INSIGHTS | P E R S P E C T I V E S
MATERIALS SCIENCE
Understanding friction in layered materials A scanning probe technique provides a clearer picture of friction at the nanometer scale
632
sciencemag.org SCIENCE
8 MAY 2015 • VOL 348 ISSUE 6235
Published by AAAS
Downloaded from www.sciencemag.org on May 7, 2015
A
The challenge of measuring relatively weak forces between components has been addressed by a number of instrumented indentation and lateral loading devices, which all make use of probes that are brought into contact or near contact with the target surface while the normal and lateral (friction) load on and displacement of the probe are measured. With sharp enough probes, it is possible to observe oscillations in the lateral force-displacement response whose maxima and minima, respectively, coincide with the onset of slipping and attachment of the probe to the target surface and were commensurate with the in-
ILLUSTRATION: P. HUEY/SCIENCE
teratomic spacing of the mica surface that was being probed (3). The novelty of the s we move to the digital age, we may experiment by Koren et al. lies in the use of forget that a lot of correspondence an atomic force microscope to apply a latwas accomplished by pencil and paeral shear force to disks (mesas) of HOPG. per. Nonetheless, when we do still The disks, 50 nm thick and with radii that write with “lead” pencils, we are ranged from 50 to 300 nm, were sheared making use of the weak interlayer along the basal plane of the graphite, about forces in graphite to strip off tiny flakes of 10 nm from their bases. A classic frictional graphite and deposit them on paper in a lateral load-displacement response was complex interplay between adhesion and observed, which was largely reversible exfriction. Understanding the nature of such cept for dynamic dissipative events that ocweak interlayer forces and the attendant curred during sliding. They can be seen as opportunities afforded by such understandslight differences in the force-displacement ing are becoming more important with the response during sliding in opposite direcadvent of nano- and microelections. These latter events were tromechanical systems (NEMS/ related to slight rotational misMEMS); machines whose moving alignments between the graphTaking a measure of friction components are at the nanometer ite lattices above and below the Illustrating the method of Koren et al. with a toy model of a layered and micrometer scales embodied slip plane. In addition, in a secsystem under lateral shear and torque. in their names. Because the surond set of experiments, when a face area of such components is torque was applied to the memuch larger than their volume, sas, stable orientations were obsurface forces (for example, van served every 60°, in registration der Waals, electrostatic, and capwith the sixfold symmetry of the illary forces) become the domihexagonal carbon atom lattice. A Torque nant loading to be considered third set of experiments, with a in the design and reliability of structure exhibiting mechanical NEMS and MEMS devices (1). bistability, demonstrated the poOn page 679 of this issue, Koren tential to produce cell structures Torque et al. (2) have devised an innofor memory devices that are envative experiment that quantitirely mechanical in operation. fies our day-to-day experience The various equilibrium states with lead pencils, but with much of these tiny cells could then be higher fidelity than we might used as switches in such memory expect. They show that two addevices. jacent planes of carbon atoms The idea behind the method can be sheared in highly ordered developed by Koren et al. can pyrolytic graphite (HOPG) and be illustrated more easily if we observe the interplay between adconsider, for example, a toy syshesion and friction forces in this tem consisting of ping-pong balls layered material. This experiment glued together in a hexagonal Torque with HOPG can be considered as a pattern (close packed) in two Torque model for two planes of graphene planes that are placed on top of or other so-called two-dimenone another (see the figure). The sional materials sliding over one in-plane attachment is much another. The findings are exciting stronger than the interaction bebecause they point to new ideas tween the planes, just as in layLateral shear for data storage at small scales, as ered, two-dimensional materials. well as novel actuation concepts One can imagine sliding or roin electromechanical devices. tating the planes relative to one another and finding a series of Center for the Mechanics of Solids, Structures equilibrium positions. and Materials, Department of Aerospace Another opportunity afforded Engineering and Engineering Mechanics, by the experiment of Koren et University of Texas at Austin, Austin TX 78712, Lateral shear USA. E-mail: [email protected] al. is that it may present a close By Kenneth M. Liechti
link to multiscale modeling from atomistic to continuum scales in that the interface or slip plane was free of any contamination and the geometry was relatively simple, which is not usually the case for contact pairs. Although atomistic or molecular analyses have provided insights into deformation and dissipation mechanisms and other effects that are difficult to separate out experimentally, one-to-one comparisons are limited by the high shear velocities and nanometer-scale probe tips that are currently accessible to analysis. Some progress in this direction has been provided by quartz microbalance experiments (4) and molecular analyses (5) of krypton monolayers sliding on gold, where the close agreement between simulation and experiment identified mechanisms for phononic contributions to friction. Molecular self-assembled monolayers provide another, slightly more complex avenue for linking experiments and analysis in addition to having technological relevance to MEMS. The issue of time and spatial scaling in atomistic analyses of friction between self-assembled monolayers has been identified as one of the current challenges in single asperity friction (6). One possible way of addressing this issue is through hybrid analyses with schemes that combine molecular analyses with phenomenological potentials (7, 8) that have allowed typical sliding speeds to be accessed. Koren et al. have come up with a novel experiment that provides a better understanding of friction in layered materials. Although graphite was the subject of this study, the extension to the plethora of twodimensional materials that are becoming available is clear. The authors have also provided much food for thought in developing potential applications of the observed phenomena to novel memory devices and machines. ■
ILLUSTRATION: C. SMITH/SCIENCE
REFERENCES AND NOTES
1. B. Bhushan, J. N. Israelachvili, U. Landman, Nature 374, 607 (1995). 2. E. Koren, E. Lörtscher, C. Rawlings, A. W. Knoll, U. Duerig, Science 348, 679 (2015). 3. R. W. Carpick, N. Agraït, D. F. Ogletree, M. Salmeron, Langmuir 12, 3334 (1996). 4. J. Krim, D. H. Solina, R. Chiarello, Phys. Rev. Lett. 66, 181 (1991). 5. M. Cieplak, E. D. Smith, M. O. Robbins, Science 265, 1209 (1994). 6. I. Szlufarska, M. Chandross, R. W. Carpick, J. Phys. D Appl. Phys. 41, 123001 (2008). 7. T. Ohzono, J. N. Glosli, M. Fujihira, Jpn. J. Appl. Phys. 37 (part 1, no. 12A), 6535 (1998). 8. T. Ohzono, J. N. Glosli, M. Fujihira, Jpn. J. Appl. Phys. 38 (part 2, no. 6A/B), L675 (1999). ACKNOWLEDGMENTS
Supported by the National Science Foundation and the Basic Energy Sciences program of the U.S. Department of Energy. 10.1126/science.aab0930
INFECTIOUS DISEASE
More than just bugs in spit Advances in imaging help to explain tuberculosis treatment failures and develop better drugs By Clifton Barry1,2,3
T
ease-free status in patients 1 year after completion of treatment. Because determining relapse-free cure requires such a long time, end points in phase 2 clinical trials focus on killing bacteria in expectorated sputum over the first few months of treatment. It has long been clear that the end points in these two different stages of clinical evaluation are not well correlated, as emphasized again recently by the failure of three large phase 3 studies that aimed to shorten the standard 6-month regimen to four months by including a fluoroquinolone as part of a four-drug regimen (1–3). Despite achieving
he development of new drugs for tuberculosis (TB) is not for the faint of heart. In addition to all the typical hurdles to developing a new antiinfective, additional complications result from the complex pathology induced by the disease. Because the bacillus is surrounded by a thick, waxy coat, hit rates in whole-cell screens are typically low; the resulting hits are typically so lipophilic that they do not offer good chemical starting points. Despite extensive research efforts, no TB drug has been developed using target-based approaches. Advances in imaging at both a macroscopic and microscopic level offer new insights and strategies for getting urgently needed new drugs to the clinic. TB is a disease of the lungs: It is transmitted by inhalation of an aerosol droplet containing the bacterium, which implants in small alveoli and then invades and multiplies in the interstitial spaces in the lung. Here, it forms granulomas—that is, nodules of immune cells recruited as the body attempts to wall off the invading bacteria. Eventually, the granulomas undergo necrosis to form cavities and leak out into the airways, spilling bacteria into the lungs and inducing coughing to enable transmission to a new host. A single patient with TB can earlier sterilization of sputum than convenhave dozens of individual lesions, all at diftional therapy in preceding phase 2 studies ferent stages of maturation. and in the phase 3 trials themselves, more TB therapy is often very protracted. patients in the 4-month fluoroquinolone Standard medical care for pulmonary TB arms suffered relapse after treatment was patients globally involves 6 months of treatdiscontinued. The fact that these patients ment. Although full treatment is successful were obviously not cured shows that bacin 95% of patients, in reality many patients teria in the sputum do not tell a complete fail to complete such a long course of therstory. Nor would one expect them to do so, apy; mobilizing the resources to ensure given that only certain lesions produce bacfull compliance is nearly impossible in reteria that are represented in the airways. source-poor countries where the disease is Curing all patients with a shortened TB endemic. Shorter courses of treatment are treatment regimen requires developing an one of the major goals of global research understanding of what leads to a relapse— efforts because they would aldefined as recurrent disease low for more supervision, less with the same bacterial strain recurrent disease, and correafter apparent initial cure—in spondingly less opportunity for a subset of treated patients. developing drug resistance. There are two main hypotheses U.S. and European regulatory for why relapse occurs. One bodies mandate that end points argues that relapse is due to a in phase 3 clinical trials of new subset of bacteria with unique TB drugs focus on achieving disphysiology (imposed by drugs, INFECTIOUS DISEASE SERIES
“All these exciting techniques have tremendous potential to accelerate TB drug development by improving the quality of drugs and drug combinations used in clinical trials to target specific lesion types.”
SCIENCE sciencemag.org
8 MAY 2015 • VOL 348 ISSUE 6235
Published by AAAS
633
Understanding friction in layered materials Kenneth M. Liechti Science 348, 632 (2015); DOI: 10.1126/science.aab0930
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/348/6235/632.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/348/6235/632.full.html#related This article cites 8 articles, 2 of which can be accessed free: http://www.sciencemag.org/content/348/6235/632.full.html#ref-list-1
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2015 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on May 7, 2015
The following resources related to this article are available online at www.sciencemag.org (this information is current as of May 7, 2015 ):
MATERIALS SCIENCE
Understanding friction in layered materials A scanning probe technique provides a clearer picture of friction at the nanometer scale
632
sciencemag.org SCIENCE
8 MAY 2015 • VOL 348 ISSUE 6235
Published by AAAS
Downloaded from www.sciencemag.org on May 7, 2015
A
The challenge of measuring relatively weak forces between components has been addressed by a number of instrumented indentation and lateral loading devices, which all make use of probes that are brought into contact or near contact with the target surface while the normal and lateral (friction) load on and displacement of the probe are measured. With sharp enough probes, it is possible to observe oscillations in the lateral force-displacement response whose maxima and minima, respectively, coincide with the onset of slipping and attachment of the probe to the target surface and were commensurate with the in-
ILLUSTRATION: P. HUEY/SCIENCE
teratomic spacing of the mica surface that was being probed (3). The novelty of the s we move to the digital age, we may experiment by Koren et al. lies in the use of forget that a lot of correspondence an atomic force microscope to apply a latwas accomplished by pencil and paeral shear force to disks (mesas) of HOPG. per. Nonetheless, when we do still The disks, 50 nm thick and with radii that write with “lead” pencils, we are ranged from 50 to 300 nm, were sheared making use of the weak interlayer along the basal plane of the graphite, about forces in graphite to strip off tiny flakes of 10 nm from their bases. A classic frictional graphite and deposit them on paper in a lateral load-displacement response was complex interplay between adhesion and observed, which was largely reversible exfriction. Understanding the nature of such cept for dynamic dissipative events that ocweak interlayer forces and the attendant curred during sliding. They can be seen as opportunities afforded by such understandslight differences in the force-displacement ing are becoming more important with the response during sliding in opposite direcadvent of nano- and microelections. These latter events were tromechanical systems (NEMS/ related to slight rotational misMEMS); machines whose moving alignments between the graphTaking a measure of friction components are at the nanometer ite lattices above and below the Illustrating the method of Koren et al. with a toy model of a layered and micrometer scales embodied slip plane. In addition, in a secsystem under lateral shear and torque. in their names. Because the surond set of experiments, when a face area of such components is torque was applied to the memuch larger than their volume, sas, stable orientations were obsurface forces (for example, van served every 60°, in registration der Waals, electrostatic, and capwith the sixfold symmetry of the illary forces) become the domihexagonal carbon atom lattice. A Torque nant loading to be considered third set of experiments, with a in the design and reliability of structure exhibiting mechanical NEMS and MEMS devices (1). bistability, demonstrated the poOn page 679 of this issue, Koren tential to produce cell structures Torque et al. (2) have devised an innofor memory devices that are envative experiment that quantitirely mechanical in operation. fies our day-to-day experience The various equilibrium states with lead pencils, but with much of these tiny cells could then be higher fidelity than we might used as switches in such memory expect. They show that two addevices. jacent planes of carbon atoms The idea behind the method can be sheared in highly ordered developed by Koren et al. can pyrolytic graphite (HOPG) and be illustrated more easily if we observe the interplay between adconsider, for example, a toy syshesion and friction forces in this tem consisting of ping-pong balls layered material. This experiment glued together in a hexagonal Torque with HOPG can be considered as a pattern (close packed) in two Torque model for two planes of graphene planes that are placed on top of or other so-called two-dimenone another (see the figure). The sional materials sliding over one in-plane attachment is much another. The findings are exciting stronger than the interaction bebecause they point to new ideas tween the planes, just as in layLateral shear for data storage at small scales, as ered, two-dimensional materials. well as novel actuation concepts One can imagine sliding or roin electromechanical devices. tating the planes relative to one another and finding a series of Center for the Mechanics of Solids, Structures equilibrium positions. and Materials, Department of Aerospace Another opportunity afforded Engineering and Engineering Mechanics, by the experiment of Koren et University of Texas at Austin, Austin TX 78712, Lateral shear USA. E-mail: [email protected] al. is that it may present a close By Kenneth M. Liechti
link to multiscale modeling from atomistic to continuum scales in that the interface or slip plane was free of any contamination and the geometry was relatively simple, which is not usually the case for contact pairs. Although atomistic or molecular analyses have provided insights into deformation and dissipation mechanisms and other effects that are difficult to separate out experimentally, one-to-one comparisons are limited by the high shear velocities and nanometer-scale probe tips that are currently accessible to analysis. Some progress in this direction has been provided by quartz microbalance experiments (4) and molecular analyses (5) of krypton monolayers sliding on gold, where the close agreement between simulation and experiment identified mechanisms for phononic contributions to friction. Molecular self-assembled monolayers provide another, slightly more complex avenue for linking experiments and analysis in addition to having technological relevance to MEMS. The issue of time and spatial scaling in atomistic analyses of friction between self-assembled monolayers has been identified as one of the current challenges in single asperity friction (6). One possible way of addressing this issue is through hybrid analyses with schemes that combine molecular analyses with phenomenological potentials (7, 8) that have allowed typical sliding speeds to be accessed. Koren et al. have come up with a novel experiment that provides a better understanding of friction in layered materials. Although graphite was the subject of this study, the extension to the plethora of twodimensional materials that are becoming available is clear. The authors have also provided much food for thought in developing potential applications of the observed phenomena to novel memory devices and machines. ■
ILLUSTRATION: C. SMITH/SCIENCE
REFERENCES AND NOTES
1. B. Bhushan, J. N. Israelachvili, U. Landman, Nature 374, 607 (1995). 2. E. Koren, E. Lörtscher, C. Rawlings, A. W. Knoll, U. Duerig, Science 348, 679 (2015). 3. R. W. Carpick, N. Agraït, D. F. Ogletree, M. Salmeron, Langmuir 12, 3334 (1996). 4. J. Krim, D. H. Solina, R. Chiarello, Phys. Rev. Lett. 66, 181 (1991). 5. M. Cieplak, E. D. Smith, M. O. Robbins, Science 265, 1209 (1994). 6. I. Szlufarska, M. Chandross, R. W. Carpick, J. Phys. D Appl. Phys. 41, 123001 (2008). 7. T. Ohzono, J. N. Glosli, M. Fujihira, Jpn. J. Appl. Phys. 37 (part 1, no. 12A), 6535 (1998). 8. T. Ohzono, J. N. Glosli, M. Fujihira, Jpn. J. Appl. Phys. 38 (part 2, no. 6A/B), L675 (1999). ACKNOWLEDGMENTS
Supported by the National Science Foundation and the Basic Energy Sciences program of the U.S. Department of Energy. 10.1126/science.aab0930
INFECTIOUS DISEASE
More than just bugs in spit Advances in imaging help to explain tuberculosis treatment failures and develop better drugs By Clifton Barry1,2,3
T
ease-free status in patients 1 year after completion of treatment. Because determining relapse-free cure requires such a long time, end points in phase 2 clinical trials focus on killing bacteria in expectorated sputum over the first few months of treatment. It has long been clear that the end points in these two different stages of clinical evaluation are not well correlated, as emphasized again recently by the failure of three large phase 3 studies that aimed to shorten the standard 6-month regimen to four months by including a fluoroquinolone as part of a four-drug regimen (1–3). Despite achieving
he development of new drugs for tuberculosis (TB) is not for the faint of heart. In addition to all the typical hurdles to developing a new antiinfective, additional complications result from the complex pathology induced by the disease. Because the bacillus is surrounded by a thick, waxy coat, hit rates in whole-cell screens are typically low; the resulting hits are typically so lipophilic that they do not offer good chemical starting points. Despite extensive research efforts, no TB drug has been developed using target-based approaches. Advances in imaging at both a macroscopic and microscopic level offer new insights and strategies for getting urgently needed new drugs to the clinic. TB is a disease of the lungs: It is transmitted by inhalation of an aerosol droplet containing the bacterium, which implants in small alveoli and then invades and multiplies in the interstitial spaces in the lung. Here, it forms granulomas—that is, nodules of immune cells recruited as the body attempts to wall off the invading bacteria. Eventually, the granulomas undergo necrosis to form cavities and leak out into the airways, spilling bacteria into the lungs and inducing coughing to enable transmission to a new host. A single patient with TB can earlier sterilization of sputum than convenhave dozens of individual lesions, all at diftional therapy in preceding phase 2 studies ferent stages of maturation. and in the phase 3 trials themselves, more TB therapy is often very protracted. patients in the 4-month fluoroquinolone Standard medical care for pulmonary TB arms suffered relapse after treatment was patients globally involves 6 months of treatdiscontinued. The fact that these patients ment. Although full treatment is successful were obviously not cured shows that bacin 95% of patients, in reality many patients teria in the sputum do not tell a complete fail to complete such a long course of therstory. Nor would one expect them to do so, apy; mobilizing the resources to ensure given that only certain lesions produce bacfull compliance is nearly impossible in reteria that are represented in the airways. source-poor countries where the disease is Curing all patients with a shortened TB endemic. Shorter courses of treatment are treatment regimen requires developing an one of the major goals of global research understanding of what leads to a relapse— efforts because they would aldefined as recurrent disease low for more supervision, less with the same bacterial strain recurrent disease, and correafter apparent initial cure—in spondingly less opportunity for a subset of treated patients. developing drug resistance. There are two main hypotheses U.S. and European regulatory for why relapse occurs. One bodies mandate that end points argues that relapse is due to a in phase 3 clinical trials of new subset of bacteria with unique TB drugs focus on achieving disphysiology (imposed by drugs, INFECTIOUS DISEASE SERIES
“All these exciting techniques have tremendous potential to accelerate TB drug development by improving the quality of drugs and drug combinations used in clinical trials to target specific lesion types.”
SCIENCE sciencemag.org
8 MAY 2015 • VOL 348 ISSUE 6235
Published by AAAS
633
Understanding friction in layered materials Kenneth M. Liechti Science 348, 632 (2015); DOI: 10.1126/science.aab0930
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/348/6235/632.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/348/6235/632.full.html#related This article cites 8 articles, 2 of which can be accessed free: http://www.sciencemag.org/content/348/6235/632.full.html#ref-list-1
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2015 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on May 7, 2015
The following resources related to this article are available online at www.sciencemag.org (this information is current as of May 7, 2015 ):
