| 1. |
- Göhl, Johan, 1989, et al.
(författare)
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Simulations of 3D bioprinting : Predicting bioprintability of nanofibrillar inks
- 2018
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Ingår i: Biofabrication. - : IOP Publishing. - 1758-5082 .- 1758-5090. ; 10:3
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Tidskriftsartikel (refereegranskat)abstract
- 3D bioprinting with cell containing bioinks show great promise in the biofabrication of patient specific tissue constructs. To fulfil the multiple requirements of a bioink, a wide range of materials and bioink composition are being developed and evaluated with regard to cell viability, mechanical performance and printability. It is essential that the printability and printing fidelity is not neglected since failure in printing the targeted architecture may be catastrophic for the survival of the cells and consequently the function of the printed tissue. However, experimental evaluation of bioinks printability is time-consuming and must be kept at a minimum, especially when 3D bioprinting with cells that are valuable and costly. This paper demonstrates how experimental evaluation could be complemented with computer based simulations to evaluate newly developed bioinks. Here, a computational fluid dynamics simulation tool was used to study the influence of different printing parameters and evaluate the predictability of the printing process. Based on data from oscillation frequency measurements of the evaluated bioinks, a full stress rheology model was used, where the viscoelastic behaviour of the material was captured. Simulation of the 3D bioprinting process is a powerful tool and will help in reducing the time and cost in the development and evaluation of bioinks. Moreover, it gives the opportunity to isolate parameters such as printing speed, nozzle height, flow rate and printing path to study their influence on the printing fidelity and the viscoelastic stresses within the bioink. The ability to study these features more extensively by simulating the printing process will result in a better understanding of what influences the viability of cells in 3D bioprinted tissue constructs.
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| 2. |
- Göhl, Johan, 1989, et al.
(författare)
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An Immersed Boundary Based Dynamic Contact Angle Framework for Handling Complex Surfaces of Mixed Wettabilities
- 2018
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322. ; 109, s. 164-177
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Tidskriftsartikel (refereegranskat)abstract
- We propose a comprehensive immersed boundary-based dynamic contact angle framework capable of handling arbitrary surfaces of mixed wettabilities in three dimensions. We study a number of dynamic contact angle models and implement them as a boundary condition for the Continuum Surface Force method. Special care is taken to capture the contact angle hysteresis by using separate models for the advancing and receding contact lines. The framework is able to account for surfaces of varying wettability by making the contact angle dependent on the local boundary condition. We validate our framework using cases where glycerol droplets impact solid surfaces at low Weber numbers. We show how a truly dynamic contact angle model is needed for advancing contact lines and how a separate dynamic model is needed for receding contact lines. To test our framework for industrially relevant problems on a more complex surface, we simulate droplet impact on a printed circuit board. We show how the local surface properties control the final droplet deposition and that the framework is capable of handling adjacent surfaces of considerably different wettabilities.
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| 3. |
- Göhl, Johan, 1989, et al.
(författare)
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Manipulation of phase transition temperatures and supercooling of sugar alcohols based Phase Change Materials (PCMs) by urea
- 2016
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Ingår i: Proceedings of the INNOSTORAGE conference, Beer Sheva, Israel.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this work we have investigated the possibility to change the thermal characteristics of the sugar alcohols erythritol and mannitol by use of urea as additive. The results show that even small amounts of urea have a great influence on the thermal properties of the sugar alcohols, which in turn implies large structural differences between the different compositions. For both sugar alcohols both smaller and higher fractions of urea result in two melting peaks, whereas a eutectic composition is obtained at intermediate urea fractions. However, not all compositions undergo crystallization on cooling.
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| 4. |
- Konstantinidis, Konstantinos, 1991, et al.
(författare)
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Coalescence-induced jumping of droplets from superhydrophobic surfaces - the effect of contact-angle hysteresis
- 2022
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 34:11
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Tidskriftsartikel (refereegranskat)abstract
- Droplets coalesce and jump from superhydrophobic surfaces, a result that stems from the dominance of capillary and inertial forces and the presence of high contact angles. This phenomenon has been a subject of intensive numerical research mostly for cases when the degree of hydrophobicity is described by a single contact-angle value (a static contact angle). The introduction of various degrees of contact-angle hysteresis complicates the numerical modeling of the jumping process due to the sensitivity to the effective value of the contact angle. We have developed and validated a comprehensive volume-of-fluid(VOF)-immersed boundary numerical framework that accounts for the effect of hysteresis by focusing on the representation of actual values of dynamic contact angles. By comparing the behavior of jumping droplets on superhydrophobic surfaces with several degrees of hysteresis (up to 15o), we quantified the influence of hysteresis on the jumping process and identified various stages of the merged droplet's detachment and re-attachment. The latter phenomena were observed in all our simulations with droplets of different initial radii. In all the cases with hysteresis, the merged droplet eventually jumps, but we point out the decrease in the jumping velocity as compared to cases with only a static contact angle imposed. Finally, by using the Kistler dynamic contact-angle model, we demonstrate the importance of accurately capturing the dynamic receding contact angle when droplets jump from superhydrophobic surfaces with various degrees of hysteresis.
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| 5. |
- Konstantinidis, Konstantinos, 1991, et al.
(författare)
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Coalescence-induced jumping of microdroplets on superhydrophobic surfaces – A numerical study
- 2022
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Ingår i: Canadian Journal of Chemical Engineering. - : Wiley. - 1939-019X .- 0008-4034. ; 100:12, Special Issue Article, s. 3517-3530
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Tidskriftsartikel (refereegranskat)abstract
- We develop a numerical framework for simulating the coalescence and jumping of microdroplets on superhydrophobic surfaces. The framework combines the VOF method with models for advancing and receding contact angles on a number of superhydrophobic surfaces. We demonstrate the temporal and spatial convergence of the framework and show agreement between our numerical results and other experimental studies. The capillary-inertial scaling is investigated together with the existence of a cut-off behaviour frequently observed in the lower size-range of that regime. We investigate findings in some of the previous studies that the cut-off behaviour can be attributed to viscosity effects and dissipation due to interaction with surface microstructures. We exemplify specific features related to the jumping process and the corresponding energy budget analysis when microdroplets coalesce and jump. We have tested droplets of a radius as small as 0.5 μm that are still jumping but recorded a decrease in the jumping velocity and the degree of energy conversion compared to the jumping of larger droplets. We argue and prove that strong capillary forces originating from the high curvature oscillations dissipate the energy of the system significantly faster in the case of microdroplets.
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| 6. |
- Mårtensson, Gustaf, 1972, et al.
(författare)
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Simulation of jet printing of solder paste for surface mounted technology
- 2021
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Ingår i: Soldering and Surface Mount Technology. - : Emerald. - 1758-6836 .- 0954-0911. ; In Press
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Tidskriftsartikel (refereegranskat)abstract
- Purpose The purpose of this study is to propose a novel simulation framework and show that it captures the main effects of the deposition process, such as droplet shape, volume and speed. Design/methodology/approach In the framework, the time-dependent flow and the fluid-structure interaction between the suspension, the moving piston and the deflection of the jetting head is simulated. The system is modelled as a two-phase system with the surrounding air being one phase and the dense suspension the other. The non-Newtonian suspension is modelled as a mixed single phase with properties determined from material testing. The simulations were performed with two coupled in-house solvers developed at Fraunhofer-Chalmers Centre; IBOFlow, a multiphase flow solver; and LaStFEM, a large strain FEM solver. Experimental deposition was performed with a commercial jet printer and quantitative measurements were made with optical profilometry. Findings Jetting behaviour was shown to be affected by not only piston motion, fluid rheology and head deformation but also the viscous energy loss in the jetting head nozzle. The simulation results were compared to experimental data obtained from an industrial jetting head and found to match characteristic lengths, speed and volume within ca 10%. Research limitations/implications The simulations are based on a rheological description using the Carreau model that does not include a time-dependent relaxation of the fluid. This modelling approach limits the descriptive nature of the deposit after impact on the substrate. The simulation also adopts a continuum approach to the suspension, which will not accurately model the break-off of the droplet filament under the characteristic diameter of the particles in the suspension. Practical implications The ability to accurately simulate the deposition of functional materials will enable the efficient development of novel product designs with a minimum of used resources and minimised product development duration. Social implications The ability to accurately simulate the deposition of functional materials will enable the efficient development of novel product designs with a minimum of used resources and therefore an improvement from a sustainability perspective. The ability to plan deposition strategies virtually will also enable a decrease in consumables at manufacturers which will in turn decrease their carbon foot print. Originality/value While basic fluid dynamic simulations have been performed to simulate flow through nozzles, the ability to include both fluid-structure interaction and multiphase capability together with a more accurate rheological description of the suspension and with a substrate for surface mount applications has not been published to the knowledge of the authors.
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| 7. |
- Paberit, Robert, 1987, et al.
(författare)
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Cycling stability of Poly(Ethylene Glycol) of six molecular weights: influence of thermal conditions for energy applications
- 2020
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Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 3:11, s. 10578-10589
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Tidskriftsartikel (refereegranskat)abstract
- Utilizing energy storage technologies is beneficial for bridging the gap between supply and demand of energy, and for increasing the share of renewable energy in the energy system. Phase change materials (PCM) offer higher energy density and compact storage design compared to conventional sensible heat storage materials. Over the past years, polyethylene glycol (PEG) gained attention in the PCM field, and several new composites of PEGs are developed for thermal energy storage purposes. PCMs are investigated at a given heating/cooling rate to evaluate their phase change temperature and enthalpy. In the case of PEG, some molecular weights show a melting behavior that depends on the thermal history, such as the crystallization conditions. This study investigates the relationship between the molecular weight of PEGs (400 to 6000 g/mol), cooling/heating rates, and the behavior during phase transitions. To evaluate the performance of PEGs as a PCM under various thermal conditions. Experiments were performed using differential scanning calorimeter (DSC) and the transient plane source method (TPS). All PEG molecular weights were subjected to the same cooling and heating conditions, cooling and heating rate and number of cycles, to decouple the thermal effects from molecular weight effects. The behavior of phase transition for different thermal conditions was thoroughly analyzed and discussed. It was found that the melting temperature range of PEGs with different molecular weight was between 5.8 °C and 62 °C (at 5 °C/min). Each PEG showed unique responses to the cooling and heating rates. Generally, the behavior of the crystallization is changing most between the thermal cycles, while the melting peak is stable regardless of the molecular weight. Finally, it is recommended that the characterization of PEGs and their composites should be conducted at a heating and cooling rate close to the thermal conditions of the intended thermal energy storage application.
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