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1. Dilatancy, shear jamming, and a generalized jamming phase diagram of frictionless sphere packings

   https://doi.org/10.1039/D0SM02186E  

   Babu, Varghese; Pan, Deng; Jin, Yuliang; Chakraborty, Bulbul; Sastry, Srikanth (March 2021, Soft Matter)

   null (Ed.)

   Granular packings display the remarkable phenomenon of dilatancy, wherein their volume increases upon shear deformation. Conventional wisdom and previous results suggest that dilatancy, also being the related phenomenon of shear-induced jamming, requires frictional interactions. Here, we show that the occurrence of isotropic jamming densities ϕ j above the minimal density (or the J-point density) ϕ J leads both to the emergence of shear-induced jamming and dilatancy in frictionless packings. Under constant pressure shear, the system evolves into a steady-state at sufficiently large strains, whose density only depends on the pressure and is insensitive to the initial jamming density ϕ j . In the limit of vanishing pressure, the steady-state exhibits critical behavior at ϕ J . While packings with different ϕ j values display equivalent scaling properties under compression, they exhibit striking differences in rheological behaviour under shear. The yield stress under constant volume shear increases discontinuously with density when ϕ j > ϕ J , contrary to the continuous behaviour in generic packings that jam at ϕ J . Our results thus lead to a more coherent, generalised picture of jamming in frictionless packings, which also have important implications on how dilatancy is understood in the context of frictional granular matter. 

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2. Contact criterion for suspensions of smooth and rough colloids

   https://doi.org/10.1039/D0SM00072H  

   Pradeep, Shravan; Hsiao, Lilian C. (June 2020, Soft Matter)

   We report a procedure to obtain the search distance used to determine particle contact in dense suspensions of smooth and rough colloids. This method works by summing physically relevant length scales in an uncertainty analysis and does not require detailed quantification of the surface roughness. We suspend sterically stabilized, fluorescent poly(methyl methacrylate) colloids in a refractive index-matched solvent, squalene, in order to ensure hard sphere-like behavior. High speed centrifugation is used to pack smooth and rough colloids to their respective jamming points, ϕ J . The jammed suspensions are subsequently diluted with known volumes of solvent to ϕ < ϕ J . Structural parameters obtained from confocal laser scanning micrographs of the diluted colloidal suspensions are extrapolated to ϕ J to determine the mean contact number at jamming, 〈 z 〉 J . Contact below jamming refers to nearest neighbors at a length scale below which the effects of hydrodynamic or geometric friction come into play. Sensitivity analyses show that a deviation of the search distance by 1% of the particle diameter results in 〈 z 〉 changing by up to 10%, with the error in contact number distribution being magnified in dense suspensions ( ϕ > 0.50) due to an increased number of nearest neighbors in the first coordination shell. 

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3. We Can Hear Your PIN Drop: An Acoustic Side-Channel Attack on ATM PIN Pads

   Kiran Balagani, Matteo Cardaioli (September 2022, ESORICS 2022)

   Personal Identification Numbers (PINs) are the most common user authentication method for in-person banking transactions at ATMs. The US Federal Reserve reported that, in 2018, PINs secured 31.4 billion transactions in the US, with an overall worth of US$ 1.19 trillion. One well-known attack type involves the use of cameras to spy on the ATM PIN pad during PIN entry. Countermeasures include covering the PIN pad with a shield or with the other hand while typing. Although this protects PINs from visual attacks, acoustic emanations from the PIN pad itself open the door for another attack type. In this paper, we show the feasibility of an acoustic side-channel attack (called PinDrop) to reconstruct PINs by profiling acoustic signatures of individual keys of a PIN pad. We demonstrate the practicality of PinDrop via two sets of data collection experiments involving two commercially available metal PIN pad models and 58 participants who entered a total of 5,800 5-digit PINs. We simulated two realistic attack scenarios: (1) a microphone placed near the ATM (0.3 m away) and (2) a real-time attacker (with a microphone) standing in the queue at a common courtesy distance of 2 m. In the former case, we show that PinDrop recovers 96% of 4-digit, and up to 94% of 5-digits, PINs. Whereas, at 2 m away, it recovers up to 57% of 4-digit, and up to 39% of 5-digit PINs in three attempts. We believe that these results are both 

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4. This PIN Can Be Easily Guessed: Analyzing the Security of Smartphone Unlock PINs

   https://doi.org/10.1109/SP40000.2020.00100  

   Markert, Philipp; Bailey, Daniel V.; Golla, Maximilian; Durmuth, Markus; AviG, Adam J. (May 2020, 2020 IEEE Symposium on Security and Privacy (SP))

   In this paper, we provide the first comprehensive study of user-chosen 4- and 6-digit PINs ($$\mathbf{n=1220}$$) collected on smartphones with participants being explicitly primed for device unlocking. We find that against a throttled attacker (with 10, 30, or 100 guesses, matching the smartphone unlock setting), using 6-digit PINs instead of 4-digit PINs provides little to no increase in security, and surprisingly may even decrease security. We also study the effects of blacklists, where a set of ``easy to guess'' PINs is disallowed during selection. Two such blacklists are in use today by iOS, for 4-digits (274 PINs) as well as 6-digits (2910 PINs). We extracted both blacklists compared them with four other blacklists, including a small 4-digit (27 PINs), a large 4-digit (2740 PINs), and two placebo blacklists for 4- and 6-digit PINs that always excluded the first-choice PIN. We find that relatively small blacklists in use today by iOS offer little or no benefit against a throttled guessing attack. Security gains are only observed when the blacklists are much larger, which in turn comes at the cost of increased user frustration. Our analysis suggests that a blacklist at about 10\,\% of the PIN space may provide the best balance between usability and security. 

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5. Finite element modeling and modal testing of a wind turbine lattice tower component with interference pin connections

   https://doi.org/10.1177/10775463241266638  

   Glazier, Kyle; Yuan, Ke; Will, David_Thomas; Zhu, Weidong; Xu, Yongfeng (July 2024, Journal of Vibration and Control)

   Fatigue failures at fastener holes in structures are undesirable as they can lead to catastrophic mechanical failures. Interference pins create interference fits with joined components to reduce stresses around fastener holes and extend the fatigue life of a structure. In this research, a novel method for finite element (FE) modeling of interference pin connections in a wind turbine lattice tower component was developed. The installation of interference pins was modeled using a two-stage process that causes local stiffness changes in joined members of the component. The local stiffness changes were accounted for in the FE model by using cylinders to represent the interference pins. An experimental setup, including a three-dimensional (3D) scanning laser Doppler vibrometer (SLDV) and a mirror, was used to measure out-of-plane and in-plane natural frequencies and mode shapes of the component. Ten out-of-plane modes and one in-plane mode from the FE model are compared with the experimental results to validate the accuracy of the FE modeling approach. The maximum percent difference between the theoretical and experimental natural frequencies of the component is 3.21%, and the modal assurance criterion (MAC) values between the theoretical and experimental mode shapes are 0.92 or greater, showing good agreement between the theoretical and experimental modal parameters of the component. 

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