On acoustic cavitation of slightly subcritical bubbles

The classical Blake threshold indicates the onset of quasistatic evolution leading to cavitation for gas bubbles in liquids. When the mean pressure in the liquid is reduced to a value below the vapor pressure, the Blake analysis identifies a critical radius which separates quasistatically stable bubbles from those which would cavitate. In this work, we analyze the cavitation threshold for radially symmetric bubbles whose radii are slightly less than the Blake critical radius, in the presence of time-periodic acoustic pressure fields. A distinguished limit equation is derived that predicts the threshold for cavitation for a wide range of liquid viscosities and forcing frequencies. This equation also yields frequency-amplitude response curves. Moreover, for fixed liquid viscosity, our study identifies the frequency that yields the minimal forcing amplitude sufficient to initiate cavitation. Numerical simulations of the full Rayleigh-Plesset equation confirm the accuracy of these predictions. Finally, the implications of these findings for acoustic pressure fields that consist of two frequencies will be discussed.Comment: 14 pages, Presented at APS/DFD conference in Philadelphia 199

Akonni Biosystems: Wicking in Microchannels on Biochips

Microfluidics is the science of designing and manufacturing devices and processes for manipulation of extremely small volumes of fluid, typically micro to nanoliters.The most mature application of microfluidics technology is ink-jet printing, which uses orifices less than 100 μm in diameter to generate drops of ink. The complex devices now being developed for biological applications involving the analysis of DNA (in genetics and genomics) and proteins (in proteomics) and bio-defense typically involve aqueous solutions and channels 30 to 300 μm in diameter. Unlike microelectronics, in which the current emphasis is on reducing the size of transistors, microfluidics is focusing on making more complex systems of channels with more sophisticated fluid-handling capabilities, rather than reducing the size of the channels. Although micro- and macro-fluidic systems require similar components including pumps, valves, mixers, filters, and separators, the small size of microchannels causes their flow to behave differently. At micron scales, fluid motions are primarily dominated by surface tension and viscous forces. In the problem under consideration, the issue is one of wicking or leaking of the sample from the reaction reservoir to the waste region at elevated temperatures. A mechanism responsible for this phenomenon was thought to be the "wedge effect," which refers to the tendency of liquids to move along a sharp corner by capillary effects if the conditions are right. The analysis performed during the workshop also mainly focused on this effect. While a definitive solution to this challenging problem posed in the workshop was not identified, it was felt that using a manufacturing process that can affect the corner angles in the channels may hold the most promise, allowing the wicking mechanism to be controlled without surface treatments that insert hydrophobic stops in the channel. For instance by "rounding" the side walls to increase the corner angles from 90 toward 180 degrees, the leaking of the sample away from the reaction chamber might be delayed

Shape Oscillations of Gas Bubbles With Newtonian Interfacial Rheological Properties

The oscillation frequency and damping rate for small-amplitude axisymmetric shape modes of a gas bubble in an ideal liquid are obtained, in the limit when the bubble interface possesses Newtonian interfacial rheology with constant surface shear and dilatational viscosities. Such results permit the latter surface properties to be measured by analyzing experimental data on frequency shift and damping rate of specific shape modes of suspended bubbles in the presence of surfactants

Procyclical Monetary Policy and Governance

Weak governance adversely affects firm’s net worth and consequently the value of its collateral. This negative impact on the collateral reduces the external credit available for importing inputs constraining potential output. As a result, a stronger procyclical monetary policy stance is adopted for protecting the exchange rate and hence arresting the degradation in the collateral constraint.Collateral Constraints; Governance; Monetary Policy

RtE Act and inclusive and discrimination-free schools

Children being non-essential or a weak constituency have no ‘ballot value’ for policy makers and perhaps this is one of the reasons why it took so long to legislate on universal elementary education. They are non-actors and have very little room to negotiate in an indirect and representative democracy. Despite the fact that children are the only beneficiaries of this fundamental right, they are often at the receiving end. It makes evolution of an inclusive education system a very difficult task. Their needs are often conceived from the point of view of parents, families, caste, tribes, community and society, who generally are carriers of ethos, values, mores and culture but not the only ones. They may not necessarily always be in sync with the constitutional ethos and usually patriarchal, feudal, caste-ist and communal/religious and are based on exclusion and largely discriminatory in nature. The Position Paper on SC and ST and Girl’s Education, by NCF-NCERT and Justice Rajindar Sachar Commission’s Reports attest that SC, ST, religious minorities and girl child are subjected to discriminatory practices in elementary schools leading to low enrollment and high dropout

Frequency of irritable bowel syndrome in college students.

Background: This study was carried out to investigate the presence of irritable bowel syndrome (IBS) in college students and compare its distribution in the non-medical college and medical college students. Methods: We collected data from 508 students by inviting them to fill out a questionnaire based on Rome II criteria for the diagnosis of IBS from November 2001 to February 2004. Two major cities Karachi and Bahawalpur were selected and it was coordinated by the section of Gastroenterology, Department of Medicine, Aga Khan University Hospital and Quaid-e-Azam Medical College, Bahawalpur. Results: A total of 508 subjects with males 43 %(220/508) and female 57 % (288/508) mean age 22 ± 2.8 years responded to the questionnaire.The overall frequency of IBS among college students was 34 % (171/508).There were 41 % (107/263) non-medical college and 26 % (64/245) medical college students with IBS. Abdominal pain was present in 100 % (171/171) with altered frequency of stool in 58 % (100/171) (p\u3c 0.001,OR 12.5, CI 7.9-20.0) of students with IBS. The medical advice was sought by 38 % (65/171) in IBS group with 46 % (30/65) non-medical college school and 54 % (35/65) medical student (p=0.001, OR 1.9, CI 1.3-2.8). Conclusion IBS is common in younger adult population of Pakistan. Its prevalence is higher in non-medical college students but health care seeking behavior was more common in medical students

Irritable bowel syndrome and health seeking behaviour in different communities of Pakistan

OBJECTIVE: To investigate the frequency of irritable bowel syndrome (IBS) and health seeking behaviour in patients belonging to different ethnic groups and their squatter settlements in two cities of Pakistan. METHODS: Questionnaire based on Rome II criteria for the diagnosis of IBS was sent to 1167 persons living in, Karachi and Bahawalpur and their squatter settlements in Baloch Colony and Tibba Badar Sher respectively. About 90% (1048) completed the questionnaire. RESULTS: The overall frequency of IBS was 14% with a mean age of 30 +/- 12.5 years and range of 16-85 years. Of the IBS positive patients, 56% (82/146) were males. IBS was significantly more common (p = 0.05) in males belonging to age group 16-30 years. The most common presenting symptoms were abdominal pain (100%), altered bowel habits (51%), and diarrhoea (54%). There was no difference in the prevalence of IBS in Karachi and Baloch Colony as compared to Bahawalpur and Tibba Badar Sher. IBS patients seeking health care advice were 17.6% as compared to 12.6 % non healthcare seekers. Of the healthcare seekers there were 29 (35.4%) males (p = 0.002). The overall mean age of health care seeking IBS patients was 32.8 +/-13.8 years (p \u3c 0.001). All IBS patients seeking healthcare advice had abdominal pain (p \u3c 0.001), while 61% had altered stool consistency (p \u3c 0.001) and 46% had stool frequency (p \u3c 0.001) as compared to IBS non healthcare seekers. CONCLUSION: IBS is seen in both urban and suburban communities. Health seeking behaviour is common in males and with abdominal pain