in roll coating are reviewed, with particular emphasis on the transfer between two rolls in the a... more in roll coating are reviewed, with particular emphasis on the transfer between two rolls in the absence of a substrate. For this situation, good agreement has been achieved between extensive experimental data on liquid fluxes (and hence film thicknesses) and theoretical predictions based on the lubrication approximation together with appropriate separation conditions. This agreement indicates that the theoretical approach would also be valid for the calculation of the separating force between rolls and the torques required to drive the rolls. However, much of the work has been restricted to Newtonian behaviour and there is scope for further analyses of the influence of shear thinning characteristics and viscoelasticity. * Dedicated to the memory of Professor J.G. Oldroyd.
Recycling of polymers is a necessity in our intensively consuming polymer world but the nature of... more Recycling of polymers is a necessity in our intensively consuming polymer world but the nature of polymers is such that they are prone to thermal degradation when re-extruded and this poses technical challenges to recycling. This papers describes research that seeks to rebuild the structure of degraded PA6. We present data from controlled experiments with pristine pPA6 extruded to form a base recycle rPA6 to which we added two chain extenders, separately: one with anhydride multifunctionality (ANHY), highly reactive with amide groups and one with epoxy multifunctionality (EPOX), less reactive. We found from rheological data carried out in the linear viscoelastic region (so as to study structural changes) a striking difference in the ability of the chain extenders to rebuild structure: 306% increase in the complex viscosity of rPA6/ANHY compared to 25% in that of rPA6/EPOX of the base rPA6. Mechanical and thermal (DSC and TGA) tests confirmed the superior efficacy of the multifunctional anhydride chain extender. Beside the practical benefit that ensues from this research, it also provides a strategic platform to develop chain extenders for other degrading polymers on the basis of understanding the degradation chemical reaction and targeting the most reactive end group of the split chains.
The coating flow examined here, labelled angled dip coating, is that where a substrate enters a p... more The coating flow examined here, labelled angled dip coating, is that where a substrate enters a pool of liquid forming an angle with the vertical so that it intersects the liquid along a wetting line which is not perpendicular to the direction of its motion. This flow situation is distinctly different from that where the substrate, inclined in the other dimension by the so-called angle of entry , intersects the liquid surface perpendicularly to its motion. Experiments were carried out with various liquids to determine the effect of on the substrate velocity at which air is entrained into the liquid. It was observed that as this angle departs from zero, air entrainment is delayed to higher speeds. The data show that the speed which is relevant to air entrainment is not the velocity of the substrate itself but its component normal to the wetting line. This result has important practical implications and suggests that this fundamental principle is also applicable to other coating flows.
This work deals with all aspects of mixing solids, liquids and gases in single and multi-phase fl... more This work deals with all aspects of mixing solids, liquids and gases in single and multi-phase flow and therefore reflects the modern use of non-intrusive methods and simulations by computational fluid dynamics.
YesTensioned web-roll coating is widely used but has surprisingly received little research attent... more YesTensioned web-roll coating is widely used but has surprisingly received little research attention. Here, a new semi-empirical model that predicts film transfer from applicator roller to web is developed and tested against data collected from a pilot coating line. The film transfer is found to vary linearly with web to applicator speed ratio S. Flow stability investigations revealed three types of defects: rivulets, air entrainment due to dynamic wetting failure and cascade, occurring at different values of S and capillary number Ca. Rivulets occurred at Ca 0.71-0.81, air entrainment at Ca>0.4 and S>0.71-0.83 and cascades at S>1.1 for Ca up to 6. Web speeds at which dynamic wetting failure occurred were, for the same Ca, comparatively higher than those that occur in dip coating. The data show that such hydrodynamic assistance is due to the coating bead being confined, more so with increasing web wrap angle β.The authors acknowledge the support of the Films R&D Centre of Toyobo Co. Ltd., Otsu, Japan and of the Thin Liquid Films Research Group of the University of Bradford, UK.The full-text of this article will be released for public view at the end of the publisher embargo on 26th March 2022
Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, ... more Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, which had a rough side and a comparatively smoother side. When we compared the air entrainment speeds on both sides, we found a switch in behaviour at a critical viscosity. At viscosity lower than a critical value, the rough side entrained air at lower speeds than the smooth side. Above the critical viscosity the reverse was observed, the smooth side entraining air at lower speed than the rough side. Only substrates with significant roughness showed this behaviour. Below a critical roughness, the rough side always entrained air at lower speeds than the smooth side. These results have both fundamental and practical merits. They support the hydrodynamic theory of dynamic wetting failure and imply that one can coat viscous fluids at higher speeds than normal by roughening substrates. A mechanism and a model are presented to explain dynamic wetting failure on rough surfaces.
Three-dimensional CFD simulations of turbulent flow in baffled stirred tanks - an assessment of t... more Three-dimensional CFD simulations of turbulent flow in baffled stirred tanks - an assessment of the current position CFD to fluidmixing needs realistic physical data the influence of rotation on the atmospheric mixing of a buoyant plume from a tall stack resistance tomography imaging of stirred vessel mixing dispersed phase distribution in liquid-liquid mixing in an agitated tank. (Part contents).
This experimental study of dynamic wetting in metering and pre-metered forward roll coating shows... more This experimental study of dynamic wetting in metering and pre-metered forward roll coating shows that the dimensionless applicator flow rate A, is the key parameter as it defines whether dynamic failure occurs by air entrainment ( A > A, trans ) or cascade ( A < A, trans ). A, trans is the flow rate when the regime switches from metering to pre-metered and A, min , the minimum flow rate in the pre-metered regime. When Ca was varied in the range 0.25, A, trans and A, min were in the range 1.401.29 and 1.201.18 respectively. These values compare very well with theoretical predictions. The air entrainment speeds V * M were found to be similar to the corresponding speeds V * plunging , measured in plunging tape flow but the cascade speeds V ** M can be smaller or larger depending on A . This led to exploring hydrodynamic assistance and drawing an analogy with curtain coating, which was supported by the data. It was also observed that cascade occurred in the pre-metered regime because the dynamic wetting line X D , moves near the minimum gap position just as in reverse roll coating. Indeed the cascade speeds conform to correlations developed for reverse roll coating. Finally the study gives data on X D , and compares them with theoretical predictions. The agreement in this case is only qualitative and this points to deficiencies in current modeling of the dynamic wetting region.
Reverse roll coating is probably the most widely used coating operation, yet its full potential h... more Reverse roll coating is probably the most widely used coating operation, yet its full potential has not been exploited as it is shown in this paper which considers operation with a negative gap. We demonstrate through a wide range of experimental data that such operation can yield very thin and stable films with no ribbing or cascade instabilities when low viscosity fluids are used. Typically, stable film thickness less than 5µm can be obtained at speeds up to 150 m/min when a rubber roller is used at -100 µm gap with fluids of viscosity in the range 10-200 mPa.s. These film thicknesses can be made to decrease further down to 1 or 2 microns with a judicious choice of speed ratios (applicator to metering roller) and rubber hardness. Such new findings make this simple coating method an attractive roll to roll technique for application in the newer coating technologies, such as in the production of solar cells and plastic electronics. The data obtained in this study have been underpinned by a model based on the classical lubrication theory, well developed for such flow situations. Essentially it is shown that the film thickness non dimensionalised with respect to the set negative gap is controlled through a single parameter, the elasticity number Ne which combines all the operating parameters. Of course, this flow problem has complexities, particularly at high speed ratios and at zero gap so the data obtained here can serve as a basis for more comprehensive modelling of this classical fluid mechanic problem.
This study assesses experimentally the role of gas viscosity in controlling the minimum film thic... more This study assesses experimentally the role of gas viscosity in controlling the minimum film thickness in slot coating in both the slot over roll and tensioned web modes. The minimum film thickness here is defined with respect to the onset of air entrainment rather than rivulets, the reason being that rivulets are an extreme form of instabilities occurring at much higher speeds. The gas viscosity effects are simulated experimentally by encasing the coaters in a sealed gas chamber in which various gases can be admitted. An appropriate choice of two gases was used to compare performances: air at atmospheric pressure and helium at sub-ambient pressure (25mbar), which we establish has a significantly lower "thin film" viscosity than atmospheric air. A capacitance sensor was used to continuously measure the film thickness on the web, which was ramped up in speed at a fixed acceleration whilst visualizations of the film stability were recorded through a viewing port in the chamber. The data collected show clearly that by coating in rarefied helium rather that atmospheric air we can reduce the minimum film thickness or air/gas entrainment low-flow limit. We attribute this widening of the stable coating window to the enhancement of dynamic wetting that results when the thin film gas viscosity is reduced. These results have evident practical significance for slot coating, the coating method of choice in many new technological applications, but it is their fundamental merit which is new and one that should be followed with further data and theoretical underpinning.
Abstract A mathematical model of the reverse and metering roll coating flow of Newtonian fluids i... more Abstract A mathematical model of the reverse and metering roll coating flow of Newtonian fluids is presented. It is based on the lubrication approximation and simple physical notions regarding the separation region. These physical notions discard the effect of surface tension on the dynamics of the system. The results of this model give the dimensionless net flux through the nip, λL, as a function of the speed ratio, rs and the radius ratio rr. This function is a weak one over the range of rr and rr normally encountered in reverse and metering roll coating operations and is in good agreement with experimental data[1] obtained previously. The results of this present study have also been related to the findings from a previous analysis of forward roll coating[2].
Thermoplastic polyurethane (TPU)/multiwalled carbon nanotube (MWNT) composites were prepared thro... more Thermoplastic polyurethane (TPU)/multiwalled carbon nanotube (MWNT) composites were prepared through mixing and extrusion process in a minimixer equipped with a single screw plus twin shaft paddles. In order to improve the dispersion of MWNT and the compatibility between MWNT and polyurethane, the MWNT were functionalised with polytetramethylene ether glycol (PTMEG) through a grafting process to get the MWNT-g-PTMEG. The relationship between the structure and properties of the TPU/MWNT composites was examined. The results showed that MWNT-g-PTMEG can be uniformly dispersed in the TPU matrix, and the tensile strength and elongation at break were both improved. For 1 wt-%MWNT-g-PTMEG/TPU composites, the tensile strength and elongation a break were enhanced to y41 MPa and 568% respectively, while for pristine TPU, they were y34 MPa and 508% respectively. Dynamic mechanical analysis showed that the storage modulus at room temperature increased, and the maximum tan d decreased with increasing MWNT-g-PTMEG contents. The simultaneous small and wide angle X-ray scattering measurements showed that with the incorporation of MWNT-g-PTMEG, the degree of crystallinity, interdomain distance, hard domain thickness as well as the degree of microphase separation increased. It appears that the functionalised MWNT-g-PTMEG promotes the formation of the ordered lamellar structure of polyurethane hard segments.
An experimental investigation of the effect of fluid rheology on air entrainment in plunging tape... more An experimental investigation of the effect of fluid rheology on air entrainment in plunging tape experiments (dip coating) was carried out. The critical velocity at which the dynamic wetting line breaks up, allowing air to entrain into the liquid, was measured for a range of viscoelastic polymer solutions in glycerin/water mixtures. Variations of the critical velocities with various solutions were found to be unexpectedly small in comparison with the large viscosity variations. The results suggest that dynamic wetting failure and subsequent air entrainment are governed by nonhydrodynamic phenomena occurring at the molecular scale at the wetting line. It was also found that fluid elasticity gives rise to flow instabilities that may lead to a different, hydrodynamic mechanism of air entrainment.
This study assesses experimentally the role of gas viscosity in determining dynamic wetting failu... more This study assesses experimentally the role of gas viscosity in determining dynamic wetting failure which hitherto has been speculated on but not measured. In this paper we used dip coating as the model experimental flow and simulated gas viscosity effects by encasing this coater in a vacuum chamber in which we admitted various gases. An appropriate choice of gases (air, carbon dioxide and helium) coupled with low pressure conditions from atmospheric down to 50 mbar enables us to test whether gas viscosity is indeed a key determinant in dynamic wetting. The data presented here are an intricate tracking of the break-up of the dynamic wetting line into the classical ''vees'' shape. Using a novel extension of otherwise well established visualisation techniques, we measured precisely the size and the angles of these vees, the speed of the onset of bubble entrainment, and calculated the maximum speed of wetting. Our data show clearly that by reducing gas viscosity to a moderate value, we can postpone and easily double the bubble entrainment speed and the classical maximum speed of wetting normally measured with air under atmospheric pressure. Very importantly, we show that the concept of the maximum speed of wetting holds only in as far as it is maximal with air at atmospheric pressure. With other gases and other viscosities, a different maximum wetting speed is measured. As the gas viscosity is reduced to approach zero, the trend in the data suggests that this maximum wetting speed and the gas entrainment speed will increase rapidly. These results have evidently practical significance but it is their fundamental merit which is new.
Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, ... more Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, which had a rough side and a comparatively smoother side. When we compared the air entrainment speeds on both sides, we found a switch in behaviour at a critical viscosity. At viscosity lower than a critical value, the rough side entrained air at lower speeds than the smooth side. Above the critical viscosity the reverse was observed, the smooth side entraining air at lower speed than the rough side. Only substrates with significant roughness showed this behaviour. Below a critical roughness, the rough side always entrained air at lower speeds than the smooth side. These results have both fundamental and practical merits. They support the hydrodynamic theory of dynamic wetting failure and imply that one can coat viscous fluids at higher speeds than normal by roughening substrates. A mechanism and a model are presented to explain dynamic wetting failure on rough surfaces.
Abstract Experimental measurements are reported for the flow of Newtonian liquids through the nip... more Abstract Experimental measurements are reported for the flow of Newtonian liquids through the nip of two co- and counter-rotating rolls of various size and speed ratios. Results are presented in a dimensionless form for the total volumetric flux through the nip and for the distribution of this flux between the two rolls. Simple correlations have been developed and these should be useful in the design and analysis of roll coating equipment. Some preliminary data are also presented for a shear thinning inelastic non-Newtonian liquid. The forms of the correlating equations developed for the Newtonian systems are still applicable but the constants in the equations are slightly modified.
in roll coating are reviewed, with particular emphasis on the transfer between two rolls in the a... more in roll coating are reviewed, with particular emphasis on the transfer between two rolls in the absence of a substrate. For this situation, good agreement has been achieved between extensive experimental data on liquid fluxes (and hence film thicknesses) and theoretical predictions based on the lubrication approximation together with appropriate separation conditions. This agreement indicates that the theoretical approach would also be valid for the calculation of the separating force between rolls and the torques required to drive the rolls. However, much of the work has been restricted to Newtonian behaviour and there is scope for further analyses of the influence of shear thinning characteristics and viscoelasticity. * Dedicated to the memory of Professor J.G. Oldroyd.
Recycling of polymers is a necessity in our intensively consuming polymer world but the nature of... more Recycling of polymers is a necessity in our intensively consuming polymer world but the nature of polymers is such that they are prone to thermal degradation when re-extruded and this poses technical challenges to recycling. This papers describes research that seeks to rebuild the structure of degraded PA6. We present data from controlled experiments with pristine pPA6 extruded to form a base recycle rPA6 to which we added two chain extenders, separately: one with anhydride multifunctionality (ANHY), highly reactive with amide groups and one with epoxy multifunctionality (EPOX), less reactive. We found from rheological data carried out in the linear viscoelastic region (so as to study structural changes) a striking difference in the ability of the chain extenders to rebuild structure: 306% increase in the complex viscosity of rPA6/ANHY compared to 25% in that of rPA6/EPOX of the base rPA6. Mechanical and thermal (DSC and TGA) tests confirmed the superior efficacy of the multifunctional anhydride chain extender. Beside the practical benefit that ensues from this research, it also provides a strategic platform to develop chain extenders for other degrading polymers on the basis of understanding the degradation chemical reaction and targeting the most reactive end group of the split chains.
The coating flow examined here, labelled angled dip coating, is that where a substrate enters a p... more The coating flow examined here, labelled angled dip coating, is that where a substrate enters a pool of liquid forming an angle with the vertical so that it intersects the liquid along a wetting line which is not perpendicular to the direction of its motion. This flow situation is distinctly different from that where the substrate, inclined in the other dimension by the so-called angle of entry , intersects the liquid surface perpendicularly to its motion. Experiments were carried out with various liquids to determine the effect of on the substrate velocity at which air is entrained into the liquid. It was observed that as this angle departs from zero, air entrainment is delayed to higher speeds. The data show that the speed which is relevant to air entrainment is not the velocity of the substrate itself but its component normal to the wetting line. This result has important practical implications and suggests that this fundamental principle is also applicable to other coating flows.
This work deals with all aspects of mixing solids, liquids and gases in single and multi-phase fl... more This work deals with all aspects of mixing solids, liquids and gases in single and multi-phase flow and therefore reflects the modern use of non-intrusive methods and simulations by computational fluid dynamics.
YesTensioned web-roll coating is widely used but has surprisingly received little research attent... more YesTensioned web-roll coating is widely used but has surprisingly received little research attention. Here, a new semi-empirical model that predicts film transfer from applicator roller to web is developed and tested against data collected from a pilot coating line. The film transfer is found to vary linearly with web to applicator speed ratio S. Flow stability investigations revealed three types of defects: rivulets, air entrainment due to dynamic wetting failure and cascade, occurring at different values of S and capillary number Ca. Rivulets occurred at Ca 0.71-0.81, air entrainment at Ca>0.4 and S>0.71-0.83 and cascades at S>1.1 for Ca up to 6. Web speeds at which dynamic wetting failure occurred were, for the same Ca, comparatively higher than those that occur in dip coating. The data show that such hydrodynamic assistance is due to the coating bead being confined, more so with increasing web wrap angle β.The authors acknowledge the support of the Films R&D Centre of Toyobo Co. Ltd., Otsu, Japan and of the Thin Liquid Films Research Group of the University of Bradford, UK.The full-text of this article will be released for public view at the end of the publisher embargo on 26th March 2022
Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, ... more Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, which had a rough side and a comparatively smoother side. When we compared the air entrainment speeds on both sides, we found a switch in behaviour at a critical viscosity. At viscosity lower than a critical value, the rough side entrained air at lower speeds than the smooth side. Above the critical viscosity the reverse was observed, the smooth side entraining air at lower speed than the rough side. Only substrates with significant roughness showed this behaviour. Below a critical roughness, the rough side always entrained air at lower speeds than the smooth side. These results have both fundamental and practical merits. They support the hydrodynamic theory of dynamic wetting failure and imply that one can coat viscous fluids at higher speeds than normal by roughening substrates. A mechanism and a model are presented to explain dynamic wetting failure on rough surfaces.
Three-dimensional CFD simulations of turbulent flow in baffled stirred tanks - an assessment of t... more Three-dimensional CFD simulations of turbulent flow in baffled stirred tanks - an assessment of the current position CFD to fluidmixing needs realistic physical data the influence of rotation on the atmospheric mixing of a buoyant plume from a tall stack resistance tomography imaging of stirred vessel mixing dispersed phase distribution in liquid-liquid mixing in an agitated tank. (Part contents).
This experimental study of dynamic wetting in metering and pre-metered forward roll coating shows... more This experimental study of dynamic wetting in metering and pre-metered forward roll coating shows that the dimensionless applicator flow rate A, is the key parameter as it defines whether dynamic failure occurs by air entrainment ( A > A, trans ) or cascade ( A < A, trans ). A, trans is the flow rate when the regime switches from metering to pre-metered and A, min , the minimum flow rate in the pre-metered regime. When Ca was varied in the range 0.25, A, trans and A, min were in the range 1.401.29 and 1.201.18 respectively. These values compare very well with theoretical predictions. The air entrainment speeds V * M were found to be similar to the corresponding speeds V * plunging , measured in plunging tape flow but the cascade speeds V ** M can be smaller or larger depending on A . This led to exploring hydrodynamic assistance and drawing an analogy with curtain coating, which was supported by the data. It was also observed that cascade occurred in the pre-metered regime because the dynamic wetting line X D , moves near the minimum gap position just as in reverse roll coating. Indeed the cascade speeds conform to correlations developed for reverse roll coating. Finally the study gives data on X D , and compares them with theoretical predictions. The agreement in this case is only qualitative and this points to deficiencies in current modeling of the dynamic wetting region.
Reverse roll coating is probably the most widely used coating operation, yet its full potential h... more Reverse roll coating is probably the most widely used coating operation, yet its full potential has not been exploited as it is shown in this paper which considers operation with a negative gap. We demonstrate through a wide range of experimental data that such operation can yield very thin and stable films with no ribbing or cascade instabilities when low viscosity fluids are used. Typically, stable film thickness less than 5µm can be obtained at speeds up to 150 m/min when a rubber roller is used at -100 µm gap with fluids of viscosity in the range 10-200 mPa.s. These film thicknesses can be made to decrease further down to 1 or 2 microns with a judicious choice of speed ratios (applicator to metering roller) and rubber hardness. Such new findings make this simple coating method an attractive roll to roll technique for application in the newer coating technologies, such as in the production of solar cells and plastic electronics. The data obtained in this study have been underpinned by a model based on the classical lubrication theory, well developed for such flow situations. Essentially it is shown that the film thickness non dimensionalised with respect to the set negative gap is controlled through a single parameter, the elasticity number Ne which combines all the operating parameters. Of course, this flow problem has complexities, particularly at high speed ratios and at zero gap so the data obtained here can serve as a basis for more comprehensive modelling of this classical fluid mechanic problem.
This study assesses experimentally the role of gas viscosity in controlling the minimum film thic... more This study assesses experimentally the role of gas viscosity in controlling the minimum film thickness in slot coating in both the slot over roll and tensioned web modes. The minimum film thickness here is defined with respect to the onset of air entrainment rather than rivulets, the reason being that rivulets are an extreme form of instabilities occurring at much higher speeds. The gas viscosity effects are simulated experimentally by encasing the coaters in a sealed gas chamber in which various gases can be admitted. An appropriate choice of two gases was used to compare performances: air at atmospheric pressure and helium at sub-ambient pressure (25mbar), which we establish has a significantly lower "thin film" viscosity than atmospheric air. A capacitance sensor was used to continuously measure the film thickness on the web, which was ramped up in speed at a fixed acceleration whilst visualizations of the film stability were recorded through a viewing port in the chamber. The data collected show clearly that by coating in rarefied helium rather that atmospheric air we can reduce the minimum film thickness or air/gas entrainment low-flow limit. We attribute this widening of the stable coating window to the enhancement of dynamic wetting that results when the thin film gas viscosity is reduced. These results have evident practical significance for slot coating, the coating method of choice in many new technological applications, but it is their fundamental merit which is new and one that should be followed with further data and theoretical underpinning.
Abstract A mathematical model of the reverse and metering roll coating flow of Newtonian fluids i... more Abstract A mathematical model of the reverse and metering roll coating flow of Newtonian fluids is presented. It is based on the lubrication approximation and simple physical notions regarding the separation region. These physical notions discard the effect of surface tension on the dynamics of the system. The results of this model give the dimensionless net flux through the nip, λL, as a function of the speed ratio, rs and the radius ratio rr. This function is a weak one over the range of rr and rr normally encountered in reverse and metering roll coating operations and is in good agreement with experimental data[1] obtained previously. The results of this present study have also been related to the findings from a previous analysis of forward roll coating[2].
Thermoplastic polyurethane (TPU)/multiwalled carbon nanotube (MWNT) composites were prepared thro... more Thermoplastic polyurethane (TPU)/multiwalled carbon nanotube (MWNT) composites were prepared through mixing and extrusion process in a minimixer equipped with a single screw plus twin shaft paddles. In order to improve the dispersion of MWNT and the compatibility between MWNT and polyurethane, the MWNT were functionalised with polytetramethylene ether glycol (PTMEG) through a grafting process to get the MWNT-g-PTMEG. The relationship between the structure and properties of the TPU/MWNT composites was examined. The results showed that MWNT-g-PTMEG can be uniformly dispersed in the TPU matrix, and the tensile strength and elongation at break were both improved. For 1 wt-%MWNT-g-PTMEG/TPU composites, the tensile strength and elongation a break were enhanced to y41 MPa and 568% respectively, while for pristine TPU, they were y34 MPa and 508% respectively. Dynamic mechanical analysis showed that the storage modulus at room temperature increased, and the maximum tan d decreased with increasing MWNT-g-PTMEG contents. The simultaneous small and wide angle X-ray scattering measurements showed that with the incorporation of MWNT-g-PTMEG, the degree of crystallinity, interdomain distance, hard domain thickness as well as the degree of microphase separation increased. It appears that the functionalised MWNT-g-PTMEG promotes the formation of the ordered lamellar structure of polyurethane hard segments.
An experimental investigation of the effect of fluid rheology on air entrainment in plunging tape... more An experimental investigation of the effect of fluid rheology on air entrainment in plunging tape experiments (dip coating) was carried out. The critical velocity at which the dynamic wetting line breaks up, allowing air to entrain into the liquid, was measured for a range of viscoelastic polymer solutions in glycerin/water mixtures. Variations of the critical velocities with various solutions were found to be unexpectedly small in comparison with the large viscosity variations. The results suggest that dynamic wetting failure and subsequent air entrainment are governed by nonhydrodynamic phenomena occurring at the molecular scale at the wetting line. It was also found that fluid elasticity gives rise to flow instabilities that may lead to a different, hydrodynamic mechanism of air entrainment.
This study assesses experimentally the role of gas viscosity in determining dynamic wetting failu... more This study assesses experimentally the role of gas viscosity in determining dynamic wetting failure which hitherto has been speculated on but not measured. In this paper we used dip coating as the model experimental flow and simulated gas viscosity effects by encasing this coater in a vacuum chamber in which we admitted various gases. An appropriate choice of gases (air, carbon dioxide and helium) coupled with low pressure conditions from atmospheric down to 50 mbar enables us to test whether gas viscosity is indeed a key determinant in dynamic wetting. The data presented here are an intricate tracking of the break-up of the dynamic wetting line into the classical ''vees'' shape. Using a novel extension of otherwise well established visualisation techniques, we measured precisely the size and the angles of these vees, the speed of the onset of bubble entrainment, and calculated the maximum speed of wetting. Our data show clearly that by reducing gas viscosity to a moderate value, we can postpone and easily double the bubble entrainment speed and the classical maximum speed of wetting normally measured with air under atmospheric pressure. Very importantly, we show that the concept of the maximum speed of wetting holds only in as far as it is maximal with air at atmospheric pressure. With other gases and other viscosities, a different maximum wetting speed is measured. As the gas viscosity is reduced to approach zero, the trend in the data suggests that this maximum wetting speed and the gas entrainment speed will increase rapidly. These results have evidently practical significance but it is their fundamental merit which is new.
Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, ... more Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, which had a rough side and a comparatively smoother side. When we compared the air entrainment speeds on both sides, we found a switch in behaviour at a critical viscosity. At viscosity lower than a critical value, the rough side entrained air at lower speeds than the smooth side. Above the critical viscosity the reverse was observed, the smooth side entraining air at lower speed than the rough side. Only substrates with significant roughness showed this behaviour. Below a critical roughness, the rough side always entrained air at lower speeds than the smooth side. These results have both fundamental and practical merits. They support the hydrodynamic theory of dynamic wetting failure and imply that one can coat viscous fluids at higher speeds than normal by roughening substrates. A mechanism and a model are presented to explain dynamic wetting failure on rough surfaces.
Abstract Experimental measurements are reported for the flow of Newtonian liquids through the nip... more Abstract Experimental measurements are reported for the flow of Newtonian liquids through the nip of two co- and counter-rotating rolls of various size and speed ratios. Results are presented in a dimensionless form for the total volumetric flux through the nip and for the distribution of this flux between the two rolls. Simple correlations have been developed and these should be useful in the design and analysis of roll coating equipment. Some preliminary data are also presented for a shear thinning inelastic non-Newtonian liquid. The forms of the correlating equations developed for the Newtonian systems are still applicable but the constants in the equations are slightly modified.
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