Browsing by Subject "Force"

Now showing 1 - 6 of 6

Open Access
Forces and torques on the nanoscale: from measurement to applications
(SPIE, 2012) Volpe,Giovanni
The possibility of measuring microscopic forces down to the femtonewton range has opened new possibilities in fields such as biophysics and nanophotonics. I will review some of the techniques most often employed, namely the photonic force microscope (PFM) and the total internal reflection microscope (TIRM), which are able to measure tiny forces acting on optically trapped particles. I will then discuss several applications of such nanoscopic forces: from plasmonic optical manipulation, to self-propelled microswimmers, to self-organization in large ensembles of particles.
Open Access
Noncontact atomic force microscopy: an emerging tool for fundamental catalysis research
(American Chemical Society, 2015) Altman, E. I.; Baykara, M. Z.; Schwarz, U. D.
ConspectusAlthough atomic force microscopy (AFM) was rapidly adopted as a routine surface imaging apparatus after its introduction in 1986, it has not been widely used in catalysis research. The reason is that common AFM operating modes do not provide the atomic resolution required to follow catalytic processes; rather the more complex noncontact (NC) mode is needed. Thus, scanning tunneling microscopy has been the principal tool for atomic scale catalysis research. In this Account, recent developments in NC-AFM will be presented that offer significant advantages for gaining a complete atomic level view of catalysis.The main advantage of NC-AFM is that the image contrast is due to the very short-range chemical forces that are of interest in catalysis. This motivated our development of 3D-AFM, a method that yields quantitative atomic resolution images of the potential energy surfaces that govern how molecules approach, stick, diffuse, and rebound from surfaces. A variation of 3D-AFM allows the determination of forces required to push atoms and molecules on surfaces, from which diffusion barriers and variations in adsorption strength may be obtained. Pushing molecules towards each other provides access to intermolecular interaction between reaction partners. Following reaction, NC-AFM with CO-terminated tips yields textbook images of intramolecular structure that can be used to identify reaction intermediates and products.Because NC-AFM and STM contrast mechanisms are distinct, combining the two methods can produce unique insight. It is demonstrated for surface-oxidized Cu(100) that simultaneous 3D-AFM/STM yields resolution of both the Cu and O atoms. Moreover, atomic defects in the Cu sublattice lead to variations in the reactivity of the neighboring O atoms. It is shown that NC-AFM also allows a straightforward imaging of work function variations which has been used to identify defect charge states on catalytic surfaces and to map charge transfer within an individual molecule.These advances highlight the potential for NC-AFM-based methods to become the cornerstone upon which a quantitative atomic scale view of each step of a catalytic process may be gained. Realizing this potential will rely on two breakthroughs: (1) development of robust methods for tip functionalization and (2) simplification of NC-AFM instrumentation and control schemes. Quartz force sensors may offer paths forward in both cases. They allow any material with an atomic asperity to be used as a tip, opening the door to a wide range of surface functionalization chemistry. In addition, they do not suffer from the instabilities that motivated the initial adoption of complex control strategies that are still used today.
Embargo
On the identification and finite element treatment of macroscopic stress in Kohn–Sham density functional theory
(Elsevier BV, 2025-02-15) Temizer, İlker
The macroscopic stress formulation for periodic systems in Kohn–Sham density functional theory is critically examined. The identification of the stress through the partial variation of the energy with respect to cell deformation is cast in a strictly large deformation context. The nature of the non-uniqueness in the stress expression which emanates from this variation is extensively discussed. The possible lack of symmetry in this expression is highlighted and the conditions under which different expressions deliver the same tensorial value are derived. These observations are demonstrated through a finite element framework that is validated towards energy, force and stress calculations.
Open Access
A portable microfluidic system for rapid measurement of the erythrocyte sedimentation rate
(Royal Society of Chemistry, 2016) Isiksacan, Z.; Erel, O.; Elbuken, C.
The erythrocyte sedimentation rate (ESR) is a frequently used 30 min or 60 min clinical test for screening of several inflammatory conditions, infections, trauma, and malignant diseases, as well as non-inflammatory conditions including prostate cancer and stroke. Erythrocyte aggregation (EA) is a physiological process where erythrocytes form face-to-face linear structures, called rouleaux, at stasis or low shear rates. In this work, we proposed a method for ESR measurement from EA. We developed a microfluidic opto-electro-mechanical system, using which we experimentally showed a significant correlation (R2 = 0.86) between ESR and EA. The microfluidic system was shown to measure ESR from EA using fingerprick blood in 2 min. 40 μl of whole blood is filled in a disposable polycarbonate cartridge which is illuminated with a near infrared emitting diode. Erythrocytes were disaggregated under the effect of a mechanical shear force using a solenoid pinch valve. Following complete disaggregation, transmitted light through the cartridge was measured using a photodetector for 1.5 min. The intensity level is at its lowest at complete disaggregation and highest at complete aggregation. We calculated ESR from the transmitted signal profile. We also developed another microfluidic cartridge specifically for monitoring the EA process in real-time during ESR measurement. The presented system is suitable for ultrafast, low-cost, and low-sample volume measurement of ESR at the point-of-care.
Open Access
Theoretical study of crossed and parallel carbon nanotube junctions and three-dimensional grid structures
(American Physical Society, 2004) Dag, S.; Senger, R. T.; Çıracı, Salim
This work presents a first-principles study of parallel and crossed junctions of single-wall carbon nanotubes (SWNT). The crossed junctions are modeled by two-dimensional grids of zigzag SWNTs. The atomic and electronic structure, stability, and energetics of the junctions are studied for different magnitudes of contact forces pressing the tubes towards each other and hence inducing radial deformations. Under relatively weak contact forces the tubes are linked with intertube bonds which allow a significant conductance through the junction. These interlinking bonds survive even after the contact forces are released and whole structure is fully relaxed. Upon increasing contact force and radial deformation the tube surfaces are flattened but the interlinking bonds are broken to lead to a relatively wider intertube spacing. The intertube conductance through such a junction diminish because of finite potential barrier intervening between the tubes. The linkage of crossing tubes to form stable junctions is enhanced by a vacancy created at the contact. The three-dimensional grid structure formed by SWNTs is also investigated as a possible framework in device integration.
Open Access
Wireless monitoring of a structural beam to be used for post-earthquake damage assessment
(IEEE, 2018-05-06) Özbey, B.; Kurc, O.; Demir, Hilmi Volkan; Ertürk, Vakur B.; Altıntaş, Ayhan
Wireless monitoring of a standard reinforced concrete beam is shown in a simply supported beam experiment. The passive nested split-ring resonator (NSRR) probes are attached on the reinforcing bars (rebars) within the beam, and an antenna interrogates the probes from outside the beam. The results of the experiment show that the plastic deformation region strain/displacement can be detected by the wireless sensing system. The data collected by the system constitutes an important input for the assessment of the damage that can be observed after earthquakes.