Passive micromixer platform for size- and shape-controllable

Subscriber access provided by Nottingham Trent University

Process Systems Engineering

Passive micromixer platform for size- and shape-controllable preparation of ultrafine HNS Shuangfei Zhao, Cong Chen, Peng Zhu, Huanming Xia, Jinyu Shi, Fanyuhui Yan, and Ruiqi Shen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02396 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Passive micromixer platform for size- and shape-controllable preparation of ultrafine HNS Shuangfei Zhao[a], Cong Chen[a], Peng Zhu*[a], Huanming Xia[b], Jinyu Shi[a], Fanyuhui, Yan[a], Ruiqi Shen[a] [a]School

of Chemical Engineering, Nanjing University of Science and Technology,

Nanjing 210094, China [b]School

of Mechanical Engineering, Nanjing University of Science and Technology,

Nanjing 210094, China *Corresponding author: Peng Zhu, E-mail address: [email protected]

Abstract: A passive micromixer platform, with the assistance of controllable mixing performance, was used for the preparation and morphology control of ultrafine explosives. This method is characterized by high mixing efficiency, safe and easy processing, low solvent consumption, and fast preparation and screening. The preparation of HNS (hexanitrostilbene) in the micromixer platform has obvious advantages over the beaker in terms of particle size and particle size distribution. The particle size of HNS prepared in the platform ranges from 91nm to 255nm, while it ranges from 106nm to 615nm for the beaker at the same anti-solvent/solvent ratio. The particle size of HNS can be controlled to about 100nm, 150nm and 200nm, and the crystal shape can be controlled to nanoscale particle, 2D nanosheet and short rod crystal. This study demonstrated the feasibility of a safe and efficient way of controlling the particle size and crystal shape of explosives.

Keywords: Microfluidics, Nano-particles, Hexanitrostilbene, Crystal morphology control, Passive micromixer

1 Introduction Explosives have received continuous attention in recent centuries due to its wide applications

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

in military and civil fields. The physical properties such as particle size and crystal shape of explosives have great influences on its detonation performance and sensitivities

1, 2.

There are

several ways to prepare explosives with tunable sizes and shapes, including micro-emulsion method 3, 4,

mechanical ball milling

5, 6,

spray process

7- 9,

supercritical fluid technology

10-12,

and nozzle

assisted solvent/anti-solvent process 13-15. However, these methods are all at the macro scale. At this scale, it is difficult to control the crystallization environment accurately. Microfluidic technology provides a unique environment for the investigation of crystallization processes at nano or microscale. Compared with traditional methods, microfluidic technology offers a controllable crystallization environment which is conducive to forming high-quality crystals, potentially realizing the control of particle size and shape 16, 17. Recently, microfluidic technology has been widely used for preparation of microscale and nanoscale particles

18-20,

due to its precisely controlled reaction parameter, rapid mixing and

heat/mass transfer, minimal reagent consumption and fast screening. Passive micromixer platform has advantages of less energy consumption, easier fabrication and wider scope of application. Passive micromixer platforms as one of the microfluidic devices utilize no energy input other than the pressure head used to drive the fluid flow at a constant rate

21-23,

which makes it safer for

preparation of explosives. Using the passive micromixer platform, efficient and controllable mixing can be easily achieved by tuning flow rate, mixing length and residence time. With controllable mixing efficiency, nucleation and crystal growth is easier to study 24, which provides the possibility of controlling particle size and crystal shape. Furthermore, the development of microfabrication technology makes it easier to fabricate and cheaper compared with the active micromixer platforms. Therefore, passive micromixer platforms have been widely used for tuning size, particle size distribution (PSD) and crystal morphology 25-28. However, there have been no reports about size and shape control of explosives using passive micromixer platform. The target material is HNS (hexanitrostibene), which is widely used because of its good thermal stability and high short duration shock pulses sensitivity. We have used an active platform consisting of an oscillator and a swirling microchip for preparation and screening of nanoscale HNS 29.

The mixing of the platform is mainly undertaken by the oscillator in an active way. In this study,

we will use a passive way for size and shape control of ultrafine HNS. A two-layer crossing channels micromixer (TLCCM) was first designed by our group in 2005 30. The new micromixer (NTLCCM)


Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


optimized by adding re-orientation structures to avoid dead corner for mixing will be used for preparation of ultrafine HNS. The particle size will be controlled by adjusting mixing efficiency and supersaturation. The mixing performance of the NTLCCM will be tuned by total flow rate, mixing length and residence time, and the supersaturation will be tuned by the concentration and the flow rate ratio. According to the effects of these factors on particle size, a size control mechanism of HNS based on the passive micromixer platform will be established. CL-20 (2,4,6,8,10,12-hexanitro2,4,6,8,10,12-hexaazaisowurtzitane,another explosive) with much higher solubility in DMSO (dimethyl sulfoxide) compared with HNS will be used as shape control additive. High solubility prevents the precipitation of CL-20 in the recrystallization system, which will ensure the purity and explosion performance of HNS.

2 Experiment 2.1 Materials Raw HNS and CL-20 was provided from Hubei Dongfang Chemical Industry Co., Ltd. The purity of raw HNS and CL-20 are 99.0% and 99.3%, respectively. Dimethyl sulfoxide (DMSO, AR grade) was used as the solvent for dissolving HNS provided from Aladdin Co., Ltd. Polyvinyl pyrrolidone (PVP, AR grade) was used as surfactant provided from Sinopharm Chemical Reagent Co., Ltd. The anti-solvent and washing solution used ultra-pure water produced byfilter system (EPED-10TJ, China).

2.2 Experimental Methods 2.2.1 Experimental devices The recrystallization of HNS is a fast crystallization process. The mixing efficiency of solvent and anti-solvent has a great influence on particle size and size distribution of HNS. Therefore, the passive micromixer with two-layer crossing channels was developed to enhance the mixing efficiency of solvent and anti-solvent. Figure 1(a) displays the schematic of the passive micromixer platform. The recrystallization platform includes a compressed nitrogen source, a digital pressure gauge (718300G, Fluke Crop., USA), a tank for storage of anti-solvent, a syringe driven by syringe



pump (YSP-101, YMC CO., Ltd., Japan), a passive NTLCCM, a beaker for sample collection and connecting PTFE (polytetrafluoroethylene) tubes with inner diameter of 800μm and outside diameter of 1600μm.The anti-solvent is driven by the compressed nitrogen source, while the solvent is driven by the syringe pump. The HNS solution mixes together with the anti-solvent in the microchannel with zig-zag geometry. The flow mechanism of the NTLCCM platform is shown in Figure 1(b). Through this special structure, the HNS solution and anti-solvent can be mixed rapidly in the microchannel, which is beneficial to the preparation of ultrafine HNS with narrow particle size distribution. Furthermore, the mixing strength and mixing time of the platform can be controlled by the flow rate and the length of the mixing zone, which makes it easier to realize the sizecontrollable preparation of HNS particles. The layer-by-layer structure of the micromixer can be fabricated with mechanical engraving method and thermal bonding techniques. The micromixer made of PMMA is safe for the preparation of explosives. The structures of top layer and bottom layer are shown in Figure 1(c), and the picture of the passive micromixer is shown in Figure 1(d). The length, width and thickness of the micromixer are 82mm, 32mm and 3mm, respectively. The width and depth of the crossing channels are both 500μm. The length (l) of mixing zone can be changed to tune the mixing efficiency.


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Schematic diagram of the passive micromixer platform (a), the flow mechanism of the NTLCCM platform (b), the structures of top layer and bottom layer (c) and the picture of NTLCCM micromixer (d).



2.2.2 Preparation of ultrafine HNS In this study, solvent/anti-solvent recrystallization was used for preparing ultrafine HNS. Raw HNS was dissolved into DMSO with various concentrations and then inhaled in a syringe. The syringe pump drove the solvent in the syringe flowing along the tube with the different flow rates. PVP was added to anti-solvent used as the surfactant with fixed concentration of 0.1g/L. The antisolvent was drove by the compressed nitrogen source. Before recrystallization experiments, the flow rates were calibrated at different pressures to control the total flow rate and flow rate ratio. After mixing of solvent and anti-solvent in the micromixer, white colloidal liquid containing ultrafine HNS flowed along a 20cm PTFE tube from the mixing zone to the beaker. The whole recrystallization process was carried out at room temperature (20°C). Then the white colloidal solution was put into the vacuum freeze-drying machine for 5h pre-freezing and 36h freeze drying at -65°C. Finally ultrafine HNS particles were obtained after freeze-drying process. In order to prove the superiority of the passive micromixer in preparation of ultrafine HNS, the comparison between the passive platform and the beaker was made under the same experimental parameters. To control the particle size, experiments with various mixing length, total flow rate of solvent and anti-solvent, concentration of HNS in DMSO and flow rate ratio of anti-solvent to solvent were conducted. The mixing length ranges from 12mm to 48mm. The total flow rate ranges from 25ml/min to 70ml/min. The concentration ranges from o.5g/l to 20g/l. The flow rate ratio ranges from 5 to 60. To control the crystal shape of HNS, CL-20 as the additive with concentrations of 2.5%, 5% and 7.5% were added to the recrystallization system of HNS.

2.3 Characterization method To study size and shape control mechanism of HNS, the crystal types were measured by X-ray diffraction (XRD, Bruker D8-advanced diffractometer equipped with a Cu-Kα X-ray source operating at 40 kV and 40 Ma, USA). XRD measurements were conducted in intervals of 0.014° between 2θ = 5° and 40°, with a counting time of 1s per step and rotation of the sample holder. The morphology and particle size of the recrystallized HNS were observed by scanning electron microscope (SEM, Zeiss Merlin at 10kV). Particle size distributions were determined by dynamic light scattering (DLS) using a Zetasizer Nano ZS9 (Malvern Instruments GmbH, Germany).


Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 Results and Discussion 3.1 Mixing performance at high Re of the NTLCCM Mixing length and total flow rate as well as residence time are important parameters affecting the mixing performance of the NTLCCM. The mixing performances at high Re of the NTLCCM with different mixing lengths and total flow rates were studied by numerical simulation using ANSYS FLUENT 17.0. In numerical simulation, the ratio of anti-solvent to solvent is 60. The proportion of anti-solvent is about 98.4%, which is much larger than that of solvent. In the recrystallization experiment, the anti-solvent is deionized water and the solvent is DMSO. Water and DMSO have similar diffusion coefficients. Therefore, water is regarded as the simulated solute. The density and kinematic viscosity of the solute are 9.998×102kg/m3 and 1.0 × 10-6m2/s, respectively. All meshes in the simulations were composed of hexahedral cells. The mixing length varies from 12mm to 48mm. The diffusion coefficient (D) of water in the mixture is 1×10-11m2s-1. The total flow rates were set at 25ml/min, 36ml/min, 45ml/min, 60ml/min and 70ml/min, respectively, the corresponding total flow velocities are 1.7m/s, 2.4m/s, 3.0m/s, 4.0m/s and 4.7m/s. The Reynolds number and Pe number can be defined as: Re = ud0/ν

(1)

Pe = ud0/D

(2)

where u is the average velocity at the inlets, and d0 is the hydraulic diameter of the channel, which is 500μm in the micromixer; ν is the kinematic viscosity. Therefore, the Res at these total flow rates are 850, 1200, 1500, 2000 and 2350, respectively, and the Pes are 8.5×107, 1.2×108, 1.5×108, 2.0×108 and 2.35×108. At the inlets, the fluids are laminar. After continuous stretching, folding, splitting and recombination in the passive micromixer, chaotic advection at low Reynolds numbers was induced, and hence which significantly promote fluid mixing. Therefore, k-ε model was used. The turbulence intensity (I), turbulent kinetic energy (k) and turbulent dissipation rate (ε) can be calculated by Eq. (3), Eq. (4) and Eq. (5), respectively. I = 0.16(Re) -1/8

(3)

k = 3(uI)2/2

(4)

ε = C3/4k3/2/l

(5)



Page 8 of 26

Where C is the constant 0.09; l is the turbulence length scale, l = 0.07 d0 = 35μm. The turbulence intensity at flow rate of 1.7m/s, 2.4m/s, 3.0m/s, 4.0m/s and 4.7m/s are 0.0689, 0.0660, 0.0641, 0.0619 and 0.0606, respectively, and the corresponding turbulent dissipation rate are 13.8m2/s3, 34.2m2/s3, 61.4m2/s3, 130.7m2/s3 and 199.6m2/s3. When interval size was set to 0.05, the total number of mesh cells is 4204800. When interval size decreases to 0.03, the number of mesh cells increases to 8518070, and the relative change rate of the final result is about 0.13%. Therefore the numerical solution is independent with mesh. The mass fraction of anti-solvent was set as 1, and it was set as 0 for solvent. The mixing performance can be represented by the mixing index σ31. The value of σ ranges from 0 for non-mixing to 1 for complete mixing. The greater σ value means the better mixing quality. The value of σ can be calculated using:

(

N σ = 1 ― ∑i = 1

Ci - Cmix

2

) /N

Cunmix - Cmix

(6)

where n is the number of cells of the outlet area, Ci is the concentration of one cell in the outlet area, Cmix is the concentration after completely mixing and Cunmix is the concentration before mixing (the value of Cunmix is 0). The simulation results are shown in Figure 2. Figure 2(a) shows the effects of mixing length on the mixing time and mixing index at total flow rate of 45 ml/min and flow rate ratio of 60. It can be clearly seen that the mixing performance enhances with the increase of mixing length. When mixing length is increased to 48mm, σ is above 0.9, at which it can be approximated as complete mixing. Continue to increase mixing length, mixing efficiency changes indistinctly. The changes of mixing efficiency may lead to the changes of particle size of ultrafine HNS. The mixing efficiencies with the total flow rate ranging from 25ml/min to 70ml/min were also simulated with the mixing length of 48mm. Figure 2(b) shows the effects of total flow rate on the mixing time and mixing index at mixing length of 48mm and flow rate ratio of 60. With the increase of the total flow rate, the mixing efficiency increases gradually, but when the total flow rate reaches to 45ml/min (Re = 1500), the mixing efficiency decreases slightly. Mixing in the micromixer consists of convection and free diffusion. Convection depends on the structure of Micromixer and the Re of flows, whereas free diffusion depends on the mixing time (t). The mixing time becomes shorter with the increase of total flow rate. Short mixing time weakens the free diffusion of the fluids. Therefore, 45ml/min is the optimal total flow rate to obtain the best mixing, which is likely to be beneficial to the


Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


preparation of ultrafine HNS.

Figure 2. Mixing performance of the NTLCCM. (a) Effects of mixing length on the mixing time and mixing index at total flow rate of 45 ml/min and flow rate ratio of 60. (b) Effects of total flow rate on the mixing time and mixing index at mixing length of 48mm and flow rate ratio of 60.

3.2 Comparison of the passive platform and the beaker Mixing efficiency and uniformity have great influence on crystal shape and particle size. The recrystallization of HNS is a fast crystallization process, which requires higher mixing efficiency to prepare HNS with more uniform crystal shape and particle size. Conventionally, ultrafine HNS is prepared by batch method such as in the beaker. In order to exploit the difference, preparation of ultrafine HNS in the passive micromixer platform was compared with in the beaker. The concentration of HNS solved in DMSO was fixed at 20g/l, and ratio of anti-solvent to solvent was fixed at 20. For the passive micromixer preparation, the mixing length was fixed at 48mm and total flow rate was fixed at 45ml/min. The X-ray diffraction (XRD) patterns of HNS particles prepared in the NTLCCM platform and in the beaker are summarized in Figure 3(a).The peaks on both of the XRD patterns can be mainly



indexed to HNS, which was consistent with the values in the standard card (PDF#42-1919). It implies that HNS prepared in the platform has the similar crystal structure as that in the beaker. However, basically all peak strengths of HNS prepared in the platform are obviously weaker than those in the beaker, suggesting that the particle sizes are different. Furthermore, some peaks of the HNS recrystallized in the platform are very weak or even disappear, which implies that the growth of some crystal planes is very weak, such as the crystal planes (-504), (511), (020), (800) and (902). Therefore, it can be predicted that the particle size of HNS prepared in the platform is smaller. In order to further compare the particle size and crystal morphology of HNS prepared by this two methods, particle size analyses and SEM tests of HNS were conducted. From the particle size distribution (PSD) analyses (see Figure 3(b)), the particle size of the HNS prepared in the platform ranges from 91nm to 255nm, while it ranges from 106nm to 615nm for beaker. It can be clearly seen that the particle size of the HNS prepared in the platform is smaller and the PSD is narrower than that of beaker. SEM images were used to analyze the shape of the recrystallized HNS (see Figure 3(c and d)). The shapes of HNS particles prepared in the platform and beaker are both sheet. However, the width and length of HNS prepared in beaker are larger and the distribution is wider. The results of SEM are consistent with those of PSD. Due to the faster and more uniform mixing, mass transfer in the passive micromixer platform was faster than in the beaker. Especially for the precipitation recrystallization, faster mass transfer means a more uniform distribution of supersaturation which leads to smaller particle size and narrower PSD.


Page 10 of 26

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. XRD patterns (a), PSDs (b) and SEM images (c, d) of ultrafine HNS prepared by in the beaker and in the NTLCCM platform.

3.3 Effect of mixing length on particle size The mixing efficiencies of the passive micromixer with different mixing length have been studied at section 3.1. The changes of mixing efficiency may lead to the difference of particle size. Therefore, ultrafine HNS particles were prepared with mixing length of 12mm, 24mm, 36mm, 48mm and 60mm, respectively. The total flow rate and flow rate ratio were fixed at 45ml/min and 20. The concentration of HNS in DMSO was fixed at 3g/l. Figure 4(a) shows the PSD results of ultrafine HNS prepared with different mixing length. For 12mm, the particle size ranges from 60.5nm to 460.9nm. For 24mm, it ranges from 50.4nm to 420.9nm. For 36mm, it ranges from 45.6nm to 401.8nm. For 48mm, it ranges from 42.3nm to 369.8nm. For 60mm, it ranges from 40.4nm to 360.7nm. The influence of mixing length on particle size of HNS was shown in Figure 4(b). D10, D50 and D90 represent the particle sizes with cumulative distribution of 10%, 50% and 90%, respectively. It can be clearly seen that the particle size decreases and the PSD becomes narrower obviously, when the mixing length increases from 12mm to 48mm.



But when the mixing length increases from 48mm to 60mm, the particle size and PSD change slightly. Considering the mixing performance and the residence time, the effect of mixing length on particle size can be well explained. The reason for particle size reduces and PSD narrows with mixing length is the enhancement of the mixing efficiency. With the fixed total flow rate, the mixing residence times of 12mm, 24mm, 36mm, 48mm and 60mm are 4ms, 8ms, 12ms, 16ms and 20ms, respectively. Increasing mixing residence time leads to the enhancement of mixing efficiency, which is contributed to reduce the particle size and narrow PSD of ultrafine HNS. While when the mixing residence time reaches 16ms, the mixing efficiency is so high that the change of particle size is not obvious with the continual increase of mixing residence time. For efficient preparation of ultrafine HNS, 48mm is the optimal mixing length of the micromixer.

Figure 4. (a) PSDs of HNS prepared in the micromixer with mixing length of 12mm, 24mm, 36mm, 48mm and 60mm, and (b) influence of mixing length on HNS particle size.


Page 12 of 26

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.4 Effect of the total flow rate on particle size The mixing performance of the NTLCCM micromixer at different total flow rate has been characterized in section 3.1. The overall recrystallization effect needs to be determined and the final optimal flow rate needs to be screened by experiments. Therefore, experiments at 20g/l and 3g/l with total flow rates of 25ml/min, 45ml/min, 60ml/min and 70ml/min, respectively, were conducted. The flow rate ratio was fixed at 20. Figure 5 shows the PSD results of ultrafine HNS prepared with different total flow rate at 20g/l. For 25ml/min, the particle size ranges from 90.5nm to 420.8nm. For 45ml/min, it ranges from 65.5nm to 290.6nm.For 60ml/min, it ranges from 82.6nm to 310.7nm.For 70ml/min, it ranges from 80.5nm to 401.7nm.When the total flow rate increases from 25ml/min to 45ml/min, the size decreases and the PSD becomes narrower. But when the total flow rate changes from 45ml/min to 70ml/min, the particle size becomes larger and PSD becomes wider. With the increase of flow rate, the Re of flow increases, and the cross-convection between solvent and anti-solvent becomes stronger, which contributes to improve the mixing efficiency. The rapid mixing of solvent and anti-solvent using the micromixer can effectively accelerate the nucleation of HNS, which contributes to reduction of particle size and PSD. However, the residence time of solvents and anti-solvents in the mixing region is inversely proportional to the total flow rate. Therefore, the optimal total flow rate to prepare ultrafine HNS is 45mL/min. This result is consistent with the simulation of mixing performance. The ultrafine HNS prepared at 3g/l has the same rule. But the changes of particle size and PSD are less than 20g/l. The reason may be the effect of supersaturation. When ultrafine HNS is prepared in the NTLCCM platform by solvent/antisolvent recrystallization, there are two main factors affecting supersaturation, i.e. concentration of HNS in DMSO and ratio of anti-solvent to solvent. Therefore, the effect of the concentration and the ratio will be studied.



Figure 5. PSD and influences of total flow rate on particle size with total flow rate of 25ml/min, 45ml/min, 60ml/min and 70ml/min at 20g/l (a, b) and 3g/l (c, d).

3.5 Effect of concentrations on particle size Ultrafine HNS particles were prepared with various amounts of HNS solved in the solvent in the passive platform. The total flow rate was fixed at 45ml/min and the flow rate ratio was fixed at 10. The concentrations of HNS are 0.5g/l, 1g/l, 3g/l, 5g/l, 7g/l, 10g/l, 15g/l and 20g/l, respectively. Figure 6(a) shows the PSDs of ultrafine HNS prepared with different concentrations. To analyze the influence of the concentration on HNS particle size, the relationship between concentration and the PSD results (mass-based values ofD10, D50 and D90) was built showing in Figure 6(b). The particle sizes were easy found to decrease firstly and then increase. Additionally, the particle size distribution narrows firstly and then widens. The reason for this phenomenon can be explained by the mechanism of crystal nucleation and growth.


Page 14 of 26

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. (a) PSDs of HNS prepared in the micromixer with concentration of 0.5g/l, 1g/l, 3g/l, 5g/l, 7g/l, 10g/l, 15g/l and 20g/l, and (b) influence of concentrations on HNS particle size. The recrystallization of HNS is very fast, consisting of nucleation process and crystal growth process. The particle size of HNS is determined by the nucleation rate and growth rate. Both nucleation and growth can be divided into two processes: molecular diffusion and precipitation reaction 32. The molecular diffusion is determined by mixing efficiency, and precipitation reaction is determined by supersaturation. As described by Fick’s first law of diffusion, the diffusion rate of molecules can be written as

dn dt

d𝑐

= ― Dd𝑥

(7)

where n is the amount of substance of solute; t is diffusion time; D is the diffusion coefficient of solute; c is the concentration of solute in solvent; x is the distance between molecular and particle.



Page 16 of 26

For the diffusion of solute at a surface of spherical particle at steady state, the above equation can be written as dn dt

4πr(r + δ)D

=

δ

(C - 𝐶i)

(8)

where δ is the thickness of the absorption layer; r is the radius of the nucleus; C is the concentration of explosive molecules in the liquid phase; Ci is the concentration of HNS at the surface of the solid/liquid. Similarly, equations can be written for the rate of the surface precipitation reaction, dn dt

= 4kπr2(𝐶i - 𝐶 ∗ )

(9)

where k is the mass transfer coefficient, C*is the saturation concentration. If the diffusion is the limiting factor, the nucleation growth can be expressed by Eq. 8. If the surface precipitation reaction is the limiting factor, the nucleation growth can be expressed by Eq. 9. If the nucleation growth process is controlled neither by diffusion nor surface reaction, then it can be expressed by Eq. 10. If the particle size changes with time, Eq. 8, Eq. 9 and Eq. 10 can be extracted as Eq. 11, Eq. 12 and Eq. 13.

dn dt

=

dr dt dr dt

=

4πr(r + δ + 𝑘𝑟𝛿)D δ

(C - 𝐶i)

Dν r

(C - Ci)

= kν(Ci - C * )

dr dt

Dν

= r + D/k(C - Ci)

(10) (11) (12) (13)

Where ν is the molar volume of particles. Combined with this law, the effect of concentration on the particle size of HNS prepared in the micromixer can be explained. When the concentration of HNS below 5g/l, the lower supersaturation makes the precipitation reaction rate slower relative to the diffusion rate. It is reaction-limited nucleation growth process at low concentration of HNS. Therefore, the particle size decreases with the increase of concentration in this concentration region. When the concentration of HNS increases from 5g/l to 10g/l, the precipitation reaction rate is still much smaller than the diffusion rate. However, the higher mixing efficiency promotes both nucleation and crystal growth, which leads to


Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the increase of particle size. When the concentration is above 10g/l, the concentration is close to the solubility. The high supersaturation greatly increases the rate of precipitation reaction. The diffusion rate and the reaction rate are comparable. In this concentration region, the nucleation growth process is controlled neither by diffusion nor surface reaction, leading to the particle size raises slowly. In order to further study the effect of supersaturation on particle size, HNS particles were prepared with different flow rate ratio.

3.6 Effect of the flow rate ratio of particle size Solvent/anti-solvent recrystallization is based on the principle of solubility difference between two reagents. The ratio of anti-solvent to solvent determines the solubility of solute in mixed reagent, and then affects the growth of crystal. In the recrystallization process in the passive micromixer, the ratio of non-solvent to solvent can be controlled by the flow rate ratio (R). Ultrafine HNS particles were prepared with different flow rate ratios in the passive platform. The total flow rate was fixed at 60ml/min and the concentration was fixed at 20g/l. The flow rate ratios are at 5, 10, 20, 30 and 60, respectively. Figure 7(a) shows the PSDs of HNS prepared with different R. For R = 5, the particle size of HNS ranges from 78nm to 426nm. For R = 10, it ranges from 80nm to 330nm. For R = 20, it ranges from 75nm to 256nm. For R = 30, it ranges from 72nm to 250nm.For R = 60, it ranges from 68nm to 238nm. The relationships between R and the PSD results (D10, D50 and D90) are shown in Figure 7(b). It can be clearly seen that the PSD becomes narrower obviously with increase of R appropriately. When R continues to increase, the PSDs change slowly. Furthermore, the D50s of HNS prepared with R of 5, 10, 20, 30 and 60 are 202.2nm, 172.7nm, 158.2nm, 160.3nm and 165.0nm, respectively. With the variations of R, the particle size has the same law with PSD. In order to find out the reasons for this phenomenon, HNS prepared in the beaker with R of 5, 10, 20 and 40 were conducted for comparison with in the platform. The relationship between R and particle size shows in Figure 7(c). It indicates that PSD narrows and D50 reduces with the increase of R. The reasons for this phenomenon can be explained from the perspective of mixing efficiency. For devices (like the beaker) with poor mixing performance, high flow rate ratio is beneficial to increase the distance between molecules and alleviate the diffusion and aggregation of HNS



molecules. For the platform, the mixing efficiency is so high that the nucleation rate is very fast, which contributes to small size and narrow PSD. Appropriate raise of R is beneficial to reduce the particle size and narrow the PSD. Therefore, the beaker with bad mixing is more suitable for recrystallization with high R, while the passive platform is more suitable for low R. Considering that a large number of screening experiments are needed for size and morphology control recrystallization, low R means less anti-solvent consumption, less waste and water pollution. For example, to prepare ultrafine HNS with D50 of about 200 nm, about 95.8% anti-solvent and 66.7% solvent can be saved by using the platform to prepare 200 nm particles with the same amount as the beaker. Additionally, the rate of production of ultrafine HNS prepared in the micromixer with flow rate of 5, 10, 20, 30 and 60 are 76%, 60%, 43%, 33% and 15% respectively. While the rate of production of ultrafine HNS prepared in the beaker with flow rate of 5, 10, 20 and 40 are 72%, 59%, 46% and 27%, respectively. Although the rate of production in the micromixer has no obvious advantage at the same flow rate ratio, it decreases with the increase of flow rate ratio. However, when preparing superfine HNS with the same particle size, the passive microfluidic platform has obvious rate of production advantages. For example, to prepare ultrafine HNS with D50 of about 200 nm, the flow rate ratio of the micromixer and the beaker are 5 and 40, respectively, and the corresponding rates of production are 76% and 27%. It is because the volatile water and solvent will take away part of the HNS molecule when ultra-fine HNS is dried by vacuum freeze-drying technology. Therefore, ultrafine HNS prepared using the passive platform is an efficient way. The above results reveal that the particle size and PSD can be controlled by mixing length, flow rate ratio, total flow rate and concentration by the passive micromixer platform. The average particle size can be controlled for about 200nm, 150nm and 100nm. Particles larger than 200nm can be easily obtained by adjusting mixing length, total flow rate, concentration and flow rate ratio. However, reducing the particle size to less than 100nm requires longer mixing length and higher flow rate ratio at the appropriate total flow rate and concentration.


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. PSDs (a) of HNS prepared in the NTLCCM platform with flow rate ratio of 5, 10, 20, 30 and 60; influence of flow rate ratio on particle size of HNS prepared in the platform (b) and in the beaker (c).

3.7 Shape control by CL-20 additive The preparation of ultrafine HNS with low flow rate ratio makes it possible to control the



crystal shape by explosives with different solubilities. As a high-energy explosive, CL-20 will not produce impurities without explosive power when used as a crystal control agent. Furthermore, the solubility of CL-20 in DMSO is quite different from that in HNS especially at low ratio of antisolvent to solvent. High solubility prevents the precipitation of CL-20 in the recrystallization of ultrafine HNS prepared by at low flow rate ratio, which will ensure the purity of HNS. The effects of CL-20 on HNS recrystallization were studied by increasing the amount of CL-20 gradually. The flow rate ratio was fixed at 5 and total flow rate was fixed at 25ml/min. The samples were analyzed by XRD and SEM. XRD patterns in Figure 8 illustrate the bulk structural information on recrystallized HNS obtained with CL-20 of 0, 2.5%, 5% and 7.5%. All the diffraction peaks can be accurately indexed to HNS, which was consistent with the values in the standard card (PDF#42-1919). Additionally, no characteristic diffraction peaks from other phases or impurities were found. This means that CL20 does not co-crystallize with HNS and CL-20 did not crystallize and precipitate. The intensity of characteristic diffraction peaks were not the same, such as the intensities at 2θ = 8.373° which indicates that the morphology and particle size of the HNS crystal can appear distinct from each other.

Figure 8. XRD patterns of recrystallized HNS obtained by MAR method with CL-20 of 0, 2.5%, 5% and 7.5%. Figure 9 shows the SEM images of recrystallized HNS with CL-20 of 0, 2.5%, 5% and 7.5%. The length, width and thickness of recrystallized HNS without CL-20 are about 150nm, 80nm and 30nm, respectively. This indicates that the HNS prepared without CL-20 is a kind of nanoscale


Page 20 of 26

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


particle. When CL-20 was added to the recrystallization system with content of 2.5%, the HNS particles have two sizes of crystals. One is a nanoscale particle, the other is a 2Dnanosheet. The nanoscale particle is the same as HNS prepared without CL-20. The 2D nanosheet is with the length, width and thickness of about 2000nm, 300nm and 50nm. For 5%, there are some short rod-like crystals in Figure 9(c) in addition to the above two HNS particles obtained with 2.5% CL-20. The short rod-like crystal has the length, width and thickness of about 3000nm, 500nm and 150nm. When the content of CL-20 increased to 7.5%, the crystal morphology of recrystallized HNS is basically short rod-like. This short rod of HNS recrystallized with 7.5% CL-20 is a little bigger than that of 5%. The length, width and thickness of HNS crystal produced with 7.5% CL-20 are about 5000nm, 1000nm and 300nm, respectively.

Figure 9. Scanning electron microscopy of recrystallized HNS obtained with CL-20 of 0, 2.5%, 5% and 7.5%. The effects of CL-20 on the recrystallization of HNS can be reflected from the results of SEM.Figure10 shows the mechanism of controlling the morphology of HNS crystal by CL-20. The addition of CL-20 affects the recrystallization system of HNS and changes the crystal morphology of HNS. Because the content of CL-20 in recrystallization system is small, it is difficult for CL-20



to recrystallize and precipitate. CL-20 exists as a molecule in recrystallized system.CL-20 has no effect on the formation of HNS seeds.CL-20 molecules act on the crystal growth process of HNS and induce the growth of HNS seeds to form layered crystals. When a small amount of CL-20 was added, the number of CL-20 molecules was limited. Some HNS seeds were induced by CL-20 molecules to grow and form 2D nanosheets, while others recrystallized forming small nanoscale particles. With the increase of CL-20, the number of CL-20 molecules increased. More HNS seeds were induced to grow into 2D nanosheets and rod-like crystals, and the number of nanoscale particles decreased. When enough CL-20 was added, almost all HNS seeds were induced to form uniform rod-like crystals. Therefore, appropriate amount of CL-20 can control the formation of 2Dnanosheets of HNS, and sufficient CL-20 can control the formation of short rod-like structure of HNS through CL-20 molecule affects the growth process of HNS seeds.

Figure 10. The mechanism of controlling the morphology of HNS crystal by CL-20.

4 Conclusion Mixing efficiency and homogeneity have great influence on particle size, PSD and crystal shape of ultrafine HNS. A passive NTLCCM platform was applied on preparation of ultrafine HNS. The NTLCCM made of PMMA is easy to manufacture and safe for explosives preparation. The platform has high mixing efficiency, and the mixing efficiency can be controlled by adjusting the mixing length and total flow rate. The results of numerical simulation and recrystallization experiments show that the optimal mixing length is 48 mm and the total flow rate is 45 ml/min. At the ratio of anti-solvent to solvent of 20, the particle size of HNS prepared in the platform ranges


Page 22 of 26

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


from 91nm to 255nm, while it ranges from 106nm to 615nm for the beaker. Compared with preparation in the beaker, ultrafine HNS prepared in the platform has smaller particle size, narrower PSD, fewer reagents consumption, fewer pollutants production, higher rate of production, faster and more efficient preparation. By tuning the rate molecular diffusion and precipitation reaction, the particle size of HNS can be controlled by mixing length and flow rate ratio, and optimized by total flow rate and concentration. The average particle size of HNS can be controlled for about 200nm, 150nm and 100nm. High solubility difference makes it possible to control the crystal shape by CL20, which ensures the purity of HNS. With different content of CL-20, the shape of HNS can be controlled to nanoscale particles, 2D nanosheets and short rod crystals. All the findings reported provide useful references for preparation of other ultrafine particles using passive micromixer platform.

Author Information Corresponding Authors *E-mail address: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgement This work was supported by Shanghai Aerospace Science and Technology Innovation Fund (No. SAST2017-124) and the National Natural Science Foundation of China (No. 51575282).

Reference 1.

Zhang, Y. X.; Liu, D. B.; Lv, C. X., Preparation and characterization of reticular nano-HMX.

Propellants, Explos., Pyrotech. 2005, 30, 438-441. 2.

Song, X.; Yi, W.; An, C.; Guo, X.; Li, F., Dependence of particle morphology and size on the

mechanical sensitivity and thermal stability of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. J. Hazard. Mater. 2008, 159, 222-229. 3.

Mandal, A. K.; Thanigaivelan, U.; Pandey, R. K.; Asthana, S.; Khomane, R. B.; Kulkarni, B. D.,



Preparation of Spherical Particles of 1,1-Diamino-2,2-dinitroethene (FOX-7) Using a Micellar Nanoreactor. Org. Process Res. Dev. 2012, 16, 1711–1716. 4.

Bayat, Y.; Zarandi, M.; Zarei, M. A.; Soleyman, R.; Zeynali, V., A novel approach for preparation

of CL-20 nanoparticles by microemulsion method. J. Mol. Liq. 2014, 193, 83-86. 5.

Guo, X.; Ouyang, G.; Liu, J.; Li, Q.; Wang, L.; Gu, Z.; Li, F., Massive Preparation of Reduced-

Sensitivity Nano CL-20 and Its Characterization. J. Energ. Mater. 2015, 33, 24-33. 6.

An, C. W.; Xu, S.; Zhang, Y. R.; Ye, B. Y.; Geng, X. H.; Wang, J. Y., Nano-HNS Particles:

Mechanochemical Preparation and Properties Investigation. J. Nanomater. 2018, 7. 7.

Wang, J.; Li, J.; An, C.; Hou, C.; Xu, W.; Li, X., Study on Ultrasound- and Spray-Assisted

Precipitation of CL-20. Propellants, Explos., Pyrotech. 2012, 37, 670–675. 8.

Risse, B.; Spitzer, D.; Hassler, D.; Schnell, F.; Comet, M.; Pichot, V.; Muhr, H., Continuous

formation of submicron energetic particles by the flash-evaporation technique. Chem. Eng. J. 2012, 203, 158-165. 9. Spitzer, D.; Baras, C.; Schaefer, M. R.; Ciszek, F.; Siegert, B., Continuous Crystallization of Submicrometer Energetic Compounds. Propellants, Explos., Pyrotech. 2011, 36, 65-74. 10. Kim, C. K.; Lee, B. C.; Lee, Y. W.; Kim, H. S., Solvent effect on particle morphology in recrystallization of HMX (cyclotetramethylenetetranitramine) using supercritical carbon dioxide as antisolvent. Korean J. Chem. Eng. 2009, 26, 1125-1129. 11. Kim, S. J.; Lee, B. M.; Lee, B. C.; Kim, H. S.; Kim, H.; Lee, Y. W., Recrystallization of cyclotetramethylenetetranitramine (HMX) using gas anti-solvent (GAS) process. J. Supercrit. Fluids 2011, 59, 108-116. 12. Kim, S.; Lee, S. J.; Seo, B.; Lee, Y. W.; Lee, J. M., Optimal Design of a Gas Antisolvent Recrystallization Process of Cyclotetramethylenetetranitramine (HMX) with Particle Size Distribution Model. Ind. Eng. Chem. Res. 2015, 54, 11087-11096. 13. Wang, J.; Huang, H.; Xu, W. Z.; Zhang, Y. R.; Lu, B.; Xie, R. Z.; Wang, P.; Yun, N., Prefilming twin-fluid nozzle assisted precipitation method for preparing nanocrystalline HNS and its characterization. J. Hazard. Mater. 2009, 162, 842-847. 14. Gupta, S.; Kumar, P. D.; Jindal, D.; Sharma, S.; Agarwal, A.; Lata, P., Micro Nozzle Assisted Spraying Process for Re-crystallization of Submicrometer Hexanitrostilbene Explosive. Propellants, Explos., Pyrotech. 2018, 43, 721-731. 15. Gupta, S.; Kumar, P. D.; Sharma, S.; Kaur, G.; Agarwal, A.; Lata, P., Pressurized Nozzle-Based Solvent/Anti-Solvent Process for Making Ultrafine ϵ-CL-20 Explosive. Propellants, Explos., Pyrotech. 2017, 42. 16. Puigmarti-Luis, J., Microfluidic platforms: a mainstream technology for the preparation of crystals. Chem. Soc. Rev. 2014, 43, 2253-2271. 17. Shi, H. H.; Xiao, Y.; Ferguson, S.; Huang, X.; Wang, N.; Hao, H. X., Progress of crystallization in microfluidic devices. Lab Chip 2017, 17, 2167-2185. 18. Thiele, M.; Soh, J. Z. E.; Knauer, A.; Malsch, D.; Stranik, O.; Müller, R.; Csáki, A.; Henkel, T.; Köhler, J. M.; Fritzsche, W., Gold nanocubes – Direct comparison of synthesis approaches reveals the need for a microfluidic synthesis setup for a high reproducibility. Chem. Eng. J. 2016, 288, 432-440. 19. Liu, Y.; Jiang, X., Why microfluidics? Merits and trends in chemical synthesis. Lab Chip 2017, 17, 3960. 20. Yang, T. J.; Choo, J.; Stavrakis, S.; de Mello, A., Fluoropolymer-Coated PDMS Microfluidic Devices for Application in Organic Synthesis. Chem. - Eur. J. 2018, 24, 12078-12083.


Page 24 of 26

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21. Hessel, V.; Lowe, H.; Schonfeld, F., Micromixers—a review on passive and active mixing principles. Chem. Eng. Sci. 2005, 60, 2479-2501. 22. Lee, C. Y.; Wang, W. T.; Liu, C. C.; Fu, L. M., Passive mixers in microfluidic systems: A review. Chem. Eng. J. 2016, 288, 146-160. 23. Cai, G.; Li, X.; Zhang, H.; Lin, J., A Review on Micromixers. Micromachines 2017, 8, 274. 24. Lindenberg, C.; Mazzotti, M., Continuous Precipitation of L-Asparagine Monohydrate in a Micromixer: Estimation of Nucleation and Growth Kinetics. AIChE J. 2011, 57, 942-950. 25. Hui, W.; Wang, C.; Zeng, C.; Zhang, L., Preparation of Barium Sulfate Nanoparticles in an Interdigital Channel Configuration Micromixer SIMM-V2. Ind. Eng. Chem. Res. 2013, 52, 5313-5320. 26. Wang, J.; Fan, Z.; Wang, Y.; Luo, G.; Cai, W., A Size-Controllable Preparation Method for Indium Tin Oxide Particles Using a Membrane Dispersion Micromixer. Chem. Eng. J. 2016, 293, 1-8. 27. Polyzoidis, A.; Altenburg, T.; Schwarzer, M.; Loebbecke, S.; Kaskel, S., Continuous microreactor synthesis of ZIF-8 with high space–time-yield and tunable particle size. Chem. Eng. J. 2016, 283, 971977. 28. Trzenschiok, H.; Distaso, M.; Peukert, W., A new approach for the stabilization of amorphous drug nanoparticles during continuous antisolvent precipitation. Chem. Eng. J. 2019, 361, 428-438. 29. Zhao, S.; Wu, J.; Zhu, P.; Xia, H.; Chen, C.; Shen, R., Microfluidic Platform for Preparation and Screening of Narrow Size-Distributed Nanoscale Explosives and Supermixed Composite Explosives. Ind. Eng. Chem. Res. 2018, 57, 13191-13204. 30. Xia, H. M.; Wan, S. Y.; Shu, C.; Chew, Y. T., Chaotic micromixers using two-layer crossing channels to exhibit fast mixing at low Reynolds numbers. Lab Chip 2005, 5, 748-755. 31. Chia-Yen, L.; Chang, C. L.; Wang, Y. N.; Fu, L. M., Microfluidic Mixing: A Review. Int. J. Mol. Sci. 2011, 12, 3263. 32. Wu, Z. H.; Yang, S. L.; Wu, W., Shape control of inorganic nanoparticles from solution. Nanoscale 2016, 8, 1237-1259.



Table of Contents Graphic

A passive micromixer platform was used for size and shape control of HNS.


Page 26 of 26

Passive micromixer platform for size- and shape-controllable

Recommend Documents