RESUMO

From Mars rovers to buildings, objects eventually sink and tilt into a fluidized granular bed due to gravity. Despite the irregular shape of realistic granular intruders, most research focus on the settling of "perfect" objects like spheres and cylinders. Here, we systematically explore the penetration of "imperfect" solids-from stones to bodies with carefully controlled asymmetries-into granular beds. A cylinder with two halves of different roughnesses rotates toward the granular region next to the smoother surface and deviates from the vertical direction. We demonstrate that even small irregularities in the surface of an object may produce substantial changes in the penetration process. Using computer simulations, we show that defects concentrate granular force chains, thus producing decisive forces on the intruder. Furthermore, we demonstrate that tilting and migration of sinking bodies can be correctly predicted by a simple mechanical model based on a unified force law.

RESUMO

During the COVID-19 pandemic, the relevance of evaluating the effectiveness of face masks-especially those made at home using a variety of materials-has become obvious. However, quantifying mask protection often requires sophisticated equipment. Using a frugal stain technique, here we quantify the "ballistic" droplets reaching a receptor from a jet-emitting source which mimics a coughing, sneezing or talking human-in real life, such droplets may host active SARS-CoV-2 virus able to replicate in the nasopharynx. We demonstrate that materials often used in home-made face masks block most of the droplets. Mimicking situations eventually found in daily life, we also show quantitatively that less liquid carried by ballistic droplets reaches a receptor when a blocking material is deployed near the source than when located near the receptor, which supports the paradigm that your face mask does protect you, but protects others even better than you. Finally, the blocking behavior can be quantitatively explained by a simple mechanical model.

Assuntos

RESUMO

The sodium-modified form of fluorohectorite nanoclay (NaFh) is introduced as a potential drug carrier, demonstrating its ability for the controlled release of the broad-spectrum antibiotic Ciprofloxacin through in vitro tests. The new clay-drug composite is designed to target the local infections in the large intestine, where it delivers most of the incorporated drug thanks to its pH-sensitive behavior. The composite has been conceived to avoid the use of coating technology and to decrease the side-effects commonly associated to the burst-release of the ciprofloxacin at the stomach level. NaFh was obtained from lithium-fluorohectorite by ion exchange, and its lack of toxicity was demonstrated by in vivo studies. Ciprofloxacin hydrochloride (Cipro) was encapsulated into the clay at different values of the pH, drug initial concentration, temperature and time. Systematic studies by X-ray diffraction (XRD), infrared and visible spectrophotometry (FT-IR and UV-vis), and thermal analysis (TGA) indicated that the NaFh host exhibits a high encapsulation efficiency for Cipro, which reaches a 90% of the initial Cipro in solution at 65 oC, with initial concentration of drug in solution of 1.36 x 10-2 mol L-1 at acid pH. XRD revealed that a true intercalation of Cipro takes place between clay layers. TG showed an increased thermal stability of the drug when intercalated into the clay, as compared to the "free" Cipro. IR suggested a strong clay-Cipro interaction via ketone group, as well as the establishment of hydrogen bonds between the two materials. In vitro drug release tests revealed that NaFh is a potentially efficient carrier to deliver Cipro in the large intestine, where the release process is mediated by more than just one mechanism.

Assuntos

RESUMO

While "vibrational noise" induced by rotating components of machinery is a common problem constantly faced by engineers, the controlled conversion of translational into rotational motion or vice-versa is a desirable goal in many scenarios ranging from internal combustion engines to ultrasonic motors. In this work, we describe the underlying physics after isolating a single degree of freedom, focusing on devices that convert a vibration along the vertical axis into a rotation around this axis. A typical Vibrot (as we label these devices) consists of a rigid body with three or more cantilevered elastic legs attached to its bottom at an angle. We show that these legs are capable of transforming vibration into rotation by a "ratchet effect", which is caused by the anisotropic stick-slip-flight motion of the leg tips against the ground. Drawing an analogy with the Froude number used to classify the locomotion dynamics of legged animals, we discuss the walking regime of these robots. We are able to control the rotation frequency of the Vibrot by manipulating the shaking amplitude, frequency or waveform. Furthermore, we have been able to excite Vibrots with acoustic waves, which allows speculating about the possibility of reducing the size of the devices so they can perform tasks into the human body, excited by ultrasound waves from the outside.