Electrospinning is an economical and relatively simple method to produce continuous and uniform nanofibers from almost any synthetic and many natural polymers. Because of the high specific surface area, tunable pore size, and flexibility, the nanofibrous membranes are finding an increasingly wide range of applications. Some particular attention has been devoted to antibacterial nanofibers for applications such as wound dressings. A variety of biocides, e.g., antibiotics, quaternary ammonium salts, triclosan, biguanides, (silver, titanium dioxide, and zinc oxide) nanoparticles and chitosan have been incorporated by various techniques into nanofibers that exhibit strong antibacterial activity in standard assays. However, the small diameters of the nanofibers also mean that the incorporated biocides are often burst released once the materials are submerged in an aqueous solution. Nevertheless, several strategies, such as coresheath structure of the nanofiber, covalent bonding of the biocide on the fiber surface and adsorption of the biocide in nanostructures, can be utilized to sustain the release over several days. This review summarizes recent development in the fabrication of antibacterial nanofibers, the release profiles of the biocides and their applications in in vivo systems.
The effect of suction, applied through a short porous wall strip, on a low Reynolds number self-preserving turbulent boundary layer has been quantified by measuring the local wall shear stress and the main Reynolds stresses downstream of the strip. When the suction rate is sufficiently high, pseudo-relaminarization occurs almost immediately downstream of the strip. Farther downstream, transition occurs followed by a slow return to a fully turbulent self-preserving state. During relaminarization, the measured skin friction coefficient cf falls below the level corresponding to the no suction value, reaching a minimum where transition begins. An empirical cf distribution is proposed that groups together results obtained at different streamwise stations and different suction rates. Of all the measured Reynolds stresses, the longitudinal turbulence intensity recovers relatively quickly from the change in boundary conditions while the wall-normal turbulence intensity and the Reynolds shear stress are significantly affected by the suction. The Reynolds shear stress, which is negligible during relaminarization, has the slowest recovery.
The process by which a liquid jet falling into a liquid pool entrains air is studied
experimentally and theoretically. It is shown that, provided the nozzle from which
the jet issues is properly contoured, an undisturbed jet does not entrap air even at
relatively high Reynolds numbers. When surface disturbances are generated on the
jet by a rapid increase of the liquid flow rate, on the other hand, large air cavities are
formed. Their collapse under the action of gravity causes the entrapment of bubbles
in the liquid. This sequence of events is recorded with a CCD and a high-speed
camera. A boundary-integral method is used to simulate the process numerically with
results in good agreement with the observations. An unexpected finding is that the
role of the jet is not simply that of conveying the disturbance to the pool surface.
Rather, both the observed energy budget and the simulations imply the presence of a
mechanism by which part of the jet energy is used in creating the cavity. A hypothesis
on the nature of this mechanism is presented.
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