Silica Microtube Research Articles

Microscale wetting effect of oil-water in silica microtubes

Pore wetting is significant for understanding fluid behaviour in porous media. Due to the lack of measurement techniques, pore wettability was estimated by using contact angles measured on flat plate. This results in that the effect of pore size on wettability is rarely studied. In this paper, the oil-water static contact angles in the silica microtubes with inner diameter (ID) range of 1–523 μm were measured by the cryo-Focused Ion Beam-Scanning Electron Microscopy (cryo-FIB-SEM) and optical microscope. The experimental results show that the oil (silicone oil, hexadecane) and water contact angles in the silica microtubes have the microscale wetting effect, that is the contact angles increase monotonically with the decrease of the microtubular ID. Moreover, the oil-water contact angle of water-filled first is always smaller than that of oil-filled first. The static pore contact angle of oil-water in a silica microtube is different from the contact angle on a silica plate. This paper adds to our current understanding of the pore scale wettability phenomenon in the porous media and provides us with an experimental method for measuring micron scale pore wettability.

Experimental investigations of local friction factors of laminar and turbulent gas flows in smooth micro-tubes

Local friction factors of laminar and turbulent gas flows including choked flows through micro-tubes were experimentally investigated under atmospheric back pressure and variable inlet pressure. The experiments were carried out using two glass micro-tubes with D = 265.7 and 399.9 μm and a fused silica micro-tube with D = 531.2 μm. Two to three pressure tap holes were drilled into the micro-tube wall at intervals of 5 ~ 6 mm to measure local pressures. The local Mach numbers, temperatures and friction factors were obtained by using stagnation temperatures and pressures, local pressures and mass flow rates. The results for wide range of Reynolds numbers and Mach numbers were obtained including choked flows. Both the local Fanning and the local Darcy friction factors were obtained under the assumption of both Fanno flow (adiabatic wall) and isothermal flow, respectively. Since the measured values of the inner relative surface roughness of the micro-tubes were less than 0.02 %, only the effect of compressibility on friction factors was assessed. The difference between Fanning and Darcy friction factors was described and compared with the ff and fd correlation as a function of Mach number. The friction factor difference obtained under the assumption of Fanno and isothermal flows was also compared with the available literature and numerical results. In the turbulent flow region including the choked flow, the local Fanning friction factors under the assumption of Fanno flow nearly coincided with Blasius correlation.

Synthesis of Hybrid Silica-Carbon Tubular Structures by Chemical Vapor Deposition with Methane or Ethene

Silica microtube and carbon nanotube hybrid structures have been synthesized by catalytic chemical vapor deposition using either methane or ethene as the carbon source, and cobalt-grafted or impregnated silica tubes (200–800 nm) as catalyst. The cobalt-grafted catalyst shows a high resistance to reduction (>1000 °C) and selectivity to single-wall carbon nanotubes (SWCNT). While ethene deposition produces more carbonaceous material, methane experiments show higher selectivity for SWCNT. After removing the silica with an excess of HF, the carbon nanostructure endured, resulting in a coaxial carbon nanostructure. The novel hybrid nanostructures obtained consist of a submicron-sized tube, with walls that are formed by a succession of carbon/silica/carbon layers to which multiwall (20–25 nm) and/or single-wall (0.6–2.0 nm) carbon nanotubes are attached. This synthesis approach combines the mechanical properties of carbon nanotubes and the thermal properties of silica tubes into a synergetic nanostructured material, opening further possibilities for polymer reinforcement and potential applications in catalysis.

Miniature refractive index fiber sensor based on silica micro-tube and Au micro-sphere

We demonstrated the refractive index sensing characteristics of a miniature fiber sensor composited by silica-hollow-tube (SHT) and Au-micro-sphere (AmS). The high sensitivity is obtained due to the evanescent field effect existing in the SHT with the inner diameter of ∼2.3 μm and the surface plasmon resonance effect excited on the surface of AmS with the diameter of ∼2 μm. Experimental results indicate that this sensor can continuously measure the glucose concentration in range of 0–60% with a good linearity. The high detection sensitivity up to 8.33 μmol/L (47.33 mW/RIU) enables its ability in determining the glucose concentration in either blood or body fluids. Furthermore, the tiny structure is promise to be integrated into the microchip or other injectable structures, and monitor the glucose concentration in real-time.

Electrical Thermo-Optic Tuning of Whispering Gallery Mode Microtube Resonator

We demonstrate a very simple electrically assisted technique for broad-range tuning of whispering gallery modes in microtube resonator, which is fabricated by embedding a FeCrAl heating wire with small resistance into a tapered silica microtube. The huge Joule heat generated by the heating wire can be effectively and thermally conducted to the microtube and consequently changes the path of the optical whispering gallery modes due to the thermo-optic effect. Experimental results show that such a device has a tuning range of 0.57 nm with excellent stability under the applied current of 200 mA, fast response (<100 ms), and low power consumption (~0.1 mW). This makes it to have potentials in tunable filters or fast optical switches.

Whispering gallery modes of dye-doped polymer microspheres in microtube

A micro-system including a silica micro-tube (inner diameter≈6.2µm) and dye-doped microspheres (diameter≈6µm) was pumped by a 532nm laser. The whispering gallery modes (WGMs) of fluorescence resonance were observed. Free Spectral Range (FSR) of single-sphere with diameters of 40μm was 3.1nm and with diameters of 6μm was 5.4nm. Two size-matched microspheres were coupled using a bi-sphere micro-system. Resonances from each sphere's evanescent field were coupled, which leads to resonance splitting. The measured resonance splitting in the spectra of the two coupled spheres is about ∆λ≈0.8nm. The scheme would be a promising component in future micro-photonic devices.

Effect of the size and pressure on the modified viscosity of water in microchannels

The phenomenon that flow resistances are higher in micro scale flow than in normal flow is clarified through the liquid viscosity. Based on the experimental results of deionized water flow in fused silica microtubes with the inner radii of 2.5 μm, 5 μm, 7.5 μm, and 10 μm, respectively, the relationship between water flow velocity and pressure gradient along the axis of tube is analyzed, which gradually becomes nonlinear as the radius of the microtube decreases. From the correlation, a viscosity model of water flow derived from the radius of microtube and the pressure gradient is proposed. The flow results modified by the viscosity model are in accordance with those of experiments, which are verified by numerical simulation software and the Hagen-Poiseuille equation. The experimental water flow velocity in a fused silica microtube with diameter of 5.03 μm, which has not been used in the fitting and derivation of the viscosity model, is proved to be comsistent with the viscosity model, showing a rather good match with a relative difference of 5.56%.

Revisiting Maxwell’s accommodation coefficient: A study of nitrogen flow in a silica microtube across all flow regimes

Gas flows have been studied quantitatively for more than a hundred years and have relevance in modern fields such as the control of gas inputs to processes, the measurement of leak rates and the separation of gaseous species. Cha and McCoy have derived a convenient formula for the flow of an ideal gas applicable across a wide range of Knudsen numbers (Kn) that approaches the Navier–Stokes equations at small Kn and the Smoluchowski extension of the Knudsen flow equation at large Kn. Smoluchowski’s result relies on the Maxwell definition of the tangential momentum accommodation coefficient α, recently challenged by Arya et al. We measure the flow rate of nitrogen gas in a smooth walled silica tube across a wide range of Knudsen numbers from 0.0048 to 12.4583. We find that the nitrogen flow obeys the Cha and McCoy equation with a large value of α, unlike carbon nanotubes which show flows consistent with a small value of α. Silica capillaries are therefore not atomically smooth. The flow at small Kn has α=0.91 and at large Kn has α close to one, consistent with the redefinition of accommodation coefficient by Arya et al., which also resolves a problem in the literature where there are many observations of α of less than one at small Kn and many equal to one at large Kn. Silica capillaries are an excellent choice for an accurate flow control system.

Sensitivity enhancement in optical micro-tube resonator sensors via mode coupling

A liquid filled, silica micro-tube with a low refractive index material inner-coating has been proposed and theoretically studied as a coupled micro-resonator sensor to greatly enhance biochemical sensor sensitivity. Its unique coupling phenomenon has been analyzed and utilized to boost the device's refractive index sensitivity to 967 nm/Refractive Index Unit (RIU). Through optimization of the coupling strength between the two micro-resonators, further improvement in refractive index sensitivity up to 1100 nm/RIU has been predicted. This mode coupling strategy allows us to design robust, thick-walled micro-tube sensors with ultra-high sensitivity which is useful in practical biochemical sensing applications.

Mass flow rate measurements in microtubes: From hydrodynamic to near free molecular regime

An experimental investigation of the reflection/accommodation process at the wall in a single silica microtube and isothermal stationary flow conditions was carried out. Several gases and different diameters were studied through various regimes. Especially for helium, the Knudsen number range was investigated as far as the free molecular regime. This kind of investigation requires a powerful experimental platform to measure mass flow rates, which we have carried out. An analytic expression of the mass flow rate, based on the Navier–Stokes equations with second order boundary condition, was used to yield the tangential momentum accommodation coefficient (TMAC) in the 0.003–0.3 Knudsen number range. Otherwise, the experimental results of the mass flow rate were compared with theoretical values calculated from kinetic approaches using variable TMAC as parameter over the 0.3–30 Knudsen number range, and an overall agreement appears through the comparison. Finally, whatever the theoretical approach the TMAC obtained from gas (nitrogen, xenon, argon, and helium)-surface (fused silica) pairs is similar and lower than unity. A tendency of the TMAC values seems to appear according to the molecular mass of the gases. In addition for each gas, the second order slip coefficient magnitude seems to decrease when the tube diameter increases.

Analysis of the sensing properties of silica microtube resonator sensors

We present a theoretical study of the sensing properties of silica microtube resonator sensors. By solving the Maxwell's equations to obtain the electric field distribution in a cylindrical coordinate system and by using the perturbation method, we have analyzed in detail the sensing properties of silica microtube resonator sensors, which include the bulk refractive index, surface, and absorption sensing sensitivities with different radial order resonant modes. We found that a type of resonant mode, different from the evanescent modes commonly employed in previous investigations, is very promising for sensing changes of the refractive index. Furthermore, the resonant mode with high electric field at the inner boundary of the microtube is ideal for surface sensing applications. The high Q factor resonant mode is useful for the absorption sensing application. These sensing properties analyzed by using the perturbation method match very well with the results from the Mie scattering method. Finally, the limitation of the perturbation method is discussed.

Micro-PIV Measurements in Micro-Tubes and Proboscis of Mosquito

Effects of the relative size of a particle to the diameter of a micro-tube on velocity profiles obtained by the micro-PIV measurement of the flow through the micro-tube were investigated to achieve the accuracy of measurements of the flow in a proboscis of a mosquito. The velocity profiles obtained for different diameter-ratios of a fluorescent particle and fused silica micro-tube were compared with the exact solution of the velocity profile of flow through a circular pipe at a low Reynolds number. A suitable diameter-ratio was found to be less than 0.04. The velocity profile of flow through the proboscis of a mosquito measured by using the suitable micro-PIV method satisfying the above condition was found to be different from the exact solution of the circular-pipe flow near the center of the proboscis. The velocity profile in the proboscis was similar to that of a rectangular cross section which was verified by a SEM observation.

A facile approach to fabricate functional 3D macroscopic silica microtube networks using N,N′-methylenediacrylamide organogel as template

Using N,N'-methylenediacrylamide organogel as a template and gamma-aminopropyltriethoxysilane as the precursor, functional silica microtube networks (MTNs) with adequate mechanical strength and different surface groups were prepared by an easy and effective approach.