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1. Faster Indentation Influences Skin Deformation To Reduce Tactile Discriminability of Compliant Objects

   https://doi.org/10.1109/TOH.2023.3253256  

   Li, Bingxu; Hauser, Steven C.; Gerling, Gregory J. (April 2023, IEEE Transactions on Haptics)

   To discriminate the compliance of soft objects, we rely upon spatiotemporal cues in the mechanical deformation of the skin. However, we have few direct observations of skin deformation over time, in particular how its response differs with indentation velocities and depths, and thereby helps inform our perceptual judgments. To help fill this gap, we develop a 3D stereo imaging method to observe contact of the skin’s surface with transparent, compliant stimuli. Experiments with human-subjects, in passive touch, are conducted with stimuli varying in compliance, indentation depth, velocity, and time duration. The results indicate that contact durations greater than 0.4 s are perceptually discriminable. Moreover, compliant pairs delivered at higher velocities are more difficult to discriminate because they induce smaller differences in deformation. In a detailed quantification of the skin’s surface deformation, we find that several, independent cues aid perception. In particular, the rate of change of gross contact area best correlates with discriminability, across indentation velocities and compliances. However, cues associated with skin surface curvature and bulk force are also predictive, for stimuli more and less compliant than skin, respectively. These findings and detailed measurements seek to inform the design of haptic interfaces. 

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2. New Insights into the Microstructural Changes During the Processing of Dual-Phase Steels from Multiresolution Spherical Indentation Stress–Strain Protocols

   https://doi.org/10.3390/met10010018  

   Khosravani, Ali; Caliendo, Charles M.; Kalidindi, Surya R. (January 2020, Metals)

   In this study, recently established multiresolution spherical indentation stress–strain protocols have been employed to derive new insights into the microstructural changes that occur during the processing of dual-phase (DP) steels. This is accomplished by utilizing indenter tips of different radii such that the mechanical responses can be evaluated both at the macroscale (reflecting the bulk properties of the sample) and at the microscale (reflecting the properties of the constituent phases). More specifically, nine different thermo-mechanical processing conditions involving different combinations of intercritical annealing temperatures and bake hardening after different amounts of cold work were studied. In addition to demonstrating the tremendous benefits of the indentation protocols for evaluating the variations within each sample and between the samples at different material length scales in a high throughput manner, the measurements provided several new insights into the microstructural changes occurring in the alloys during their processing. In particular, the indentation measurements indicated that the strength of the martensite phase reduces by about 37% when quenched from 810 °C compared to being quenched from 750 °C, while the strength of the ferrite phase remains about the same. In addition, during the 10% thickness reduction and bake hardening steps, the strength of the martensite phase shows a small decrease due to tempering, while the strength of the ferrite increases by about 50% by static aging. 

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3. Thin Films on the Skin, but not Frictional Agents, Attenuate the Percept of Pleasantness to Brushed Stimuli

   https://doi.org/10.1109/WHC49131.2021.9517259  

   Rezaei, Merat; Nagi, Saad S.; Xu, Chang; McIntyre, Sarah; Olausson, Hakan; Gerling, Gregory J. (July 2021, IEEE World Haptics Conference)

   Brushed stimuli are perceived as pleasant when stroked lightly on the skin surface of a touch receiver at certain velocities. While the relationship between brush velocity and pleasantness has been widely replicated, we do not understand how resultant skin movements – e.g., lateral stretch, stick-slip, normal indentation – drive us to form such judgments. In a series of psychophysical experiments, this work modulates skin movements by varying stimulus stiffness and employing various treatments. The stimuli include brushes of three levels of stiffness and an ungloved human finger. The skin’s friction is modulated via non-hazardous chemicals and washing protocols, and the skin’s thickness and lateral movement are modulated by thin sheets of adhesive film. The stimuli are hand-brushed at controlled forces and velocities. Human participants report perceived pleasantness per trial using ratio scaling. The results indicate that a brush’s stiffness influenced pleasantness more than any skin treatment. Surprisingly, varying the skin’s friction did not affect pleasantness. However, the application of a thin elastic film modulated pleasantness. Such barriers, though elastic and only 40 microns thick, inhibit the skin’s tangential movement and disperse normal force. The finding that thin films modulate affective interactions has implications for wearable sensors and actuation devices. 

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4. Atomistic Investigation of Material Deformation Behavior of Polystyrene in Nanoimprint Lithography

   https://doi.org/10.3390/surfaces3040043  

   Odujole, Jahlani; Desai, Salil (December 2020, Surfaces)

   null (Ed.)

   This research investigates deformation behavior of polystyrene (PS) as a thermoplastic resist material for the thermal nanoimprint lithography (T-NIL) process. Molecular dynamics modeling was conducted on a PS substrate with dimensions 58 × 65 × 61 Å that was imprinted with a rigid, spherical indenter. The effect of indenter size, force, and imprinting duration were evaluated in terms of indentation depth, penetration depth, recovery depth, and recovery percentage of the polymer. The results show that the largest indenter, regardless of force, has the most significant impact on deformation behavior. The 40 Å indenter with a 1 µN of force caused the surface molecules to descend to the lowest point compared to the other indenters. An increase in indenter size resulted in higher penetration depth, recovery depth, and recovery percentage. Higher durations of imprint cycle (400 fs) resulted in plastic deformation of the PS material with minimal recovery (4 Å). The results of this research lay the foundation for explaining the effect of several T-NIL process parameters on virgin PS thermoplastic resist material. 

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5. Molecular dynamics investigation of material deformation behavior of PMMA in nanoimprint lithography

   https://doi.org/10.1063/5.0014458  

   Odujole, Jahlani_I; Desai, Salil (September 2020, AIP Advances)

   Computational analysis of polymeric materials plays a key role in defining their tribological characteristics. This research investigates the deformation behavior of poly(methylmethacrylate) (PMMA) as a thermoplastic resist material for the thermal nanoimprint lithography (T-NIL) process. Molecular dynamics modeling was conducted on a PMMA substrate imprinted with a rigid spherical indenter. The effect of indenter size, force, and imprinting duration on the indentation depth, penetration depth, recovery depth, and recovery percentage of the polymer was evaluated. The results showed that the largest indenter, regardless of force, had the most significant impact on deformation behavior. The 40-Å indenter with a 1 μN of force caused the surface molecules to descend to the lowest point compared to the other indenters. An increase in the indenter size resulted in a higher penetration depth, recovery depth, and recovery percentage. Higher durations of imprint cycle (400 fs) resulted in plastic deformation of the PMMA material with minimal recovery (30 Å). The results of this research lay foundation for explaining the effect of several T-NIL process parameters on the virgin PMMA thermoplastic resist material. 

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