Breakup of Finite-Size Colloidal Aggregates in Turbulent Flow Investigated by Three-Dimensional (3D) Particle Tracking Velocimetry.

Aggregates grown in mild shear flow are released, one at a time, into homogeneous isotropic turbulence, where their motion and intermittent breakup is recorded by three-dimensional particle tracking velocimetry (3D-PTV). The aggregates have an open structure with a fractal dimension of ∼2.2, and their size is 1.4 ± 0.4 mm, which is large, compared to the Kolmogorov length scale (η = 0.15 mm). 3D-PTV of flow tracers allows for the simultaneous measurement of aggregate trajectories and the full velocity gradient tensor along their pathlines, which enables us to access the Lagrangian stress history of individual breakup events. From this data, we found no consistent pattern that relates breakup to the local flow properties at the point of breakup. Also, the correlation between the aggregate size and both shear stress and normal stress at the location of breakage is found to be weaker, when compared with the correlation between size and drag stress. The analysis suggests that the aggregates are mostly broken due to the accumulation of the drag stress over a time lag on the order of the Kolmogorov time scale. This finding is explained by the fact that the aggregates are large, which gives their motion inertia and increases the time for stress propagation inside the aggregate. Furthermore, it is found that the scaling of the largest fragment and the accumulated stress at breakup follows an earlier established power law, i.e., dfrag ∼ σ(-0.6) obtained from laminar nozzle experiments. This indicates that, despite the large size and the different type of hydrodynamic stress, the microscopic mechanism causing breakup is consistent over a wide range of aggregate size and stress magnitude.

Experimental characterization of breakage rate of colloidal aggregates in axisymmetric extensional flow.

Aggregates prepared under fully destabilized conditions by the action of Brownian motion were exposed to an extensional flow generated at the entrance of a sudden contraction. Two noninvasive techniques were used to monitor their breakup process [i.e. light scattering and three-dimensional (3D) particle tracking velocimetry (3D-PTV)]. While the first one can be used to measure the size and the morphology of formed fragments after the breakage event, the latter is capable of resolving trajectories of individual aggregates up to the breakage point as well as the trajectories of formed fragments. Furthermore, measured velocity gradients were used to determine the local hydrodynamic conditions at the breakage point. All this information was combined to experimentally determine for the first time the breakage rate of individual aggregates, given in the form of a size reduction rate K(R), as a function of the applied strain rate, as well as the properties of the formed fragments (i.e., the number of formed fragments and the size ratio between the largest fragment and the original aggregate). It was found that K(R) scales with the applied strain rate according to a power law with the slope being dependent on the initial fractal dimension only, while the obtained data indicates a linear dependency of K(R) with the initial aggregate size. Furthermore, the probability distribution function (PDF) of the number of formed fragments and the PDF of the size ratio between the largest fragment and the original aggregate indicate that breakage will result with high probability (75%) in the formation of two to three fragments with a rather asymmetric ratio of sizes of about 0.8. The obtained results are well in agreement with the results from the numerical simulations published in the literature.

Ondansetron, ramosetron, or palonosetron: Which is a better choice of antiemetic to prevent postoperative nausea and vomiting in patients undergoing laparoscopic cholecystectomy?

BACKGROUND: Postoperative nausea and vomiting (PONV) is a serious concern in patients undergoing laparoscopic cholecystectomy (LC), with an incidence of 46 to 72%. The purpose of this study was to compare the antiemetic efficacy of intravenous (iv) ondansetron 8 mg, ramosetron 0.3 mg, and palonosetron 0.075 mg for prophylaxis of PONV in high-risk patients undergoing LC. MATERIALS AND METHODS: In this prospective, randomized, double-blinded study, 87 female patients, 18 to 70 years of age (ASA I and II) and undergoing elective LC under general anesthesia were randomly allocated into three equal groups, the ondansetron group (8 mg iv; n=29), the ramosetron group (0.3 mg iv; n=29), and the palonosetron group (0.075 mg iv; n=29), and the treatments were given just after completion of surgery before extubation. The incidence of complete response (patients who had no PONV and needed no other rescue antiemetic medication), nausea, vomiting, retching, and need for rescue antiemetics over 24 hours after surgery were evaluated. RESULTS: The number of complete responders were 19 (65.5%) for ramosetron, 11 (37.9%) for palonosetron, and 10 (34.5%) for ondansetron, representing a significant difference overall (P=0.034) as well as between ramosetron and ondansetron (P=0.035). Comparison between ramosetron and palonosetron also showed a clear trend favoring the former (P=0.065). CONCLUSION: Ramosetron 0.3 mg iv was more effective than palonosetron 0.075 mg and ondansetron 8 mg in the early postoperative period, but there was no significant difference in the overall incidence of nausea suffered.