Marangoni Flow in Freely Suspended Liquid Films.

We demonstrate controlled material transport driven by temperature differences in thin freely suspended smectic films. Films with submicrometer thicknesses and lateral extensions of several millimeters were studied in microgravity during suborbital rocket flights. In-plane temperature differences cause two specific Marangoni effects, directed flow and convection patterns. At low gradients, practically thresholdless, flow transports material with a normal (negative) temperature coefficient of the surface tension dσ/dT<0 from the hot to the cold film edge, it accumulates at the cold film edge. In materials with dσ/dT>0, the reverse transport from the cold to the hot edge is observed. We present a model that describes the effect quantitatively. It predicts that not the temperature gradient in the film plane but the temperature difference between the thermopads is relevant for the effect.

Defect Interactions in Anisotropic Two-Dimensional Fluids.

Disclinations in liquid crystals bear striking analogies to defect structures in a wide variety of physical systems, and their straightforward optical observability makes them excellent models to study fundamental properties of defect interactions. We employ freely suspended smectic-C films, which behave as quasi-two-dimensional polar nematics. A procedure to capture high-strength disclinations in localized spots is introduced. These disclinations are released in a controlled way, and the motion of the mutually repelling topological charges with strength +1 is studied quantitatively. We demonstrate that the classical models, which employ elastic one-constant approximation, fail to describe their dynamics correctly. In realistic liquid crystals, even small differences between splay and bend constants lead to the selection of pure splay or pure bend +1 defects. For those, the models work only in very special configurations. In general, additional director walls are involved which reinforce the repulsive interactions substantially.

Rupture and recoil of bent-core liquid crystal filaments.

The recoil process of free-standing liquid crystal filaments is investigated experimentally and theoretically. We focus on two aspects, the contraction speed of the filament and a spontaneously formed undulation instability. At the moment of rupture, the filaments buckle similarly to the classical Euler buckling of elastic rods. The tip velocity decays with decreasing filament length. The wavelength of buckling affinely decreases with the retracting filament tip. The energy gain related to the decrease of the total length and surface area of the filaments is mainly dissipated by layer rearrangements during thickening of the fibre. A flow back into the meniscus is relevant only in the final stage of the recoil process. We introduce a model for the quantitative description of the filament retraction speed. The dynamics of this recoil behaviour may find relevance as a model for biology-related filaments.

Granular gases of rod-shaped grains in microgravity.

Granular gases are convenient model systems to investigate the statistical physics of nonequilibrium systems. In the literature, one finds numerous theoretical predictions, but only few experiments. We study a weakly excited dilute gas of rods, confined in a cuboid container in microgravity during a suborbital rocket flight. With respect to a gas of spherical grains at comparable filling fraction, the mean free path is considerably reduced. This guarantees a dominance of grain-grain collisions over grain-wall collisions. No clustering was observed, unlike in similar experiments with spherical grains. Rod positions and orientations were determined and tracked. Translational and rotational velocity distributions are non-Gaussian. Equipartition of kinetic energy between translations and rotations is violated.

Two-dimensional microrheology of freely suspended liquid crystal films.

Smectic liquid crystals form freely-suspended, fluid films of highly uniform structure and thickness, making them ideal systems for studies of hydrodynamics in two dimensions. We have measured particle mobility and shear viscosity by direct observation of the gravitational drift of silica spheres and smectic islands included in these fluid membranes. In thick films, we observe a hydrodynamic regime dominated by lateral confinement effects, with the mobility of the inclusion determined predominantly by coupling of the fluid flow to the fixed boundaries of the film. In thin films, the mobility of inclusions is governed primarily by coupling of the fluid to the surrounding air, as predicted by Saffman-Delbrück theory.

Bacterial clearance of biologic grafts used in hernia repair: an experimental study.

BACKGROUND: Biologic grafts used in ventral hernia repair are derived from various sources and undergo different post-tissue-harvesting processing, handling, and sterilization techniques. It is unclear how these various characteristics impact graft response in the setting of contamination. We evaluated four materials in an infected hernia repair animal model using fluorescence imaging and quantitative culture studies. METHODS: One hundred seven rats underwent creation of a chronic hernia. They were then repaired with one synthetic polyester control material (n = 12) and four different biologic grafts (n = 24 per material). Biologic grafts evaluated included Surgisis (porcine small intestinal submucosa), Permacol (crosslinked porcine dermis), Xenmatrix (noncrosslinked porcine dermis), and Strattice (noncrosslinked porcine dermis). Half of the repairs in each group were inoculated with Staphylococcus aureus at 10(4) CFU/ml and survived for 30 days without systemic antibiotics. Animals then underwent fluorescence imaging and quantitative bacterial studies. RESULTS: All clean repairs remained sterile. Rates of bacterial clearance were as follows: polyester synthetic 0%, Surgisis 58%, Permacol 67%, Xenmatrix 75%, and Strattice 92% (P=0.003). Quantitative bacterial counts had a similar trend in bacterial clearance: polyester synthetic 1×10(6) CFU/g, Surgisis 4.3×10(5) CFU/g, Permacol 1.7×10(3) CFU/g, Xenmatrix 46 CFU/g, and Strattice 31 CFU/g (P=0.001). Fluorescence imaging was unable to detect low bacterial fluorescence counts observed on bacterial studies. CONCLUSION: Biologic grafts, in comparison to synthetic material, are able to clear a Staphylococcus aureus contamination; however, they are able to do so at different rates. Bacterial clearance correlated to the level of residual bacterial burden observed in our study. Post-tissue-harvesting processing, handling, and sterilization techniques may contribute to this observed difference in ability to clear bacteria.

Transient hexagonal structures in sheared emulsions of isotropic inclusions on smectic bubbles in microgravity conditions.

We describe the collective behavior of isotropic droplets dispersed over a spherical smectic bubble, observed under microgravity conditions on the International Space Station (ISS). We find that droplets can form two-dimensional hexagonal structures changing with time. Our analysis indicates the possibility of spatial and temporal periodicity of such structures of droplets. Quantitative analysis of the hexagonal structure including the first three coordination circles was performed. A peculiar periodic-in-time ordering of the droplets, related to one-dimensional motion of droplets with non-uniform velocity, was found.

Corona patterns around inclusions in freely suspended smectic films.

We discuss the structure and physical origin of corona patterns observed around solid or liquid spherical inclusions in freely suspended smectic films. Such patterns are observed when droplets or solid beads of micrometer size are sprayed onto the films. They are found in the smectic C phase and in the smectic A phase above such a smectic C phase, but disappear, for example, at the transition into a lower-temperature smectic B phase. We show that these structures are equivalent to splay domains found in the meniscus of freely suspended films, originating from surface-induced spontaneous splay.

[Bedtime, television and computer habits of primary school children in Germany]. / Schlafenszeiten, Fernseh- und Computergewohnheiten von Grundschulkindern in Deutschland.

BACKGROUND: Pediatricians recommend daily sleep of at least ten hours for children aged 9 and 10 years. Sufficient sleep is necessary for maintaining the body's homeostasis, as well as for fixing memories and learning. Lack of sleep in children has been associated with a diminished school performance, reduced attention span, and obesity. Adulthood is influenced by childhood lifestyle habits. Data from 4th graders in 34 schools in Mainz and its surroundings were analysed in order to determine negative health lifestyle factors in German primary school children, such as lack of sleep and increased leisure time spent watching television and computer gaming. METHODS: Data from a cross-sectional study regarding cellular phone use in fourth-graders in Mainz were used for this analysis. Bedtime, television and computer use habits, as well as other factors were explored. A total of 1933 children from 34 schools participated by answering a questionnaire in the 2002-2003 school year (participation rate: 88%). Complete data for the secondary analysis were available from 1889 students (51% male, median age 10 years). RESULTS: Overall, 28% of the children reported going to bed after 9 pm on week nights, 16% reported watching television more than three hours daily, and 11% played computer or video games more than three hours daily. In the adjusted binary logistic regression model, being older, male, having older siblings, watching television (OR 1.82, 95% CI 1.28-2.61) and playing computer games (OR 3.23; 95% CI 2.24-4.67) for more than three hours daily, owning a mobile phone, and being in a school in the city were associated with going to bed after 9 pm. CONCLUSION: Every fourth child does not obtain a sleep duration of 10 hours, under the assumption that primary schoolchildren need to be awake at 7 am on weekdays. Lifestyle factors that may negatively influence a child's development determine their actual and future habits. Sufficient sleep and less television and computer leisure times should be assertively emphasized to parents and carers of primary school students, in order to prevent the negative consequences of lack of sleep, such as diminished school performance. Developing sex-specific prevention programs, which take into account the complex lifestyles and habits of primary schoolchildren, seems to be relevant.

Frustrated packing of spheres in a flat container under symmetry-breaking bias.

We study statistical properties of packings of monodisperse spheres in a flat box. After "gravitational" filling and appropriate agitation, a nearly regular (in plane) but frustrated (normal to the plane) triangular lattice forms, where beads at individual sites touch either the front or back wall. It has striking analogies to order in antiferromagnetic Ising spin models. When tilting the container, Earth's gravitational field mimics external forces similar to magnetic fields in the spin systems. While packings in vertical containers adopt a frustrated state with statistical correlations between neighboring sites, the configurations continuously approach the predictions of a random Ising model when the cell tilt is increased. Our experiments offer insights into both the influence of geometrical constraints on random granular packing and a descriptive example of frustrated ordering.

Biologic mesh use practice patterns in abdominal wall reconstruction: a lack of consensus among surgeons.

BACKGROUND: Despite the prevalence of ventral hernias, there is little agreement as to the most appropriate technique or prosthetic to repair these defects. Our objective was to determine biologic mesh practice patterns of reconstructive surgeons with regard to indications, most appropriate technique, choice of prosthetic, and experience with complications. METHODS: A survey was mailed to 2,000 practicing surgeons. Main outcome measures included surgeon experience with biologic mesh and associated complications. RESULTS: Two hundred and forty (12 %) surgeons responded to the survey. Ten were excluded, as surgeons completing the survey indicated they did not perform ventral hernia repairs or left multiple questions unanswered. Of the 230 included, 93.5 % (n = 215) of responders had experience using biologic mesh. Frequency of biologic graft use in the last year was as follows: low-volume users (<5 times in the last year) 50.7 %; medium-volume users (5-20 times in the last year) 37.3 %; high-volume users (>20 times in the last year) 11.9 %. Indications for biologic mesh use based on wound classifications (clean, clean contaminated, contaminated or dirty) were quite varied and lacked consensus among surgeons (p value < 0.05). The most commonly reported influences for use included personal experience (45 %), literature (28.3 %), and availability (17.2 %). CONCLUSIONS: Despite a lack of level 1 evidence, biologic meshes are being used under various wound classifications. Importantly, use in clean and dirty-infected settings may reflect an inappropriate overuse of these expensive materials. To better guide surgeons, prospective, randomized trials should be undertaken to evaluate the short- and long-term outcomes associated with these materials under the various surgical wound classifications.