Accumulating Particles at the Boundaries of a Laminar Flow

The accumulation of small particles is analyzed in stationary flows through channels of variable width at small Reynolds number. The combined influence of pressure, viscous drag and thermal fluctuations is described by means of a Fokker-Planck equation for the particle density. It is shown that in the limit of vanishing particle size a uniform particle distribution is always approached in the long time limit. For extended spherical particles, conditions are specified that lead to inhomogeneous densities and consequently to particle accumulation and depletion. Hereby the boundary conditions for the particle density play a decisive role: The centers of spherical particles must keep the minimal distance of their radius from the fluid boundaries. The normal components of the forces acting on the sphere then may assume finite values which are diffusively transported into the bulk of the fluid.Comment: final version, accepted by Physica

Optimization of chaotic micromixers using finite time Lyapunov exponents

In microfluidics mixing of different fluids is a highly non-trivial task due to the absence of turbulence. The dominant process allowing mixing at low Reynolds number is therefore diffusion, thus rendering mixing in plain channels very inefficient. Recently, passive chaotic micromixers such as the staggered herringbone mixer were developed, allowing efficient mixing of fluids by repeated stretching and folding of the fluid interfaces. The optimization of the geometrical parameters of such mixer devices is often performed by time consuming and expensive trial and error experiments. We demonstrate that the application of the lattice Boltzmann method to fluid flow in highly complex mixer geometries together with standard techniques from statistical physics and dynamical systems theory can lead to a highly efficient way to optimize micromixer geometries. The strategy applies massively parallel fluid flow simulations inside a mixer, where massless and noninteracting tracer particles are introduced. By following their trajectories we can calculate finite time Lyapunov exponents in order to quantify the degree of chaotic advection inside the mixer. The current report provides a review of our results published in [1] together with additional details on the simulation methodology

Exploring Internationally Where Infants Sleep and Why

Background: Guidelines for safe infant sleep around the world are inconsistent. There are few similarities that can be found between the American Academy of Pediatrics (AAP) and co-sleeping guidelines. In order to explore parent’s self-management behaviors, there needs to be more research that visually depicts where infants sleep and explains how parents make decisions. The purpose of this research study was to learn where infants sleep at night around the world. Methods: This descriptive, exploratory study asked parents with infants (\u3c12 months) to submit pictures and descriptions of where their infants sleep. Data were collected anonymously via Internet survey. Parents were recruited using Facebook, Twitter and LinkedIn. Emails, mommy bloggers, listservs and mother/parent groups were also utilized for recruitment. Parents (n=30) were primarily married/partnered with countries of origin primarily in the northern hemisphere, with babies age x= 6.7 months. Results: Pictures of sleep spaces varied from room sharing where infants were in cribs inside the bedroom or bed sharing to solitary spaces in cribs. Key influencing factors reported by parents to make decisions about where their infants slept were safety, infant’s comfort, parent’s ease, infant proximity and space constraints. Some parents expressed having no worries about their infant’s sleep location, while others expressed concern about infant temperature, rolling on the infant, and response time. Conclusion: Parents put significant, thoughtful consideration into making their infant’s sleep environment safe. There were a wide variety of infant sleep places that reflect back to their decisions. Parents acknowledged the debate about where infants sleep most safely within their responses