Increasing Gap Width Research Articles

Исследование щелевой моды в структуре, включающей линию задержки и резонирующую пьезоэлектрическую пластину

The results of the experimental and theoretical study of a structure including a delay line based on a lithium niobate plate of Y-X cut with two interdigitated transducers for exciting and receiving an acoustic wave with the shear-horizontal polarization in the frequency range 1.5-2.2 MHz are presented. The plate thickness was 0.35 mm. A resonating lithium niobate plate was located above the delay line between the transducers with a certain gap. Resonating plates of Z-X, Y-X–140° and Y-X+155° crystallographic cuts were studied. When RF voltage was applied to the input transducer, pronounced resonant absorption peaks were observed on the frequency dependence of the insertion loss. It has been established that with increasing width of the gap between the plates, the depth of the resonance peaks monotonically decreases and the peaks smoothly disappears. Moreover, for most peaks, the quality factor decreases with increasing gap width. The theoretical analysis of such structures showed good agreement between the theoretical and the experimental results. The developed structure shows promise for its use for multiparameter analysis of various liquids

DNS analysis of flow and heat transfer of SCO2 in a square subchannel–Effect of gap width

The heat transfer and flow characteristics of supercritical carbon dioxide (SCO2) in a tight lattice are problematic in the thermal-hydraulic design of reactor fuel assemblies. Herein, direct numerical simulation (DNS) was performed to investigate the heat transfer and flow behavior of SCO2 in a vertically upward square subchannel. Emphasis was placed on clarifying the effect of the pitch-to-diameter (p/d) ratio, defined as the subchannel pitch over the rod diameter, on the time-averaged heat transfer and instantaneous velocity/temperature fluctuation. It was found that the wall temperature exhibits a “V-shaped” profile along the rod circumference, which is caused by the combined effects of buoyancy and an uneven flow area. The wall temperature is lower for subchannels with smaller p/d ratios; meanwhile, the circumferential non-uniform heat transfer becomes stronger, which also requires to be considered. The time-averaged streamwise velocity and fluid temperature have similar distributions on the subchannel cross-section, that is, high velocity and temperature occur near the rod gap, while regions with low velocity and temperature are identified in the subchannel interior. With an increase in the p/d ratio, both the temperature and velocity gradients are enlarged. Large-scale flow pulsations were identified in the subchannels. The instantaneous velocity fluctuation is weakened with increasing gap width; however, the fluctuation in the wall temperature exhibits an opposite trend. The effects of the p/d ratio on the dominant frequency of velocity/temperature fluctuation are not universal. Among the three selected cases, a subchannel with a small p/d ratio had a higher dominant frequency of instantaneous fluctuations. The turbulence structure was studied using a quadrant and anisotropy analysis. It was found that the outward and inward interactions included the main turbulence structures in the subchannel interior. In addition, with an increase in the p/d ratio, the transformation from disk-like turbulence to rod-like turbulence can be identified.

Effect of the gap width in AZ31 magnesium alloy joints obtained by friction stir welding

Thin AZ31 magnesium alloy sheets, i.e., 2 mm thick, are welded by Friction Stir Welding (FSW) in butt joint configuration using gap width up to 1.15 mm. All welds present good surface finishing and no internal defects, except for the weld produced using the maximum gap width. A reduction of the weld thickness within the Stir Zone is seen with the increase in gap width, leading to a maximum thickness reduction of 8.5%. Microstructure and Vickers hardness investigations reveal no influence of the gap width on these properties. Up to a gap width of 0.51 mm, a slight decrease in the Ultimate Tensile Strength (UTS) is observed with increasing gap width. For larger gap widths, the UTS, as well as the fracture strain, are constant. To keep the metallurgical integrity, a maximum gap width of 1 mm seems acceptable for joints of thin AZ31 magnesium alloy sheets produced by FSW.

Observation of self-organized honeycomb patterns by fast photography in a liquid-anode discharge

In a rod-water geometry, self-organized patterns are formed on the water surface of an atmospheric glow discharge excited by a square-wave voltage, which include a disk with an encircling ring and concentric triple rings with varying air gap widths. The diameter of these patterns slightly increases with the increasing gap width. Although a square-wave voltage is used, waveforms of voltage and current indicate that the discharge belongs to a periodical liquid-anode discharge. By fast photography, spatial and temporal evolutions of these self-organized patterns indicate that honeycomb patterns are prone to be generated with a wider gap. Moreover, these honeycomb patterns finally lead to the formation of the concentric triple rings and the disk with an encircling ring with wider gaps. Within the scope of our knowledge, this is the first observation of honeycomb patterns in a liquid-anode discharge.

Experimental and numerical investigation for the geometrical parameters effect on the labyrinth-seal flow characteristics of fast reactor fuel assembly

The labyrinth seal is widely used in the fast reactor fuel assembly to control leakage flow rate, adjust pressure drop and cool assembly outer wall. In the current investigation, a new non-rotating labyrinth seal with right-angle trapezoidal throttle tooth is designed and manufactured, and its flow characteristics are experimentally and numerically investigated. Reynolds number from 61,512 to 81,261 is considered and experiment data were generated from hydraulic experiment conducted at constant water temperature (84 °C) in the labyrinth-seal test section. Numerical results with different turbulence model, wall-normal grid spacing, inlet turbulence intensity and stream-wise grid length are compared with experiment results to demonstrate the effectiveness of numerical simulation. Geometrical parameters have a significant effect on labyrinth-seal flow characteristics. Increasing throttle-tooth number, spacing and throttle-tooth thickness can improve the labyrinth-seal sealing performance, while increasing gap width influences the flow velocity limitation and increases the leakage. Labyrinth-seal resistance coefficient is validated to be independent of Reynolds number (61,512–81,261). It is deduced that the fully rough regime still exists in an annular-tube turbulence flow with high wall roughness. The energy dissipation caused by throttling effect, jet, vortex and secondary transverse turbulence is the main sealing mechanism. Moreover, a leakage-coefficient empirical correlation is obtained by regression analysis of experiment data, which can be used for the structure design of sodium-cooled fast reactor and preliminary estimation of leakage flow rate in similar non-rotating seals.

CFD Simulations of the Propagation of Free-Surface Waves Past Two Side-By-Side Fixed Squares with a Narrow Gap

Two-dimensional computational fluid dynamics (CFD) simulations are carried out to investigate the gap resonance phenomenon that occurs when free-surface waves travel past twin squares in tandem. The volume of fluid method is used to capture the free surface. Validation studies of the present numerical model are conducted for different incident wave frequencies. The numerical results agree well with the published experimental data in terms of the free surface elevations in the gap. The hydrodynamic characteristics of the water column in the gap are investigated at different incident wave frequencies and gap widths. It is found that the free surface elevation in the gap increases and then decreases with the increasing incident wave frequency. The horizontal force on the weather side square structure (the structure in front of the gap) reaches the peak value at a larger frequency than the gap resonance frequency, whereas the variation of the horizontal force on the lee side structure (the structure behind the gap) is in-phase with the free surface elevation in the gap. Moreover, the added mass of the water column in the gap increases with the increasing gap width, which results in the decrease of resonance amplitude and frequency in the gap. However, this does not necessarily reduce the peak value of the horizontal forces on the structures.

A simple treatment of increased gap due to fuel assembly bowing through correction of cross sections

ABSTRACT Increased fuel assembly gap due to bowing in commercial light-water reactors (LWRs) has an impact on local pin-power distribution due to increased local moderation. In order to consider the effect of increased assembly gap without explicit consideration of increased gap width, a correction method of cross sections in the gap region is proposed. In this cross section correction method, the average chord length of gap region is preserved to capture the effect of increased gap width. The validity of the present method is confirmed by verification calculations in a single assembly and 5 × 5 assembly geometries using the GENESIS code, which is a transport code based on the method of characteristics.

Gap bridging of 6061 aluminum alloy joints welded by variable-polarity cold metal transfer

6061 aluminum alloy joints at different gap widths were welded successfully with the same parameters (except wire offset). Bead formation, microhardness, shear strength, fracture characteristic, and forming mechanism were investigated. The results indicated that the shear strength of the joints decreased with increasing gap width. The gap width had little impact on the hardness of joints, and the fracture mode of the samples at different gap widths was ductile fracture. The wire offset and wire feeding speed had significant effects on the bead formation. The results of high-speed video and current waveform analysis showed that the variable-polarity cold metal transfer (CMT) Advance (C + A) process had better gap-bridging ability due to a lower heat input and higher deposition rate compared with the conventional CMT mode.

Computation of a Shrinking Interface in a Hele-Shaw Cell

In this paper, we present an adaptive rescaling method for computing a shrinking interface in a Hele-Shaw cell with a time increasing gap width b(t). We focus our study on a one-phase interior Hele...

Spoof surface plasmon polaritons on ultrathin metal strips: from rectangular grooves to split-ring structures

An ultrathin metal strip consisting of connected split-ring resonators (SRRs) is proposed which can tune the dispersion of spoof surface plasmon polaritons (SPPs) by varying the gap widths of the SRRs. It is found that the decrease of the gap width leads to a remarkable red-shift of the dispersion of spoof SPPs. The underlying mechanism can be interpreted from the resonance properties of a single SRR and two connected SRRs. As an application, we experimentally demonstrate that spoof SPPs with different frequencies can be slowed down gradually and stopped at different positions by introducing a graded SRR structure with a linearly increased gap width, and thus, a slow-light “rainbow” is observed.

Optical properties of dielectric plates coated with gapped graphene

The optical properties of dielectric plates coated with gapped graphene are investigated on the basis of first principles of quantum electrodynamics. The reflection coefficients and reflectivities of graphene-coated plates are expressed in terms of the polarization tensor of gapped graphene and the dielectric permittivity of plate material. Simple approximate expressions for the required combinations of components of the polarization tensor applicable in the wide frequency region, where the presence of a gap influences the optical properties, are found. Numerical computations of the reflectivities of graphene-coated SiO${}_2$ plates are performed for different values of the mass-gap parameter at different temperatures. It is shown that with an increasing gap width the reflectivity of a graphene-coated plate at the normal incidence decreases by up to a factor of 8 depending on the values of frequency and mass-gap parameter. The angle dependences of reflectivities for both polarizations of the incident electromagnetic waves have been computed for Si and SiO${}_2$ plates coated with gapped graphene. We demonstrate that the TM reflectivity has a minimum value at some angle of incidence depending on the mass-gap parameter, frequency and temperature, whereas the TE reflectivity depends on the angle of incidence monotonously. However, for the graphene coatings with a nonzero mass-gap parameter the reflected light cannot be fully polarized. Possible applications of the obtained results are discussed.

Redox Cycling Behavior of Catecholamines and Their Mixtures at Different Diffusion Distances: Steps Toward Quantitative Speciation

Investigation of catecholamines (dopamine: DA, norepinephrine: NE and epinephrine EP) has been of importance, because of the crucial roles that these molecules play in neurological function and related diseases. Due to the similar chemical structures and oxidation potentials of these compounds, it has been a challenge to use simple electrochemical techniques to differentiate them in mixtures.1 Redox cycling is known to have the capability to differentiate between species based on the propensity of the compounds to diffuse to and produce an electrochemical response at the neighboring electrode.2 The oxidized species generated at an electrode (generator) can diffuse to neighboring electrodes held at a reducing potential (collector) that will yield the original species. This process can occur multiple times and can produce an amplified signal. If the oxidized form of the species is electrochemically inactive, selectivity can be achieved for the electroactive molecules at the collector. Chemical reactions that can happen during the transfer between generator and collector can also convert the oxidized species to an electroinactive one and prevent a signal at the collector electrode, as well. This can be the basis of separating and detecting species using redox cycling. In their ECC’ mechanism, catecholamines undergo a two-electron oxidation (E) to yield the o-quinone.3 The o-quinone form can then go through an internal cyclization reaction (C) to yield the leucoaminochrome. The leucoaminochrome and o-quinone forms can react with each other (C’) to yield the original catecholamine and the aminochrome form. The internal cyclization reaction that is involved in the C part of the mechanism occurs at different rates for the different catecholamines. The apparent rate constants at pH=7.4 in phosphate buffer have been reported to be 0.13 ± 0.05 s-1, 0.98 ± 0.52 s-1 and 87 ± 10 s-1 for DA, NE and EP, respectively.4 Due to the difference in these rate constants, the species can be distinguished by spatial distribution based on the survival of the o-quinone from the following chemistry. Step C in the mechanism can also affect the number of times the species can go back and forth between the electrodes and therefore the amplification of the signal, depending on the species. The purpose of our research is to study the fundamental electrochemical redox cycling behavior of DA, NE and EP. By understanding this behavior, it can then be determined whether their simultaneous detection in more complex samples will be feasible. Recent developments from our laboratory will be discussed. We are using microfabricated chips with individually-addressable microelectrode arrays to activate various combinations of electrodes in the redox cycling experiments. By activating different electrodes on the array, greater distances between the collector and generator electrodes can be achieved leading to longer transit times for the molecules to move between electrodes. This study will lead to the generation of a calibration map for different gaps, ratios of catecholamines, and concentrations. There is a greater decrease in collector signal with increasing gap width (beyond the loss due to diffusion) for species with greater rate constants for step C. Epinephrine does not exhibit a reduction signal when the gap is set at 4-µm.2 Norepinephrine’s signal decreases as a function of increasing gap more sharply than dopamine’s. There is no observed collector signal for NE at the collector electrode, which is activated 28-µm away from the collector. The oxidation signal of the mixtures of catecholamines decreases even further due to the following chemistry producing the leucoaminochrome and increased concentrations of o-quinone and leucoaminochrome molecules. References Hawley, M. D.; Tatawawadi, S. V.; Piekarski, S.; Adams, R. N. Journal of the American Chemical Society 1967, 89, 447-450. Hu, Mengjia; Fritsch, Ingrid. Analytical Chemistry 2015, 87, 2029−2032.Ciolkowski, E. L.; Maness, K. M.; Cahill, P. S.; Wightman, R. M.; Evans, D. H.; Fosset, B.; Amatore, C. Analytical Chemistry 1994, 66, 3611-3617.Ciolkowski, E. L.; Cooper, B. R.; Jankowski, J. A.; Jorgenson, J. W.; Wightman, R. M. Journal of the American Chemical Society 1992, 114, 2815-2821.

Improving the spatial resolution in CZT detectors using charge sharing effect and transient signal analysis: Simulation study

Cadmium Zinc Telluride (CZT) semiconductor detectors are capable of providing superior energy resolution and three-dimensional position information of gamma ray interactions in a large variety of fields, including nuclear physics, gamma-ray imaging and nuclear medicine. Some dedicated Positron Emission Tomography (PET) systems, for example, for breast cancer detection, require higher contrast recovery and more accurate event location compared with a whole-body PET system. The spatial resolution is currently limited by electrode pitch in CZT detectors. A straightforward approach to increase the spatial resolution is by decreasing the detector electrode pitch, but this leads to higher fabrication cost and a larger number of readout channels. In addition, inter-electrode charge spreading can negate any improvement in spatial resolution. In this work, we studied the feasibility of achieving sub-pitch spatial resolution in CZT detectors using two methods: charge sharing effect and transient signal analysis. We noted that their valid ranges of usage were complementary. The dependences of their corresponding valid ranges on electrode design, depth-of-interaction (DOI), voltage bias and signal triggering threshold were investigated. The implementation of these two methods in both pixelated and cross-strip configuration of CZT detectors were discussed. Our results show that the valid range of charge sharing effect increases as a function of DOI, but decreases with increasing gap width and bias voltage. For a CZT detector of 5mm thickness, 100µm gap and biased at 400V, the valid range of charge sharing effect was found to be about 112.3µm around the gap center. This result complements the valid range of the transient signal analysis within one electrode pitch. For a signal-to-noise ratio (SNR) of ~17 and preliminary measurements, the sub-pitch spatial resolution is expected to be ~30µm and ~250µm for the charge sharing and transient signal analysis methods, respectively.

Redox Cycling Behavior of Catecholamines and Their Mixtures at Different Diffusion Distances: Steps Toward Quantitative Speciation

Catecholamines (dopamine, DA; norepinephrine, NE and epinephrine, EP) are neurotransmitters of great importance in the mammalian brain. The electrochemical oxidation of these catecholamines has been studied before and can undergo a disproportionation reaction.1,2 During electrochemical oxidation, a catecholamine undergoes a reversible 2-electron transfer process and is oxidized to its o-quinone form. At physiological pH, the o-quinone deprotonates and undergoes homogeneous intramolecular cyclization to form the leucoaminochrome. The leucoaminochrome can oxidize to form the aminochrome which is not electroactive by reacting with the o-quinone form and re-generating the starting catecholamine species. The degree of cyclization reaction will change the reversibility of the total reaction. Different catecholamines have different apparent rate constants of the cyclization reaction (kDA = 0.13 ± 0.05 s-1, kNE = 0.98 ± 0.52 s-1, kEP = 87 ± 10 s-1, in phosphate buffer at pH 7.43), which provide the possibility for distinguishing the three catecholamines from each other in an electrochemical analysis. The electrochemical method of redox cycling has the ability of recycling electrochemically reversible species, thus providing amplified signals and the selectivity of reversible species over irreversible ones.4 In the redox cycling process of catecholamines, the oxidation and subsequent reduction reactions take place at different electrodes, namely the generator and collector electrodes, respectively. The time between the two reactions is controlled by the time the species diffuse across the gap between those electrodes. The extent of reversibility of the heterogeneous electron transfer reaction during redox cycling is then determined by the relationship of diffusion time and cyclization reaction time.5 We will report the electrochemical redox cycling behaviors of individual and binary mixtures of DA, NE, and EP using microfabricated gold microelectrode arrays with various gap widths between the generators and collectors. The catecholamines showed decreasing collector signals as gap width increased with increasing cyclization rates, which is a reflection of how much o-quinone survives diffusion time from the generator. With increasing gap width, the collector signals of the three catecholamines all decreased because of decreasing collection efficiencies, as expected. However, the signals decreased at different extents, depending on the cyclization rates. The collector signal for EP became undetectable at a smaller gap width than that for NE, while the collector signal for DA was still measurable. In the binary mixture of catecholamines, o-quinone and leucoaminochrome from different catecholamines can also react, thus make the collector signal decrease further, compared to solutions containing a single kind of catecholamine. Acknowledgments Research was supported partially through the National Science Foundation (CBET-1336853) and the Arkansas Biosciences Institute, the major research component of the Arkansas Tobacco Settlement Proceeds Act of 2000.

A parametric investigation of the performance of multiple edge highway noise barriers and proposals for design guidance

The Boundary Element Method has been employed for a parametric investigation of the performance afforded by highway noise barriers with multi-edge tops and different acoustic treatment. Configurations investigated included single additional edges located on either the source or receiver sides of the barrier and two additional edges located symmetrically on each side of the barrier. The effect of treating the internal faces with a sound absorbing material was also investigated. The parameters investigated included the source to barrier distance, the receiver to barrier distance, the barrier height, the length of the additional edge and the gap between the additional edges and the face of the barrier. The values of each parameter were selected to be those appropriate to a practical installation. The performance of each edge variation was investigated for both reflective and absorptive faces. The relative insertion loss afforded by a given multi-edge configuration was found to be a function of the location of the source, the barrier and the receiver and also the height of the barrier. However, the sensitivity of the relative insertion loss to variations in most parameters was not great. For both reflective and absorptive treatments, the relative insertion loss of most additional edges was found to be only slightly greater on increasing the length of the edge above 0.5m. Although there was always an increase with increasing gap width over the range of widths investigated gap but the indications were that benefit to be obtained for gap widths in excess of 0.4m may not be an economic proposition. For source locations close to the barrier and receiver locations in the far field, as would be appropriate for a highway noise barrier, although the relative insertion loss afforded by a reflective additional edge located on the source side of the barrier is generally low, significant attenuation can be obtained when an absorptive treatment with a high coefficient of absorption at frequencies around 1000Hz is applied to the device. However, a reflective edge located on the source side of a barrier at very short source to barrier distances and/or high barriers was found to result in a negative value of relative insertion loss and an explanation based upon resonances in the gap between the additional edge and the barrier was postulated. The use of two additional edges located symmetrically either side of a barrier can be a very effective means of improving the performance of a highway noise barrier giving relative insertion loss values that are high and consistent over a large range of parameters for both reflective and absorptive surfaces. However, in the former case the performance can be severely reduced for combinations of short source to barrier distances and/or high barriers.

Hydrodynamic performance study of two side-by-side barges

Recently, hydrodynamic interaction between two vessels in a side-by-side configuration has become a research focus. Because of the interaction of multiple bodies, the fluid movement and responses of the vessels are complex. First, the hydrodynamic performances of two identical barges in close proximity in a side-by-side configuration are numerically investigated with a wave elevation in between the barges. Second, corresponding experiments are carried out to validate the numerical results, and comparison of the experimental and calculation results shows that the numerical calculation accurately predicts the resonance period despite a difference in the peak values. Both fixed barges and barges undergoing constrained motions are also compared with free-floating barges to illustrate the influence of motion on the wave elevation and resonance phenomena. Sensitivity analyses of the gap width, barge length, barge breadth, and draft are performed, revealing that the resonant wavelength increases with increasing gap width or barge dimensions.

Mitigation measures for earthquake induced pounding effects on seismic performance of adjacent buildings

Post-earthquake damages investigation in past and recent earthquakes has illustrated that the building structures are vulnerable to severe damage and/or collapse during moderate to strong ground motion. Among the possible structural damages, seismic induced pounding has been commonly observed in several earthquakes. A parametric study on buildings pounding response as well as proper seismic hazard mitigation practice for adjacent buildings is carried out. Three categories of recorded earthquake excitation are used for input excitations. The effect of impact is studied using linear and nonlinear contact force model for different separation distances and compared with nominal model without pounding consideration. The severity of the impact depends on the dynamic characteristics of the adjacent buildings in combination with the earthquake characteristics. Pounding produces acceleration and shear forces/stresses at various story levels that are greater than those obtained from the no pounding case, while the peak drift depends on the input excitation characteristics. Also, increasing gap width is likely to be effective when the separation is sufficiently wide to eliminate contact. Furthermore, it is effective to provide a shock absorber device system for the mitigation of impact effects between adjacent buildings with relatively narrow seismic gaps, where the sudden changes of stiffness during poundings can be smoothed. This prevents, to some extent, the acceleration peaks due to impact. The pounding forces exerted on the adjacent buildings can be satisfactorily reduced.

Performance analysis of axial flow pump on gap changing between impeller and guide vane

In order to study the influence on gap changing of the static and dynamic components in axial flow pump, the axial flow pump model (TJ04-ZL-06) that used in the eastern of south-to-north water diversion project was selected. Steady turbulence field with different gaps was simulated by standard κ-ε turbulence model and double-time stepping methods. Information on the pressure distribution and velocity distribution of impeller surfaces were obtained. Then, calculated results were compared with the test results and analyzed. The results show that the performance of pump is not sensitive with the axial gap width under design conditions and the large flow rate condition. With increasing gap width, it will be improved in low flow rate condition. The attack angle of impeller inlet in small flow rate condition become small and the flow separation phenomenon can be observed in this condition. The axial velocity distribution of impeller outlet is nonlinear and to increase the axial gap is to improve the flow pattern near the hub effectively. The trend of calculating results is identical with test. It will play a guiding role to the axial pump operation and design in south-to-north water diversion project.

Characteristics of a Normal Glow Discharge Excited by DC Voltage in Atmospheric Pressure Air

Atmospheric pressure glow discharges were generated in an air gap between a needle cathode and a water anode. Through changing the ballast resistor and gas gap width between the electrodes, it has been found that the discharges are in normal glow regime judged from the current-voltage characteristics and visualization of the discharges. Results indicate that the diameter of the positive column increases with increasing discharge current or increasing gap width. Optical emission spectroscopy is used to calculate the electron temperature and vibrational temperature. Both the electron temperature and the vibrational temperature increases with increasing discharge current or increasing gap width. Spatially resolved measurements show that the maxima of electron temperature and vibrational temperature appeared in the vicinity of the needle cathode.

An actin length threshold regulates adhesion maturation at the lamellipodium/lamellum interface

The mechanical coupling between adherent cells and their substrates is a major driver of downstream behavior. This coupling relies on the formation of adhesion sites and actin bundles. How cells generate these elements remains only partly understood. A potentially important mechanism, the length threshold maturation (LTM), has previously been proposed to regulate adhesion maturation and actin bundle stabilization tangential to the leading edge. The LTM describes the process by which cells integrate lamellar myosin forces to trigger adhesion maturation. These forces, cumulated over the length of an actin bundle, are balanced at the anchoring focal complexes. When the bundle length exceeds a certain threshold, the distributed lamellar forces become sufficient to trigger the stabilization of the bundle and its adhesions. In this continuing study, we experimentally challenge the LTM for the first time, by seeding cells on micropatterned substrates with various non-adhesive gaps designed to selectively trigger the LTM. While stable actin bundles were observed on all patterns, their lengths were almost exclusively above 3 μm or 4 μm depending on the cell type. Furthermore, the frequency with which gaps were bridged increased nearly as a step function with increasing gap width, indicating a substrate dependent behavioral switch. These combined observations point strongly to LTM with a threshold above 3 μm. We thus experimentally confirm with two cell types our previous theoretical work postulating the existence of a length dependent threshold mechanism that triggers adhesion maturation and actin bundle stabilization.