Daniel Cavanagh

Interfacial dynamics of stationary gas bubbles in flows in inclined tubes

We have experimentally examined the effects of bubble size (0.4 [les ] λ [les ] 2.0), inclination angle (0° [les ] α [les ] 90°), and tube material on suspended gas bubbles in flows in tubes for a range of Weber (0 [les ] We [les ] 3.6), Reynolds (0 [les ] Re [les ] 1200), and Froude (0 [les ] Fr α [les ] 1) numbers. Flow rates and associated pressure differences which allow the suspension of bubbles in glass and acrylic tubes are measured. Due to contact angle hysteresis, bubbles which dry the tube wall (i.e. form a gas–solid interface) may remain suspended over a range of flows while non-drying bubbles remain stationary for a single flow rate depending on experimental conditions. Stationary bubbles increase the axial pressure gradient with larger bubbles and steeper inclination angles leading to the greatest increase in the pressure gradient. Both the suspension flow range and pressure difference modifications are strongly dependent upon gas/liquid/solid material interactions. Stronger contact forces, i.e. smaller spreading coefficients, cause dried bubbles in acrylic tubes to remain stationary over a wider range of suspension flows than bubbles in glass tubes. Bubble deformation is governed by the interaction of interfacial, contact, and flow-derived forces. This investigation reveals the importance of bubble size, tube inclination, and tube material on gas bubble suspension.

The effects of a soluble surfactant on the interfacial dynamics of stationary bubbles in inclined tubes

We have experimentally examined the effects of a common soluble surfactant on gas bubbles in liquid flows in inclined tubes. Air bubbles of known size (λ = 0.8, 1.0, 1.5) are held stationary under minimum flow conditions in tubes inclined at fixed angles (ω = 25°, 45°, 65°, 90°). Sodium dodecyl sulphate (SDS) is infused into the bulk flow at two bulk concentrations ( C = 10% or 100% critical micelle concentration (CMC)). In addition to recording pressure and flow waveforms, we capture video images of bubbles before and during exposure to the surfactant. Modification of the interfacial properties by the surfactant results in extremely dynamic bubble behaviour including interfacial deformation, deformation plus axial translation, and bubble detachment from the wall plus translation. We measure the corresponding time-dependent pressure gradient within the tube. The surfactant mediated responses observed are dependent upon the interrelated effects of C , λ and ω. A high bulk concentration of surfactant may produce more rapid modification of bubble shape and influence wetting, thus increasing the potential for bubble detachment. The likelihood that detachment will occur increases further as bubble volume in increased. In both vertical tubes in which contact forces are absent and in non-vertical tubes, the infusion of surfactant may result in axial translation either in the direction of, or opposite to, the direction of the bulk flow. Critical to the translation and/or detachment of the bubble is the surfactant-mediated modification of contact line mechanics. Contact line velocities corresponding to rates of shrinkage of dewetted surface area are extracted from experimental data. We also explore the potential effects of surfactants on interfacial remobilization. This investigation demonstrates the potential use of surfactants to be used for dislodging dewetted gas bubbles by the intentional manipulation of interfacial properties.

Bubble detachment by diffusion-controlled surfactant adsorption

Abstract The effect of the diffusion-controlled adsorption of the surfactant Triton X-100 on detachment of gas bubbles adherent to the wall in pipe flow is studied. Air bubbles of known volume (scaled bubble dimension λ=0.8, 1.0, 1.5) are maintained in tubes inclined at 25°, 45°, 65°, or 90°. Triton X-100 is infused into the bulk flow to achieve a final mixed bulk concentration of either 10% or 100% of the critical micelle concentration. Pressure and flow waveforms are recorded, and video images of bubbles before and during exposure to the surfactant are captured. Surfactant-induced reductions of the surface tension and contact angle lead to distinct regimes of dynamic bubble behavior including static and oscillatory interfacial deformation, deformation with axial displacement, and complete bubble detachment from the wall. The surfactant-mediated responses depend on the interrelated effects of Triton X-100 concentration, bubble size, and tube angle. A high bulk concentration of Triton X-100 produces rapid changes in bubble shape and promotes wetting, increasing the potential for bubble detachment. Bubble detachment occurs more readily with larger bubble volume. For vertical tubes (i.e. no contact forces present) and non-vertical tubes (i.e. contact forces present), the bubble may be displaced axially in either the direction of or opposite to the bulk flow direction following introduction of Triton X-100. Bubble axial motion and detachment result from the surfactant effects on contact line mechanics. For moving contact lines, the contact line velocities corresponding to rates of shrinkage of de-wetted surface area (or, alternatively, rates of gas–liquid interfacial dilation) along with time-dependent contact front length are determined from the experimental data. Potential effects of Triton X-100 on interfacial remobilization are described. Soluble surfactants have the potential to be used for dislodging gas bubbles adherent to a pipe wall by intentional manipulation of interfacial shape and wetting properties.

Computational Analysis of Confined Jet Flow and Mass Transport in a Blind Tube

A computational analysis of confined nonimpinging jet flow in a blind tube is performed as an initial investigation of the underlying fluid and mass transport mechanics of tracheal gas insufflation. A two-dimensional axisymmetric model of a laminar steady jet flow into a concentric blind-end tube is put forth and the governing continuity, momentum, and convection-diffusion equations are solved with a finite element code. The effects of the jet diameter based Reynolds number (Re(j)), the ratio of the jet-to-outer tube diameters (epsilon), and the Schmidt number (Sc) are evaluated with the determined velocity and contaminant concentration fields. The normalized penetration depth of the jet is found to increase linearly with increasing Re(j) for epsilon = O(0.1). For a given epsilon, a ring vortex that develops is observed to be displaced downstream and radially outward from the jet tip for increasing Re(j). The axial shear stress profile along the inside wall of the outer tube possesses regions of fixed shear stress in addition to a local minimum and maximum in the vicinity of the jet tip. Corresponding regions of axial shear stress gradients exist between the fixed shear stress regions and the local extrema. Contaminant concentration gradients develop across the ring vortex indicating the inward diffusion of contaminant into the jet flow. For fixed epsilon and Sc and Re(j) approximately 900, normalized contaminant flow rate is observed to be approximately twice that of simple diffusion. This model predicts modest net axial contaminant transport enhancement due to convection-diffusion interaction in the region of the ring vortex.

A Novel Epidural Catheter Fixation Device

Epidurals are a method of long-term pain relief administered by injecting and continuously delivering an anesthetic via catheter in the spine. This method of pain relief is often used for patients in the Obstetrics/Gynecology unit as well as those in pre- and post-operational care. For almost 2 million singleton vaginal deliveries across 27 states in 2008 (representing 65% of all US singleton vaginal births in 2008), 61% of patients received some form of an epidural or spinal injection [1]. Additionally, this number has been increasing. For the 18 states for which 2006 and 2008 data are available, the average of the state-level increases in epidural/spinal injections is approximately 4.2% revealing an overall increase in these injections. Just between 2000 and 2010, the use of epidural injections increased by 160% [2]. Commonly, epidural catheters are inserted into the patient’s back in the appropriate location and then secured to the body with an adhesive medical dressing.Movement and subsequent dislocation of the catheter beneath the adhesive medical dressing can result in inefficient anesthetic delivery, increased patient discomfort, and repeated administration of the epidural. Secondary migration of epidural catheters is a problem responsible for failure in approximately 6.8% of epidurals administered [3]. Requiring an anesthesiologist to repeat the procedure is also an increased cost. A solution to secondary migration of epidural catheters would ensure effective delivery of the anesthetic to the patient, reduce the need for a repeated procedure, and prevent unwanted additional healthcare expenses.Copyright