Polymer Wicks Research Articles

Pool boiling heat transfer performance of micro-embossing molds for the fabrication of polymer wicks

As flexible heat dissipation devices are urgently needed for flexible electronics, polymers with the advantages of good flexibility, insulation, and low density have attracted wide attention. It would be interesting to fabricate thermal functional structures on polymers to overcome their low thermal conductivity. This study aims to design and fabricate a micro-embossing mold to process polymer three-dimensional microgroove wicks (3D-MWs), which are applied in flexible heat pipes and achieve high thermal conductivity. Three kinds of tungsten steel molds were processed by using two wire electrical discharge machining processes (WEDM), two electrical discharge shaping machining processes (EDSM), and a combination of WEDM and EDSM. The 3D-MWs of the tungsten steel mold were reprinted onto the polymer films by a two-step embossing process. To investigate the effects of microstructures on boiling heat transfer performances, tungsten steel molds fabricated by different methods were tested to reflect the heat dissipation potential of the polymer 3D-MWs. The results showed that the 3D-MWs fabricated by the combination of WEDM and EDSM exhibit good boiling heat transfer performance and high processing efficiency, with a critical heat flux of 1040.1 kW/m2 and a maximum heat transfer coefficient of 38.4 kW/m2·°C. These values are 46% and 611% higher than those of the smooth tungsten steel surface, respectively, making it a superior choice for micro-embossing molds.

Effect of PTFE wick thickness on Loop heat pipe performance

Heat leakage has long been a problem in the development of Loop heat pipe (LHP). Among the strategies to suppress heat leakage, the use of low thermal conductivity polymer materials and adjustment of wick thickness are considered as one of the effective methods. However, although there are many recent reports on the effect of metal wick thickness on performance, there are few studies on the effect of polymer wick thickness such as PTFE wick. Therefore, this paper hopes to integrate the two factors and present the best results. This paper investigates the effect of PTFE wick thickness (1 to 2.75 mm) on LHP performance and proposes the thickness effects on both thermal resistance and flow drag. In addition, since PTFE is hydrophobic, water, with its high surface tension, cannot be used as working fluid. Therefore, in this study, a tiny amount of Butanol (6 wt%) is added to water to create self-rewetting fluid to be used as working fluid, overcoming the flow drag of PTFE wick and allowing the system to operate smoothly.Regarding the wick’s porosity and permeability, the best capillary effect and flow drag occurs at wick thickness of 2 mm. The LHP performance result shows, compared to using PTFE wick with water as working fluid where the system was unable to start up, the system using self-rewetting fluid can smoothly operate, overcome heat leakage and enhance performance. The best performance occurs at wick thickness of 2 mm, the maximum heat load reaches 600 W and heat flux 30 W/cm2; the results are all consistent with the performance predictions based on the wick parameters.

Investigation on refrigerant transport by capillary effect with fleeces in an evaporator for a high temperature cooling machine

The work described in this paper is part of development of a compact cooling machine for electronics in well environments with ambient temperatures above 200 °C. It includes an evaporator, a compressor and a condenser such that the cooling machine can be operated permanently. The evaporator is designed as a capillary evaporator. Instead of metal or polymer wicks, different fleece materials based on cellulose and wool have been investigated regarding capillary wetting on heat transfer surfaces.Characteristic fleece properties have been determined experimentally. The tests include effects supporting the wetting rise, such as multiple layers, compressed layers, pre-wetted fleeces and increased temperature.Results of evaporation tests show the functionality of the evaporator principle and confirm sufficient cooling effect.The presented work can serve as basis for further investigation of wetting under evaporation influence and for optimization of functionality of evaporator design using passive capillary wetting with fleece materials.

In-service characterization of a polymer wick-based quasi-dry electrode for rapid pasteless electroencephalography.

A novel quasi-dry electrode prototype, based on a polymer wick structure filled with a specially designed hydrating solution is proposed for electroencephalography (EEG) applications. The new electrode does not require the use of a conventional electrolyte paste to achieve a wet, low-impedance scalp contact. When compared to standard commercial Ag/AgCl sensors, the proposed wick electrodes exhibit similar electrochemical noise and potential drift values. Lower impedances are observed when tested in human volunteers due to more effective electrode/skin contact. Furthermore, the electrodes exhibit an excellent autonomy, displaying an average interfacial impedance of 37±11 kΩ cm2 for 7 h of skin contact. After performing bipolar EEG trials in human volunteers, no substantial differences are evident in terms of shape, amplitude and spectral characteristics between signals of wick and commercial wet electrodes. Thus, the wick electrodes can be considered suitable to be used for rapid EEG applications (electrodes can be prepared without the presence of the patient) without the traditional electrolyte paste. The main advantages of these novel electrodes over the Ag/AgCl system are their low and stable impedance (obtained without conventional paste), long autonomy, comfort, lack of dirtying or damaging of the hair and because only a minimal cleaning procedure is required after the exam.

Development of polymer wicks for the fabrication of bio-medical sensors.

Polymer based wicking structures were fabricated by sintering powders of polycarbonate (PC), ultra-high molecular weight polyethylene and polyamide 12, aiming at selecting a suitable material for an innovative electroencephalography (EEG) bio-electrode. Preliminary experiments showed that PC based wicks displayed the best mechanical properties, therefore more detailed studies were carried out with PC to evaluate the influence of powder granulometry and processing parameters (pressure, temperature and time) on the mechanical properties, porosity, mean pore radius and permeability of the wicks. It was concluded that the mechanical properties are significantly enhanced by increasing the processing time and pressure, although at the expense of a significant decrease of porosity and mean pore diameter (and thus permeability), particularly for the highest applied pressures (74kPa). However, a good compromise between porosity/permeability and mechanical properties could be obtained by sintering PC powders of particle sizes below 500μm at 165°C for 5min, upon an applied pressure of 56kPa. Moreover, PC proved to be chemically stable in contact with an EEG common used disinfectant. Thus, wicking structures with appropriate properties for the fabrication of reusable bio-electrodes could be fabricated from the sintering of PC powders.

Darcy's law–based numerical simulation for modeling 3D liquid absorption into porous wicks

AbstractLiquid imbibition into polymer wicks, where a clear liquid front can be seen rising during the wicking process, is modeled using the concepts of flow in porous media. The flow of liquid behind the moving liquid front is modeled using the physics of single‐phase flow in a porous medium where the Darcy's law is combined with the continuity equation and a capillary suction pressure is imposed at the liquid front. A novel numerical simulation PORE‐FLOW© based on the finite element/control volume method is proposed to model such imbibitional flows in wicks of complex shapes. A validation of the simulation is obtained by achieving an excellent comparison of its predictions with an experimental result, an analytical solution, and the Washburn equation for the case of wicking against gravity in a cylindrical wick. The simulation is also used to predict a case of two‐dimensional (2D) wicking in the altered cylindrical wicks with two different cross‐sectional areas. Once again an excellent match is obtained with the experimental results, while analytical solutions for the single and double cross‐section cases along with the Washburn equation fail to predict the 2D wicking. Later, some other types of altered wicks with sharp changes in their cross‐sectional areas were analyzed numerically for their wicking behavior. It was observed that the height of liquid front in a vertical wick as a function of time, which is proportional to the history of liquid imbibed, is strongly dependent on the extent of reduction in the wick cross‐sectional area as well as its location vis‐à‐vis the wick entrance. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Design and fabrication of porous polymer wick structures

Wicks are often an integral part of fluid capacitance and transport in many systems, including heat pipes, fuel cells, and lateral flow chemical assays on cellulose paper. In this paper, we explore porous polymer monoliths as a new wick material potentially applicable to these and other applications. Polymer monolith chemistries, long used for high surface-to-volume ratio separation and filtering media in analytical chemistry, offer tremendous flexibility in resulting monolith pore structure, chemical composition, and surface chemistry (including wettability). We leverage this flexibility to design, fabricate, and characterize hydrophilic porous monoliths, with the aim of achieving high permeability wick materials. We show that variations in monomer concentration and porogen composition can affect mode pore diameters ranging from 6.3 to 10.1 μm and permeabilities ranging from 0.73 × 10 −12 to 1.9 × 10 −12 m 2. In addition, we identify a rough dependence of monolith permeability on porosity times the square of mode pore diameter and discuss key figures of merit characterizing capillary transport. As an example application, we then detail a custom injection molding procedure, where we in situ polymerize ∼150 μm thick wicks conformally onto the surface of metal channels of a polymer electrolyte fuel cell cathode.

Nanoelectrospray aerosols from microporous polymer wick sources

Nanoelectrospray aerosols were formed from microporous polymer wick sources. Current-voltage characteristics were measured as a function of solution electrical conductivity and surface tension and two distinct electrospray modes were observed. In the first mode, when the maximum capillary flow rate through the wick exceeds the electrospray flow rate, a single electrospray forms from a droplet at the end of the wick. In the second mode, when the maximum capillary flow rate is less than the electrospray flow rate, a multitude of microscopic nanoelectrospray sources are formed from within the surface of the wick tip.

Darcy's law‐based models for liquid absorption in polymer wicks

AbstractWicking of liquids in polymer wicks made of sintered polymer beads is studied experimentally where three different polymer wicks (made from polycarbonate, polyethylene, and polypropylene) and three different well‐characterized liquids (hexadecane, decane, and dodecane) are used to plot the mass of wicked liquid as a function of time. The experimental results are compared with the predictions from the Washburn equation as well as a Darcy's law‐based formulation. The suction pressure needed to pull the liquid up a wick in the formulation is modeled using a new energy balance (EB) method and a capillary method. In the former, the released surface energy during wetting is equated to the viscous losses during liquid motion; in the latter, the suction pressure is obtained by treating the wick pore‐space as a bundle of capillary tubes. The Darcy's law‐based formulation also considers the effect of gravity in its predictions. The newly proposed EB method in conjunction with gravity yields the most satisfying predictions. All parameters used in the proposed model were measured independently and no fitting parameters were used. The success of this method is especially notable for a large‐pore polypropylene wick where it was the only model to predict the final steady‐state height for the liquid column. © 2007 American Institute of Chemical Engineers AIChE J, 2007