Viscoelastic drops moving on hydrophilic and superhydrophobic surfaces.

So-called "superhydrophobic" surfaces are strongly non-wetting such that fluid droplets very easily roll off when the surface is tilted. Our interest here is in understanding if this is also true, all else held equal, for viscoelastic fluid drops. We study the movement of Newtonian and well-characterised constant-viscosity elastic liquids when various surfaces, including hydrophilic (smooth glass), weakly hydrophobic (embossed polycarbonate) and superhydrophobic surfaces (embossed PTFE), are impulsively tilted. Digital imaging is used to record the motion and extract drop velocity. Optical and SEM imaging is used to probe the surfaces. In comparison with "equivalent" Newtonian fluids (same viscosity, density surface tension and contact angles), profound differences for the elastic fluids are only observed on the superhydrophobic surfaces: the elastic drops slide at a significantly reduced rate and complex branch-like patterns are left on the surface by the drop's wake including, on various scales, beads-on-a-string-like phenomena. The strong viscoelastic effect is caused by stretching filaments of fluid from isolated islands, residing at pinning sites on the surface pillars, of order ∼30 µm in size. On this scale, the local strain rates are sufficient to extend the polymer chains, locally increasing the extensional viscosity of the solution, retarding the drop.

Stable jets of viscoelastic fluids and self-assembled cylindrical capsules by hydrodynamic focusing.

We demonstrate formation of long-lived cylindrical jets of a viscoelastic fluid using hydrodynamic focusing. A solution of polyacrylamide in water is driven coaxially with immiscible oil and subjected to strong extensional flow. At high flow rates, the aqueous phase forms jets that are 4-90 microm in diameter and several centimeters long. The liquid surfaces of these jets are then used as templates for assembly of microspheres into novel rigid and hollow cylinders.

Prosthetic joints, dental treatment, and antibiotic prophylaxis.

Dentists should be familiar with the newest antibiotic prophylaxis recommendations for joint prosthesis patients undergoing dental treatment. The guidelines identify physical conditions that place joint patients at the highest risk for joint sepsis. They also stratify dental procedures into higher- and lower-risk categories. Combining these two groupings clarifies the dentist's strategy for antibiotic prophylaxis regimens, which are greatly simplified. Of notable importance is the elimination of posttreatment antibiotic dosing. A large group of patients who do not require antibiotic prophylaxis prior to dental treatment are also identified by the guidelines. Every dentist must use clinical judgment, knowledge of the patient, and consultation with the attending physician to determine the appropriate treatment plan.