RESUMEN
The amount of ice present in mixed-phase clouds, which contain both supercooled liquid water droplets and ice particles, affects cloud extent, lifetime, particle size and radiative properties. The freezing of cloud droplets can be catalysed by the presence of aerosol particles known as ice nuclei. One of the most important ice nuclei is thought to be mineral dust aerosol from arid regions. It is generally assumed that clay minerals, which contribute approximately two-thirds of the dust mass, dominate ice nucleation by mineral dust, and many experimental studies have therefore focused on these materials. Here we use an established droplet-freezing technique to show that feldspar minerals dominate ice nucleation by mineral dusts under mixed-phase cloud conditions, despite feldspar being a minor component of dust emitted from arid regions. We also find that clay minerals are relatively unimportant ice nuclei. Our results from a global aerosol model study suggest that feldspar ice nuclei are globally distributed and that feldspar particles may account for a large proportion of the ice nuclei in Earth's atmosphere that contribute to freezing at temperatures below about -15 °C.
RESUMEN
The homogeneous freezing of water is of fundamental importance to a number of fields, including that of cloud formation. However, there is considerable scatter in homogeneous nucleation rate coefficients reported in the literature. Using a cold stage droplet system designed to minimize uncertainties in temperature measurements, we examined the freezing of over 1500 pure water droplets with diameters between 4 and 24 µm. Under the assumption that nucleation occurs within the bulk of the droplet, nucleation rate coefficients fall within the spread of literature data and are in good agreement with a subset of more recent measurements. To quantify the relative importance of surface and volume nucleation in our experiments, where droplets are supported by a hydrophobic surface and surrounded by oil, comparison of droplets with different surface area to volume ratios was performed. From our experiments it is shown that in droplets larger than 6 µm diameter (between 234.6 and 236.5 K), nucleation in the interior is more important than nucleation at the surface. At smaller sizes we cannot rule out a significant contribution of surface nucleation, and in order to further constrain surface nucleation, experiments with smaller droplets are necessary. Nevertheless, in our experiments, it is dominantly volume nucleation controlling the observed nucleation rate.
RESUMEN
BACKGROUND: Laparoscopic Roux-en-Y gastric bypass is the most common bariatric procedure performed in the United States today. The most common early complication after laparoscopic Roux-en-Y is stenosis. No randomized studies have compared the effect of the different staplers on the incidence of stenosis or on long-term weight loss. We compared the anastomoses performed with the two sizes of circular stapler in common use. METHODS: A total of 200 patients were randomized to undergo 21-mm (n = 100) or 25-mm (n = 100) circular stapled gastroenterostomy after induction of anesthesia. No other differences were allowed in the operative technique. Stenosis was defined as patient complaints of dysphagia leading to endoscopy within 10 weeks of surgery, in which a 9-mm diameter endoscope would not pass through the gastroenterostomy without dilation. RESULTS: Of the 200 patients, 19 patients in the 21-mm group and 8 in the 25-mm group underwent endoscopy for symptoms (P = 0.027). Of the 19 and 8 patients, 17 and 7, respectively, had measurable stenosis at endoscopy (p = 0.035). Patients with a 21-mm anastomosis developed symptoms an average of 4.8 +/- 1.2 weeks after surgery, and those with a 25-mm anastomosis developed symptoms an average of 8.9 +/- 3.8 weeks postoperatively (P <0.001). CONCLUSIONS: We found a significant difference in the stenosis rate between the 21-mm and 25-mm circular stapled anastomoses in laparoscopic Roux-en-Y gastric bypass. The 21-mm staplers led to significantly more endoscopies than did the 25-mm staplers. Symptoms leading to endoscopy occurred significantly later with use of a 25-mm stapler than after a 21-mm stapler.