A circle swimmer at low Reynolds number

Swimming in circles occurs in a variety of situations at low Reynolds number. Here we propose a simple model for a swimmer that undergoes circular motion, generalising the model of a linear swimmer proposed by Najafi and Golestanian (Phys. Rev. E 69, 062901 (2004)). Our model consists of three solid spheres arranged in a triangular configuration, joined by two links of time-dependent length. For small strokes, we discuss the motion of the swimmer as a function of the separation angle between its links. We find that swimmers describe either clockwise or anticlockwise circular motion depending on the tilting angle in a non-trivial manner. The symmetry of the swimmer leads to a quadrupolar decay of the far flow field. We discuss the potential extensions and experimental realisation of our model.Comment: 9 pages, 9 Figure

Macrophages, neutrophils and tumour necrosis factor-α expression in bronchiectatic airways in vivo

Bronchiectasis is increasingly being recognized as an inflammatory condition of the airways in which pathological permanent dilation occurs. We have obtained endobronchial biopsies in 14 patients with stable bronchiectasis and 15 control subjects. Airway neutrophils, macrophages and tumour necrosis factor-α (TNFα)-positive cells were stained with monoclonal antibodies and the densities of positive cells in the lamina propria were determined by using a computer image analyser. There was significantly higher neutrophil, macrophage and TNFα-positive cell densities in the lamina propria of bronchiectatic than control airways (P<0·001, P<0·001 and P=0·0002, respectively). Airway neutrophil density, in bronchiectasis but not in controls, correlated with TNFα-positive cell density (r=0·71, P=0·004). A significant correlation between airway macrophage and TNFα-positive cell densities was demonstrated in both control and bronchiectatic airways (r=0·63, P=0·016 and r=0·60, P=0·02 respectively). Neutrophil density negatively correlated with per cent forced vital capacity (FVC%) predicted among patients with bronchiectasis (r=-0·53, P=0·04). Bronchiectasis patients who were regular sputum producers had a significantly higher macrophage, but not neutrophil density, than their counterparts (P=0·02 and P=0·48 respectively). Our original findings suggest that airway macrophages could contribute to neutrophil influx into airway walls through their production of TNFα and therefore play an important role in the pathogenesis of bronchiectasis. © 2001 Harcourt Publishers Ltd.link_to_subscribed_fulltex