Abstract:We introduce the Cohesive Energy Density (CED) method, a multiple sampling Molecular Dynamics computer simulation procedure that may offer higher consistency in the estimation of Hildebrand and Hansen solubility parameters. The use of a multiple sampling technique, combined with a simple but consistent molecular force field and quantum mechanically determined atomic charges, allows for the precise determination of solubility parameters in a systematic way ( ϭ 0.4 hildebrands). The CED method yields first-principles Hildebrand parameter predictions in good agreement with experiment [root-mean-square (rms) ϭ 1.1 hildebrands]. We apply the CED method to model the Caltech electronic nose, an array of 20 polymer sensors. Sensors are built with conducting leads connected through thin-film polymers loaded with carbon black. Odorant detection relies on a change in electric resistivity of the polymer film as function of the amount of swelling caused by the odorant compound. The amount of swelling depends upon the chemical composition of the polymer and the odorant molecule. The pattern is unique, and unambiguously identifies the compound. Experimentally determined changes in relative resistivity of seven polymer sensors upon exposure to 24 solvent vapors were modeled with the CED estimated Hansen solubility components. Predictions of polymer sensor responses result in Pearson R 2 coefficients between 0.82 and 0.99.

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Geometric asymmetry and surface forces are used directly the shape transformation of two-dimensional nanoparticle (NP)-based ribbons into three-dimensional helices. The balance between elasticity and surface tension dictates the helical radius dimension. NP helical ribbons have exceptional mechanical properties, displaying high stretchability, helical shape recovery after extension, and low-strain stiffness values similar to biological helices.

For soft materials with Young's moduli below 100 kPa, quantifying mechanical and interfacial properties by small scale indentation is challenging because in addition to adhesion and elasticity, surface tension plays a critical role. Until now, microscale contact of very soft materials has only been studied by static experiments under zero external loading. Here we introduce a combination of the colloidal probe technique and confocal microscopy to characterize the force-indentation and force-contact radius relationships during microindentation of soft silicones. We confirm that the widespread Johnson-Kendall-Roberts theory must be extended to predict the mechanical contact for soft materials. Typically a liquid component is found within very soft materials. With a simple analytical model, we illustrate that accounting for this liquid surface tension can capture the contact behavior. Our results highlight the importance of considering liquid that is often associated with soft materials during small scale contact.

When a water drop is placed onto a soft polymer network, a wetting ridge develops at the drop periphery. The height of this wetting ridge is typically governed by the drop surface tension balanced by elastic restoring forces of the polymer network. However, the situation is more complex when the network is swollen with fluid, because the fluid may separate from the network at the contact line. Here we study the fluid separation and network deformation at the contact line of a soft polydimethylsiloxane (PDMS) network, swollen with silicone oil. By controlling both the degrees of crosslinking and swelling, we find that more fluid separates from the network with increasing swelling. Above a certain swelling, network deformation decreases while fluid separation increases, demonstrating synergy between network deformation and fluid separation. When the PDMS network is swollen with a fluid having a negative spreading parameter, such as hexadecane, no fluid separation is observed. A simple balance of interfacial, elastic, and mixing energies can describe this fluid separation behavior. Our results reveal that a swelling fluid, commonly found in soft networks, plays a critical role in a wetting ridge.

Tree frogs need to adhere to surfaces of various roughnesses in their natural habitats; these include bark, leaves and rocks. Rough surfaces can alter the effectiveness of their toe pads, due to factors such as a change of real contact area and abrasion of the pad epithelium. Here, we tested the effect of surface roughness on the attachment abilities of the tree frog Litoria caerulea. This was done by testing shear and adhesive forces on artificial surfaces with controlled roughness, both on single toe pads and whole animal scales. It was shown that frogs can stick 2–3 times better on small scale roughnesses (3–6 µm asperities), producing higher adhesive and frictional forces, but relatively poorly on the larger scale roughnesses tested (58.5–562.5 µm asperities). Our experiments suggested that, on such surfaces, the pads secrete insufficient fluid to fill the space under the pad, leaving air pockets that would significantly reduce the Laplace pressure component of capillarity. Therefore, we measured how well the adhesive toe pad would conform to spherical asperities of known sizes using interference reflection microscopy. Based on experiments where the conformation of the pad to individual asperities was examined microscopically, our calculations indicate that the pad epithelium has a low elastic modulus, making it highly deformable.

Commercial silicone elastomers are commonly used in soft materials research due to their easily tunable mechanical properties.However, conventional polydimethylsiloxane (PDMS) elastomers with moduli below $100 kPa contain uncrosslinked free molecules, which play a significant role in their behavior. To utilize these materials, it is important to quantify what role these free molecules play in the mechanical response before and after their removal. We present a simple and inexpensive extraction method that enables the removal of free molecules from a lightly crosslinked sheet of Sylgard 184, a commercially available PDMS elastomer. The materials can contain a majority of free molecules yet maintain a thin and flat geometry without fractures after extraction. Subsequently, we compare the modulus, maximum stretchability, and hysteresis behavior with mixing ratios ranging from 60:1 to 30:1, before and after extraction. We show that the modulus, maximum stretchability, and dissipation increase upon extraction. Moreover, our approach offers a route to prepare crosslinked silicone elastomers with a modulus as low as $20 kPa without free molecules from a commercially available kit.

Many urological studies rely on models of animals, such as rats and pigs, but their relation to the human urinary system is poorly understood. Here, we elucidate the hydrodynamics of urination across five orders of magnitude in body mass. Using high-speed videography and flow-rate measurement obtained at Zoo Atlanta, we discover that all mammals above 3 kg in weight empty their bladders over nearly constant duration of 21 ± 13 s. This feat is possible, because larger animals have longer urethras and thus, higher gravitational force and higher flow speed. Smaller mammals are challenged during urination by high viscous and capillary forces that limit their urine to single drops. Our findings reveal that the urethra is a flow-enhancing device, enabling the urinary system to be scaled up by a factor of 3,600 in volume without compromising its function. This study may help to diagnose urinary problems in animals as well as inspire the design of scalable hydrodynamic systems based on those in nature.urology | allometry | scaling | Bernoulli's principle M edical and veterinary urology often relies on simple, noninvasive methods to characterize the health of the urinary system (1, 2). One of the most easily measured characteristics of the urinary system is its flow rate (3), changes in which may be used to diagnose urinary problems. The expanding prostates of aging males may constrict the urethra, decreasing urine flow rate (4). Obesity may increase abdominal pressure, causing incontinence (5). Studies of these ailments and others often involve animal subjects of a range of sizes (6). A study of urination in zero gravity involved a rat suspended on two legs for long periods of time (7), whereas other studies involve mice (8), dogs (1), and pigs (9). Despite the wide range of animals used in urological studies, the consequences of body size on urination remain poorly understood.The bladder serves a number of functions, as reviewed by Bentley (10). In desert animals, the bladder stores water to be retrieved at a time of need. In mammals, the bladder acts as a waterproof reservoir to be emptied at a time of convenience. This control of urine enables animals to keep their homes sanitary and themselves inconspicuous to predators. Stored urine may even be used in defense, which one knows from handling rodents and pets.Several misconceptions in urology have important repercussions in the interpretation of healthy bladder function. For instance, several investigators state that urinary flow is driven entirely by bladder pressure. Consequently, their modeling of the bladder neglects gravitational forces (11-13). Others, such as Martin and Hillman (14), contend that urinary flow is driven by a combination of both gravity and bladder pressure. In this study, we elucidate the hydrodynamics of urination across animal size, showing the effects of gravity increase with increasing body size. ResultsIn Vivo Experiments. We filmed the urination of 16 animals and obtained 28 videos of urination from YouTube, listed in SI Appendix. Movies ...