy. It can be done, but it requires very expensive light sources. Our method uses a narrow-band infrared laser and costs about $100,000. The existing method uses a broadband light source and costs about $300,000.”

A third advantage, says Xu, is that PFIR obtains a mechanical as well as a chemical response from a material.

“No other spectroscopy method can do this,” says Xu. “Is a material rigid or soft? Is it inhomogeneous—is it soft in one area and rigid in another? How does the composition vary from the soft to the rigid areas? A material can be relatively rigid and have one type of chemical composition in one area, and be relatively soft with another type of composition in another area.

“Our method simultaneously obtains chemical and mechanical information. It will be useful for analyzing a material at various places and determining its compositions and mechanical properties at each of these places, at the nanoscale.”

A fourth advantage of PFIR is its size, says Xu.

“We use a table-top laser to get infrared spectra. Ours is a very compact light source, as opposed to the much larger sizes of competing light sources. Our laser is responsible for gathering information concerning chemical composition. We get mechanical information from the AFM [atomic force microscope]. We integrate the two types of measurements into one device to simultaneously obtain two channels of information.”

Although PFIR does not work with liquid samples, says Xu, it can measure the properties of dried biological samples, including cell walls and protein aggregates, achieving a 10-nm spatial resolution without staining or genetic modification.

This looks like very exciting work.

Here are links and citations for both studies mentioned in the news release (the most recently published being cited first),

Nanoscale simultaneous chemical and mechanical imaging via peak force infrared microscopy by Le Wang, Haomin Wang, Martin Wagner, Yong Yan, Devon S. Jakob, and Xiaoji G. Xu. Science Advances 23 Jun 2017: Vol. 3, no. 6, e1700255 DOI: 10.1126/sciadv.1700255

Nanoscale spectroscopic and mechanical characterization of individual aerosol particles using peak force infrared microscopy by Le Wang, Dandan Huang, Chak K. Chan, Yong Jie Li, and Xiaoji G. Xu. Chem. Commun., 2017,53, 7397-7400 DOI: 10.1039/C7CC02301D First published on 16 Jun 2017

The June 23, 2017 paper is open access while the June 16, 2017 paper is behind a paywall.

Pancake bounce

What impact does a droplet make on a solid surface? It’s not the first question that comes to my mind but scientists have been studying it for over a century. From an Aug. 5, 2015 news item on Nanowerk (Note: A link has been removed),

Studies of the impact a droplet makes on solid surfaces hark back more than a century. And until now, it was generally believed that a droplet’s impact on a solid surface could always be separated into two phases: spreading and retracting. But it’s much more complex than that, as a team of researchers from City University of Hong Kong, Ariel University in Israel, and Dalian University of Technology in China report in the journal Applied Physics Letters, from AIP Publishing (“Controlling drop bouncing using surfaces with gradient features”).

An Aug. 4, 2015 American Institute of Physics news release (also on EurekAlert), which originated the news item, describes the impact in detail,

“During the spreading phase, the droplet undergoes an inertia-dominant acceleration and spreads into a ‘pancake’ shape,” explained Zuankai Wang, an associate professor within the Department of Mechanical and Biomedical Engineering at the City University of Hong Kong. “And during the retraction phase, the drop minimizes its surface energy and pulls back inward.”

Remarkably, on gold standard superhydrophobic–a.k.a. repellant–surfaces such as lotus leaves, droplets jump off at the end of the retraction stage due to the minimal energy dissipation during the impact process. This is attributed to the presence of an air cushion within the rough surface.

There exists, however, a classical limit in terms of the contact time between droplets and the gold standard superhydrophobic materials inspired by lotus leaves.

As the team previously reported in the journal Nature Physics, it’s possible to shape the droplet to bounce from the surface in a pancake shape directly at the end of the spreading stage without going through the receding process. As a result, the droplet can be shed away much faster.

“Interestingly, the contact time is constant under a wide range of impact velocities,” said Wang. “In other words: the contact time reduction is very efficient and robust, so the novel surface behaves like an elastic spring. But the real magic lies within the surface texture itself.”

To prevent the air cushion from collapsing or water from penetrating into the surface, conventional wisdom suggests the use of nanoscale posts with small inter-post spacings. “The smaller the inter-post spacings, the greater the impact velocity the small inter-post can withstand,” he elaborated. “By contrast, designing a surface with macrostructures–tapered sub-millimeter post arrays with a wide spacing–means that a droplet will shed from it much faster than any previously engineered materials.”

What the New Results Show

Despite exciting progress, rationally controlling the contact time and quantitatively predicting the critical Weber number–a number used in fluid mechanics to describe the ratio between deforming inertial forces and stabilizing cohesive forces for liquids flowing through a fluid medium–for the occurrence of pancake bouncing remained elusive.

So the team experimentally demonstrated that the drop bouncing is intricately influenced by the surface morphology. “Under the same center-to-center post spacing, surfaces with a larger apex angle can give rise to more pancake bouncing, which is characterized by a significant contact time reduction, smaller critical Weber number, and a wider Weber number range,” according to co-authors Gene Whyman and Edward Bormashenko, both professors at Ariel University.

Wang and colleagues went on to develop simple harmonic spring models to theoretically reveal the dependence of timescales associated with the impinging drop and the critical Weber number for pancake bouncing on the surface morphology. “The insights gained from this work will allow us to rationally design various surfaces for many practical applications,” he added.

The team’s novel surfaces feature a shortened contact time that prevents or slows ice formation. “Ice formation and its subsequent buildup hinder the operation of modern infrastructures–including aircraft, offshore oil platforms, air conditioning systems, wind turbines, power lines, and telecommunications equipment,” Wang said.

At supercooled temperatures, which involves lowering the temperature of a liquid or gas below its freezing point without it solidifying, the longer a droplet remains in contact with a surface before bouncing off the greater the chances are of it freezing in place. “Our new surface structure can be used to help prevent aircraft wings and engines from icing,” he said.

This is highly desirable, because a very light coating of snow or ice–light enough to be barely visible–is known to reduce the performance of airplanes and even cause crashes. One such disaster occurred in 2009, and called attention to the dangers of in-flight icing after it caused Air France Flight 447 flying from Rio de Janeiro to Paris to crash into the Atlantic Ocean.

Beyond anti-icing for aircraft, “turbine blades in power stations and wind farms can also benefit from an anti-icing surface by gaining a boost in efficiency,” he added.

As you can imagine, this type of nature-inspired surface shows potential for a tremendous range of other applications as well–everything from water and oil separation to disease transmission prevention.

The next step for the team? To “develop bioinspired ‘active’ materials that are adaptive to their environments and capable of self-healing,” said Wang.

Here’s a link to and a citation for the paper,

Controlling drop bouncing using surfaces with gradient features by Yahua Liu, Gene Whyman, Edward Bormashenko, Chonglei Hao, and Zuankai Wang. Appl. Phys. Lett. 107, 051604 (2015); http://dx.doi.org/10.1063/1.4927055

This paper appears to be open access.

Finally, here’s an illustration of the pancake bounce,

Droplet hitting tapered posts shows “pancake” bouncing characterized by lifting off the surface of the end of spreading without retraction. Credit- Z.Wang/HKU

Droplet hitting tapered posts shows “pancake” bouncing characterized by lifting off the surface of the end of spreading without retraction. Credit- Z.Wang/HKU

There is also a pancake bounce video which you can view here on EurekAlert.

Call for papers (IEEE [Institute for Electrical and Electronics Engineers] 10th annual NEMS conference in 2015

The deadline for submissions is Nov. 15, 2014 and here’s more from the notice on the IEEE [Institute for Electrical and Electronics Engineers] website for the IEEE-NEMS [nano/micro engineered and moecular systems] 2015,

The 10th Annual IEEE International Conference on Nano/ Micro Engineered and Molecular Systems (IEEE-NEMS 2015)
Xi’an, China
April 7-11, 2015

The IEEE International Conference on Nano/Micro Engineered and Molecular Systems (IEEE-NEMS) is a series of successful conferences that began in Zhuhai, China in 2006, and has been a premier IEEE annual conference series held mostly in Asia which focuses on MEMS, nanotechnology, and molecular technology. Prior conferences were held in Waikiki Beach (USA, 2014), Suzhou (China, 2013), Kyoto (Japan, 2012), Kaohsiung (Taiwan, 2011), Xiamen (China, 2010), Shenzhen (China, 2009), Hainan Island (China, 2008), Bangkok (Thailand, 2007), and Zhuhai (China, 2006). The conference typically has ~350 attendees with participants from more than 20 countries and regions world-wide.

In 2015, the conference will be held in Xi’an, one of the great ancient capitals of China. Xi’an has more than 3,100 years of history, and was known as Chang’an before the Ming dynasty. Xi’an is the starting point of the Silk Road and home to the Terracotta Army of Emperor Qin Shi Huang.

We now invite contributions describing the latest scientific and technological research results in subjects including, but are not limited to:

  • Nanophotonics
  • Nanomaterials
  • Nanobiology, Nanomedicine, Nano-bio-informatics
  • Micro/Nano Fluidics, BioMEMS, and Lab-on-Chips
  • Molecular Sensors, Actuators, and Systems
  • Micro/Nano Sensors, Actuators, and Systems
  • Carbon Nanotube/Graphene/Diamond based Devices
  • Micro/Nano/Molecular Heat Transfer & Energy Conversion
  • Micro/Nano/Molecular Fabrication
  • Nanoscale Metrology
  • Micro/Nano Robotics, Assembly & Automation
  • Integration & Application of MEMS/NEMS
  • Flexible MEMS, Sensors and Printed Electronics
  • Commercialization of MEMS/NEMS/Nanotechnology
  • Nanotechnology Safety and Education

Important Dates:

Nov. 15, 2014 – Abstract/Full Paper Submission
Dec. 31, 2014 – Notification of Acceptance
Jan. 31, 2015 – Final Full Paper Submission

We hope to see you at Xi’an, China, in April 2015!

General Chair: Ning Xi, Michigan State University, USA
Program Chair: Guangyong Li, University of Pittsburgh, USA
Organizing Chair: Wen J. Li, City University of Hong Kong, Hong Kong
Local Arrangement Chair: Xiaodong Zhang, Xi’an Jiaotong University, China

The 2015 IEEE-NEMS webpage offers more general information about the conference,

The IEEE-NEMS is a key conference series sponsored by the IEEE Nanotechnology Council focusing on advanced research areas related to MEMS, nanotechnology, and molecular technology. … The conference typically has ~350 attendees with participants from more than 20 countries and regions world-wide.

Good luck!

Things falling apart: both a Nigerian novel and research at the Massachusetts Intitute of Technology

First the Nigerian novel ‘Things Fall Apart‘ (from its Wikipedia entry; Note: Links have been removed),

Things Fall Apart is an English-language novel by Nigerian author Chinua Achebe published in 1958 by William Heinemann Ltd in the UK; in 1962, it was also the first work published in Heinemann’s African Writers Series. Things Fall Apart is seen as the archetypal modern African novel in English, one of the first to receive global critical acclaim. It is a staple book in schools throughout Africa and is widely read and studied in English-speaking countries around the world. The title of the novel comes from William Butler Yeats’ poem “The Second Coming”.[1]

For those unfamiliar with the Yeats poem, this is the relevant passage (from Wikipedia entry for The Second Coming),

Turning and turning in the widening gyre
The falcon cannot hear the falconer;
Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world,
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;
The best lack all conviction, while the worst
Are full of passionate intensity.

The other ‘Things fall apart’ item, although it’s an investigation into ‘how things fall apart’, is mentioned in an Aug. 4, 2014 news item on Nanowerk,

Materials that are firmly bonded together with epoxy and other tough adhesives are ubiquitous in modern life — from crowns on teeth to modern composites used in construction. Yet it has proved remarkably difficult to study how these bonds fracture and fail, and how to make them more resistant to such failures.

Now researchers at MIT [Massachusetts Institute of Technology] have found a way to study these bonding failures directly, revealing the crucial role of moisture in setting the stage for failure. Their findings are published in the journal Proceedings of the National Academy of Science in a paper by MIT professors of civil and environmental engineering Oral Buyukozturk and Markus Buehler; research associate Kurt Broderick of MIT’s Microsystems Technology Laboratories; and doctoral student Denvid Lau, who has since joined the faculty at the City University of Hong Kong.

An Aug. 4, 2014 MIT news release written by David Chandler (also on EurekAlert), which originated the news item, provides an unexpectedly fascinating discussion of bonding, interfaces, and infrastructure,

“The bonding problem is a general problem that is encountered in many disciplines, especially in medicine and dentistry,” says Buyukozturk, whose research has focused on infrastructure, where such problems are also of great importance. “The interface between a base material and epoxy, for example, really controls the properties. If the interface is weak, you lose the entire system.”

“The composite may be made of a strong and durable material bonded to another strong and durable material,” Buyukozturk adds, “but where you bond them doesn’t necessarily have to be strong and durable.”

Besides dental implants and joint replacements, such bonding is also critical in construction materials such as fiber-reinforced polymers and reinforced concrete. But while such materials are widespread, understanding how they fail is not simple.

There are standard methods for testing the strength of materials and how they may fail structurally, but bonded surfaces are more difficult to model. “When we are concerned with deterioration of this interface when it is degraded by moisture, classical methods can’t handle that,” Buyukozturk says. “The way to approach it is to look at the molecular level.”

When such systems are exposed to moisture, “it initiates new molecules at the interface,” Buyukozturk says, “and that interferes with the bonding mechanism. How do you assess how weak the interface becomes when it is affected? We came up with an innovative method to assess the interface weakening as a result of exposure to environmental effects.”

The team used a combination of molecular simulations and laboratory tests in its assessment. The modeling was based on fundamental principles of molecular interactions, not on empirical data, Buyukozturk says.

In the laboratory tests, Buyukozturk and his colleagues controlled the residual stresses in a metal layer that was bonded and then forcibly removed. “We validated the method, and showed that moisture has a degrading effect,” he says.

The findings could lead to exploration of new ways to prevent moisture from reaching into the bonded layer, perhaps using better sealants. “Moisture is the No. 1 enemy,” Buyukozturk says.

“I think this is going to be an important step toward assessment of the bonding, and enable us to design more durable composites,” he adds. “It gives a quantitative knowledge of the interface” — for example, predicting that under specific conditions, a given bonded material will lose 30 percent of its strength.

Interface problems are universal, Buyukozturk says, occurring in many areas besides biomedicine and construction. “They occur in mechanical devices, in aircraft, electrical equipment, in the packaging of electronic components,” he says. “We feel this will have very broad applications.”

Bonded composite materials are beginning to be widely used in airplane manufacturing; often these composites are then bonded to traditional materials, like aluminum. “We have not had enough experience to prove the durability of these composite systems is going to be there after 20 years,” Buyukozturk says.

Here’s a link to and a citation for the research paper,

A robust nanoscale experimental quantification of fracture energy in a bilayer material system by Denvid Lau, Kurt Broderick, Markus J. Buehler, and Oral Büyüköztürk. PNAS, doi: 10.1073/pnas.1402893111 published August 5, 2014

This paper is behind a paywall.