Dissipative Particle Dynamics Study of Ultraviolet Ink Agglomeration in 3D Inkjet Printing

Due to its outstanding properties, ultraviolet (UV) ink is currently a major ink type used in 3D inkjet printing applications and additive manufacturing (AM) in general. However, there exists a major challenge which has to be addressed and overcome, namely the agglomeration issue, which can potentially lead to nozzle clogging. To understand the underlying physics and chemistry of the agglomeration phenomenon, numerical characterisation provides a low-cost high resolution solution, if the correct numerical methodology is appropriately exploited. F or this meso-scale agglomeration problem, dissipative particle dynamics (DPD) is a highly suitable simulation technique, and in this preliminary study, the commercial solver Material Studio 8.0 from Accelrys Inc is utilized. Here, the coarse-grained models are generated by directly coarse-grained from the atomistic model. Commercial UV inks used in AM applications today are usually composed of oligomers, monomers, photo-initiators, pigments, and other additives such as stabilizers and surfactants. Among these components. oligomers have the highest tendency to agglomerate, which can agitate the stability and quality of the printing fluid, and possibly lead to nozzle clogging. Specifically we study and examine the morphological characteristics of an UV ink composing of photopolymers of polystyrene (PS) and polyethylene glycol (PEG) as the main components in the simulation model. In this case, styrene is chosen as it is one of the most prevalent commercial photopolymers in present 3D inkjet applications, while ethylene glycol is a photopolymers known to improve ink viscosity. The preliminary results for different models considered show that the kind of photopolymers and their constituent ratios affect the agglomeration morphology of the system, and the existence of both oligomers and monomers results in mutual morphological benefits against agglomeration. The results also reveal the importance of other additives in the ink composition to prevent, reduce and control various forms of agglomeration to achieve enhanced print quality.ASTAR (Agency for Sci., Tech. and Research, S’pore)Published versio

Carbon nanotube arrays as multilayer transverse flow carbon nanotube membrane for efficient desalination

Although single layer transverse flow carbon nanotube (CNT) membrane (TFCM) has been shown to be ultrapermeable with high salt rejection, its physical fabrication with sub-nanometre slits remains a significant challenge to its development. This work presents the multilayer TFCM, which resembles vertically aligned CNT arrays, as an alternative candidate for efficient desalination. Using molecular dynamics, this work shows that multilayer TFCM can provide permeability and salt rejection on par with its single layer counterpart. By multilayering, the slit size between neighbouring CNTs can be increased to nanometre range while still maintaining high salt rejection. The increase in slit size counteracts the reduction in permeability due to multilayering, thereby allowing multilayer TFCM to achieve permeability performance comparable to the single layer TFCM. The effects of the number of layers n and other design parameters (interlayer distance d, CNT diameter DCNT , offset h) on the desalination performance of multilayer TFCM are investigated thoroughly using results from non-equilibrium and equilibrium molecular dynamics. It was found that the desalination performance is not sensitive to variations in d, DCNT or h. Finally, this work provides computational evidence that the multilayer TFCM, which could be fabricated using techniques for current dense vertically aligned CNT arrays, can make an efficient design for future low dimensional materials membrane

Effects of oscillating pressure on desalination performance of transverse flow CNT membrane

In parallel with recent developments in carbon nanomaterials, there is growing interest in using these nanomaterials for desalination. To date, many studies have affirmed the potential of using such nanomaterials for constant pressure desalination operation. In this work, the performance of such membrane when subjected to oscillatory pressure at sub-nanosecond is investigated in detail. Using the transverse flow CNT membrane operating at periods ranging from 0.02 to 0.1 ns, we find that oscillatory pressure operation can increase the permeability of the membrane by 16% with a salt rejection close to 100%. Detailed studies on the salt concentration profile, water orientation and water permeance behavior revealed that this increase in permeability is due to the development of resistance to reverse flow at higher periods of oscillation. Further extension of the analysis to periods on the order of 0.1 ns and beyond do not show a positive influence on water permeability. Thus, this work shows that periods on the order of 10−2 ns are required for improved performance of low dimensional nanomaterials membrane. The results from this work shows that nanomaterials membrane is suitable for oscillatory operation, such as electrodialysis reversal. Due to the nanoscale sized of the membrane channels, sub-nanoseconds pulsations are more effective in introducing instabilities to the system to positively influence the water permeance behavior of the membrane

A review on low dimensional carbon desalination and gas separation membrane designs

The widespread use of low dimensional carbon membrane for desalination and gas separation is limited by the difficulty to physically realise such membrane designs on a meaningful scale. This review aims to bring together results achieved in this field, hoping to inspire new designs or developments that could bridge this technical challenge. The focus of this paper is on sub-nanometer separation operations such as desalination or gas separation. This is because such operations consume the most energy, and there is thus much interest to reduce this cost. Three groups of low dimensional carbon materials are considered: graphene, carbon nanotubes (CNT) and graphene oxide (GO). Graphene and CNT membranes have the advantage of high permeability but are difficult to manipulate to form membranes that separate efficiently. GO, on the other hand, has the advantage of ease of fabrication but suffers in terms of separation performance. This review dives deep into the innovative ideas proposed for these low dimensional carbon membrane design, deliberating their strengths and weaknesses, in a consolidated effort to generate new ideas for further advancements