Visual Experiments of Bubble Growth and Departure at the Tip of Inclined Capillary Tubes in a Stagnant Liquid

The bubble growth and departure at the tip of inclined glass capillary tubes in stagnant fluid is experimentally investigated by using a high-speed visual system. The visual experiments show that the bubble growth experiences a sphere-like growth stage and the asymmetric growth stage at the tip of an inclined capillary tube. In the asymmetric growth stage, the bubble firstly gets an upward deflection under action of buoyancy force and then the triple contact line of the bubble detaches from the downside of the tube tip and withdraws into the tube. In the period of bubble departure, the neck of the bubble shrinks towards the upside of the tube tip and finally breaks up at the far upper portion of inner surface edge. The incursion of liquid into the tube tip has been detected after the bubble departing from tube tip. It is found that the departure size and the growth cycle period of the bubble as well as the volume of liquid incursion into the tube are decreased with an increase in the inclined angle of the tube. A smaller inner diameter of the inclined capillary tube leads to a smaller bubble departure size and a shorter cycle period of the bubble growth.     

Improvement in the hydrophilic property of inner surface of polyvinyl chloride tube by DC glow discharge plasma

A DC glow discharge plasma was generated inside polymer tube at low pressure. The polyvinyl chloride (PVC) tube with 4 mm inner diameter and 50 mm length was treated by Ar plasma generated inside the tube. The hydrophilic property of the inner surface of the PVC tube was remarkably improved after the plasma treatment. The water contact angle of the inner surface decreased to 37° from 75° for the untreated sample as the treatment time increase to 30 min. The water contact angle of plasma-treated PVC tube decreases with increasing applied voltage, but increase with increase in working pressure. When the Ar plasma-treated PVC tubes are stored in air at room temperature, the contact-angles of the inner surface quickly increases and finally reaches a plateau value which is still considerably lower than the 75° for the untreated sample.     

Contact angle hysteresis on smooth and homogenous surfaces in gravitational fields

In an experiment conducted on a space shuttle flight, contact angle hysteresis was observed for the water-glass system. Surface heterogeneities have usually been taken as the cause of the observed hysteresis. In order to examine if other factors such as adsorption could be the cause of the observed hysteresis, contact angles on homogenous, smooth surfaces of glass were examined in ground-based experiments. Ten glass cylinders with different inside diameters were constructed with a capillary tube in the centre of each cylinder. The cylinders were filled with prepared water and the meniscus heights inside the capillary tube and in the glass cylinder were measured. Using the values of these parameters, the pressure in the vapour and the contact angle at the three-phase line of each interface were calculated from the equilibrium conditions. In each of the cylinders, a difference in the contact angle at the three-phase line of the capillary and the three-phase line of the cylinder was observed. This contact angle hysteresis cannot be explained by surface heterogeneity or line tension effect. The observed contact angle hysteresis is predicted to be the result of the pressure profile in the system. This suggests adsorption and its extraordinary sensitivity to vapour-phase pressure as the source of the contact angle hysteresis.     

The shape and stability of wall-bound and wall-edge-bound drops and bubbles

The behavior of wall-bound drops and bubbles is fundamental to many natural and industrial processes. Key characteristics of such capillary systems include interface shape and stability for a variety of gravity levels and orientations. Significant solutions are in hand for axisymmetric pendent drops for a variety of uniform boundary conditions along the contact line with gravity acting normal to a planar wall. The special case of a wall-bound drop or bubble that is also pinned at an edge (i.e. a ‘wall-edge-bound’ drop) is considered here where numerical solutions are obtained for interface shape and stability as functions of drop volume, contact angle, fluid properties, and uniform gravity vector. For a semi-infinite zero-thickness planar wall (plate), a critical contact angle is identified below which wall-edge-bound drops are always stable. The critical contact angle is computed as a function of the gravity vector. The numerical procedure, which makes no account for contact angle hysteresis, predicts that such wall-edge-bound drops are unconditionally unstable for any gravity field with a component that is tangent to the wall while inwardly normal to the edge. Select experiments are conducted that support the conclusions drawn from the numerical results.     

Thermo-hydrodynamic transport phenomena in partially wetting liquid plugs moving inside micro-channels

Single-phase as well as two-phase fluid flows inside mini/micro-channels and capillary tubes are of practical importance in many miniaturized engineering systems. While several issues related to single-phase transport are fairly well understood, two-phase systems still pose challenges for engineering design. The presence of gas–liquid interfaces, dominance of surface forces, moving contact lines, wettability, dynamic contact angle hysteresis and flow in confined geometries are some of the unique features of two-phase systems, which manifest into complex transport phenomena. While Taylor plug/bubble flow is a fairly common flow pattern in several micro-fluidic devices operating at low Bond number, the ensuing transport characteristics are complex and still not fully discerned. This review paper aims at highlighting the nuances and features of a unit cell of a Taylor plug flow, especially focusing on partially wetting systems, which are more common in engineering applications. Emphasis is given to a ‘unit cell’ flow system consisting of an isolated liquid Taylor plug with adjacent gas phase, confined in a capillary tube. Such a seemingly simple flow condition poses considerable challenges for discerning and modelling local thermo-hydrodynamic transport coefficients. Relevant background information and fundamentals are carefully scrutinized while summarizing the state-of-the-art. The role of wettability and dissipation near the contact line is highlighted via available experimental and simulation results. Local momentum and heat transfer exchange processes during the motion of an isolated plug of partially wetting liquid moving inside a capillary tube are delineated.     

Effect of graphene oxide coating on bubble dynamics and nucleate pool boiling heat transfer

Materials coating has been proved to be an effective mean to increase the number of active nucleating sites, and therefore generate more vapor bubbles and lead to better pool boiling heat transfer performance. In this work, graphene oxide (GO) is coated on a boiling surface by self-assembly method, to enhance critical heat flux (CHF). The pool boiling is carried out on a smooth copper surface to study the effect of GO coating using distilled water as the working fluid along with bubble dynamic visualization. GO coating facilitates bubble nucleation by providing numerous microscale cavities. The visualization investigation of bubble dynamic behavior shows that the CO-coated surface exhibits a higher bubble departure frequency, a smaller bubble departure diameter and smaller bubble diameters in the pool, indicating greatly enhanced heat transfer effects. Meanwhile, the GO-coated surface exhibited a smaller contact angle than the copper surface, revealing that surface becomes more hydrophilic after GO coating. Consequently, GO-coated surface with a coating time of 4 h provides a CHF of 224.3 W/cm² and a heat transfer coefficient (HTC) of 8.79 W/(cm²·K), representing an improvement of 94.0% in CHF and 83.5% in HTC compared to smooth copper surface.     

The Unique Superfluid 3He A-B Interface: Surface Tension and Contact Angle

We have measured the A-B boundary wall contact angle and the phase interface surface tension in superfluid ³ He. The measurements have been made in the ballistic temperature regime at zero pressure in magnetic fields up to 400 mT. We infer the surface energies from the behaviour of the phase boundary moving in and out of a stack of glass capillary tubes. We measure the wall contact angle from the observed capillary height and obtain the interphase surface tension from the level of over- or under-magnetisation when the interface pops out of the capillaries. This is the first measurement of both the surface tension and contact angle in high magnetic fields.     

Influence of wettability on bubble formation in liquid

This paper presents experimental results of the surface phenomena effect on bubble formation from a single orifice in water and at nozzle in liquid aluminium with gas blowing at small flow rates. The usage of coated orifice in water and nozzles of different materials in the melt realized wide range of contact angles. The meaningful stages, termed (1) nucleation period, (2) under critical growth, (3) critical growth and (4) necking, were identified during bubble formation in a regime referring to simultaneous forced flow and surface tension control. It was revealed that bubble formation is substantially dominated by hysteresis of contact angle. Evolution of interface equilibrium and force balance conditions distinctive for bubble formation is clarified. X-ray fluoroscope was used to carry out in-situ observation of bubble formation in the melt. It was shown that bubble volume increased with wettability worsening both for aqueous and metallic systems. A further insight into the mechanism of the bubble formation was obtained by comparison of the bubble behaviour at the tip of the injection devices in liquid aluminium and at the orifice in water.     

Analysis of quasi-static conditions of boiling onset and bubble departure

The bubbles slowly growing at the heater can depart in two ways - either from its smooth surface or from the edge of the catity, which is the centre of boiling. The departure mode is dependent on the relation between the wetting angle and the dimensionless cavity radius. In terms of these parameters a boundary between the regions of different departure regimes was plotted. For each region there is an interpolation formula giving departure sizes of the bubble. The radius of the cavity, which is an active evaporation centre, is estimated. Theoretical conclusions are tested experimentally.     

Numerical Simulation of Nucleate Boiling Phenomenon Coupled with Thermal Response of the Solid

The present work describes the response of a heated solid surface during nucleation, growth and departure of a single bubble. Two dimensional, axisymmetric, finite difference schemes are used to solve the governing equations in the liquid, vapor and solid phases. The interface between liquid and vapor phases is tracked by a level set method. An iterative procedure is used at the interface between the solid and fluid phases in order to match temperatures and heat fluxes. Time and space invariant heat fluxes are supplied at the solid base and calculations are carried out for solids with different thermo-physical properties and thicknesses. Near the three-phase contact line, temperatures in the solid are observed to fluctuate significantly over short periods as the bubble base first expands outwards then contracts inwards before departure. The results show that waiting and growth periods can be related directly to wall superheat. The functional relationship between waiting time and wall superheat is found to agree well with the correlations based on experimental data reported in the literature.     

A scanning thermocouple probe for temperature mapping

A simple thermocouple probe with tip diameter less than 10 μm has been developed for mapping temperature distribution on surfaces. The fabrication procedures of the thermocouple tip involve fixing a fine tungsten wire inside a glass capillary tube and coating the exposed tip with gold. We have successfully used the thermocouple tip to obtain a two-dimensional temperature map on the heatsink of a high-power light-emitting diode (LED), thus demonstrating this device as a low-cost alternative to expensive infrared cameras for near-field thermal imaging     

Sheathless capillary electrophoresis/electrospray mass spectrometry using a carbon-coated tapered fused-silica capillary with a beveled edge.

A tapered capillary tip containing a beveled edge was developed for use in sheathless capillary electrophoresis/electrospray mass spectrometry (CE/ESI-MS). The optimal flow rate of a 75-microm-i.d., 90-microm-o.d. beveled tapered capillary tip was similar to a conventional flat tapered tip with a 25-microm orifice. Using a mixture of coptisine, berberine, and palmatine chloride, the sheathless CE/ ESI-MS sensitivity of a beveled 75 microm tapered tip capillary was found to be similar to a 25 microm flat tip. Although both tips offer similar CE/ESI-MS sensitivity, the beveled tapered capillary tip is more rugged and durable than a conventional 25-microm tapered capillary because of the larger outside diameter and inside diameter. To make electrical contact, the capillary tip was smeared with paint marker followed by the application of a carbon coating using a graphite pencil. Using this refined carbon-coating procedure, the capillary tip can be operated with aprotic solvents.     

Permanent Anticoagulation Blood-Vessel by Mezzo-Sized Double Re-Entrant Structure

Having a permanent omniphobicity on the inner surface of the tube can bring enormous advantages, such as reducing resistance and avoiding precipitation during mass transfer. For example, such a tube can prevent blood clotting when delivering blood composed of complex hydrophilic and lipophilic compounds. However, it is very challenging to fabricate micro and nanostructures inside a tube. To overcome these, a wearability and deformation-free structural omniphobic surface is fabricated. The omniphobic surface can repel liquids by its “air-spring” under the structure, regardless of surface tension. Furthermore, it is not lost an omniphobicity under physical deformation like curved or twisted. By using these properties, omniphobic structures on the inner wall of the tube by the “roll-up” method are fabricated. Fabricated omniphobic tubes still repels liquids, even complex liquids like blood. According to the ex vivo blood tests for medical usage, the tube can reduce thrombus formation by 99%, like the heparin-coated tube. So, the surface will soon replace typical coating-based medical surfaces or anticoagulation blood vessels.     

Adhesion control of fungal spores on solid surfaces using hydrophilic nanoparticles

Mold growth can trigger a variety of serious problems such as allergies and asthma. Designing surfaces that are unfavorable for the adhesion of fungal spores is considered an effective method to prevent fungal growth. In this study, the effect of hydrophilic surface treatment on the adhesion of fungal spores onto substrates was investigated using Aspergillus oryzae as a model fungus. The fungal spores that strongly adhered on the hydrophilic substrates under atmospheric conditions were easily removed by lightly washing by hand in water. These experimental results agreed well with thermodynamic predictions based on contact angle measurements. In addition, the removal ratio of the fungal spores on substrates coated with silica nanoparticles was higher than that on plasma-treated glass. It is believed that the contact area between a spore and substrate depended on the substrate roughness. Atomic force microscopy revealed that there was almost no adhesive force between the spores and glass substrate coated with silica nanoparticles. These results suggest that hydrophilic treatment using hydrophilic silica nanoparticles is more effective than hydrophilic plasma treatment to prevent fungal spore adhesion on glass substrates.     

Rupture of thin ductile tubes by oblique impact of blunt missiles: experiments

Impact tests at both normal and oblique angles of incidence were conducted on thin mild steel tubes using a moderate size of blunt missile (D_m/h=4.33) at various angles of obliquity (0°⩽φ₀⩽60°) from normal. The minimum impact speed that generated cracks through the thickness of the wall, termed the speed for rupture, was measured, and various types of rupture were identified. For a thin tube hit by a flat-nosed missile at a large angle of obliquity, the flat-nose initially cuts into the surface of the tube wall; through thickness rupture is due to tensile tearing that occurs in a region just below the crater. The speed for rupture of the tube is found to be a minimum at an angle of incidence equal to 45°; this speed is about 41% less than that required to rupture a tube of equal thickness by impact at a normal angle of obliquity. If the missile nose has a radius of curvature of the same order as the missile radius, the deformation is more diffuse in the region immediately adjacent to the contact surface; consequently the mode of failure changes from predominately shear at the edge of the missile to more uniform stretching under the contact surface. For oblique impact of missile with more rounded noses, this causes the observed speed for rupture to increase with increasing angle of obliquity.     

Ice crystal growth in supercooled solution

Study on the crystal growth of ice in water-ethyleneglycol solution was carried out, experimentally. The ice crystal, which was seeded on top of the capillary tube, propagated inside the tube slowly and began to grow freely at the tip of the tube in subcooled solution. The outer diameter of the tip of the capillary tube was less than 0.1 mm, which was much smaller than that of other researchers. Hence, considerable reduction of the influence of the existence of a capillary was accomplished and the initial growth of a single crystal was observed, precisely. Under the condition of subcooling of less than 8 K, the shape of the crystal was observed to be different from that of the one in pure water. The velocity of dendrite ice growth and the radius of the curvature of the tip were measured. It was found that after the dendrite ice developed to a certain size, the velocity of the dendritic growth and the radius of curvature were kept steady, and its values were dependent on the degree of subcooling and the concentration of the solution.     

Tip and inner walls modification of single-walled carbon nanotubes (3.5 nm diameter) and preparation of polyamide/modified CNT nanocomposite reverse osmosis membrane

To enhance the water flux and salt rejection of reverse osmosis membranes, thin film nanocomposite reverse osmosis membranes were prepared by incorporating functionalised single-walled carbon nanotubes (SWNTs). The functionalised SWNTs were obtained by chemical process such as mixed acid oxidation and substitution reaction. The SWNTs were characterised by HRTEM, FTIR, TGA, Raman spectra, UV-Vis and XPS, etc. The surface morphology and structure of membrane were characterised by SEM and contact angle measurement, respectively. The results showed that carboxyl, acyl, amide and amine were successfully grafted on the tip and inner wall of the single-walled carbon nanotubes. The dispersity of the functionalised SWNTs in water was tested, indicating a good hydrophilic property. The polyamide/modified CNT nanocomposite reverse osmosis membrane was prepared. Compared with the bare polyamide membrane, the SWNT-polyamide thin film nanocomposite membranes showed higher property in hydrophilic surface and its water flux, as along with salt rejection, improved dramatically. The experimental results revealed that the modified SWNTs (especially those containing hydrophilic groups such as carboxyl groups and amino groups) well dispersed in the polyamide thin film layer, and hence improved the water permeation, and the salt rejection.     

3D electron microscopy study of metal particles inside multiwalled carbon nanotubes

The location of palladium nanoparticles on and inside the multiwalled carbon nanotubes channel is presented for the first time using electron tomography (3D TEM). The palladium salt precursor was rapidly sucked inside the nanotube channel by means of capillarity that is favored by the hydrophilic character of the tube wall after acidic treatment at low temperature. Statistical analysis indicates that the palladium particles were well dispersed and the palladium particle size was relatively homogeneous, ranging from 3 to 4 nm regardless of their location within the nanotube, within the resolution limit of the technique for our experimental conditions, i.e., about 2 nm. Three-dimensional TEM analysis also revealed that introduction of foreign elements inside the tube channel is strongly influenced by the diameter of the tube inner channel, i.e., easy filling seems to occur with a tube channel >or=30 nm , whereas with tubes having a smaller channel (<15 nm), almost no filling by capillarity occurred leading to the deposition of the metal particles only on the outer wall of the tube.     

Substrates effects on the growth behavior of Copper tetra-tert-butyl phthalocyanine films studied by atomic force microscopy

Copper tetra-tert-butyl Phthalocyanine (CuTTBPc) was vacuum deposited onto substrates of hydrophilic glass, hydrophobic silanized-glass, and one layer CuTTBPc LB film. The effects of substrates on the growth behavior and film characteristics of CuTTBPc were studied by atomic force microscopy as well as XRD and dynamic contact angle analyzer. The results show that, in the early growth stage, the island density and coverage ratio of CuTTBPc are small on hydrophilic glass surface. This result is caused from the weak interaction of CuTTBPc molecules to the glass which leads to a small nucleation rate and rougher morphology in the early growth stage. On the contrary, the nucleation rate of CuTTBPc on the silanized-surface is high and thus a much smoother film comprises densely distributed fine-grain clusters was observed. This fact indicates the higher adhesive force of CuTTBPc molecules to the silanized-surface. On the LB film, the CuTTBPc molecules are arranged in aggregative domains which are separately distributed on the glass surface. These domains act as active sites for the nucleation and growth of the later deposition process and thus, high density clusters were found in the early growth stage. The XRD results demonstrate that the film grown on glass has higher degree of crystalline structure than the others which is resulted from the distinction of the initial growth stage.     

The Effect of Gas Properties on Bubble Formation,Growth, and Detachment

Bubble formation and growth play an important role in various processes and industries, where the dispersion of gas bubbles in a liquid medium occurs frequently. In this paper, the formation, growth, and detachment of gas bubbles produced from a submerged needle in water are numerically and experimentally investigated. The effect of injected gas properties on bubble characteristics, including bubble diameter, contact angle, and the frequency of bubble formation, is evaluated. In particular, the changes in bubble characteristics during the injection process are investigated for three different gases to evaluate the effect of density and surface tension on the bubble detachment criteria. The present numerical results show an acceptable agreement with experiments under different operating conditions. The results show that the increase in surface tension, and the decrease in gas density result in larger bubble sizes before detachment occurs. Moreover, the bubble generation frequency is found to strongly depend on the contact angle and the surface tension.