Droplet-droplet collision under high pressurized ambient

Under this condition, we find out the stretching coalescence, too. At the same time we do captured very clear droplet pair bouncing with very low relative velocities.

The Fig. 4.13~Fig. 4.15 is the snapshots of the head-on (b=0) droplet pair collision under high pressurized ambient (~0.55 atm) with different initial velocity (V=1000, 1250, 1375 and 1500 m/s). In Fig. 4.13, (V=1250 m/s), we can see the droplet area is increase as the beginning and then the area ruptures to a “ring”, at last the ring become a droplet again. In Fig. 4.14, (V=1375), we can see the droplet area is increase as the beginning and then the area ruptures to a “net”, at last the “net”

fragment into several bigger droplets. In Fig. 4.15, (V=1500 m/s), we can see the droplet area is increase as the beginning and then the area ruptures to a “net”, at last the “net” fragment into several small droplets. The size of fragmented droplets is still bigger than the droplets of simulation of the same condition under vacuum, and the number of fragments is more than low pressurized ambient (~ 0.055 atm). Because, there are less atoms be vaporized under pressurized ambient, higher pressurized make the droplet atoms collisions times more than others.

The Fig. 4.23~Fig. 4.26 is the snapshots of the non-head-on droplet pair collision

under high pressurized ambient (~0.55 atm) with different initial velocity and different impact parameter.

The collision behavior of Fig. 4.23 (b=0.25, V=250 m/s) is be classified in coalescence regime. In this case, the droplet pair move slowly and coalescence become a rotating “bigger” droplet which mass is almost equal the sum of droplet pair mass, as the same conditions under vacuum. The Fig. 4.44(a) is the number of atoms of the largest fragment. We can find out the number of atoms is became double of one droplet, because this is a coalescence case. Then the number of atoms is decreased function of time, because the droplet evaporation is occurring with time increasing.

The Fig. 4.44(b) is the vibrational temperature of atoms of the largest fragment, we find out the temperature rapidly increased when the droplet pair occur impact. After the droplet impact the temperature is cold down during time at 100~250ps, at this time the impact energy is completed has been transferred into temperature energy and complete the thermo-equilibrium inside the droplet. After 250ps, the temperature is increased with time, because the droplet evaporation is pulling the atoms from droplet surface. The Fig. 4.44(c) is the rotational energy of the largest fragment, we find out the rotational energy rapidly increased, because the initial relative translation contributed to rotational energy. And the Fig. 4.44(d) is the angular momentum of the largest fragment in different directions. In this case, the Fig. 4.44 is almost the same

with Fig.4.40. Because the relative velocity is too small, in this case we can’t find out the pressurized ambient effect.

The collision behavior of Fig. 4.24 (b=0.25, V=750 m/s) is be classified in stretching coalescence regime. In this case, the droplet be stretched and rotated during the process, at final stage, the droplet become into a rotating ball, and there no breakup occur. The Fig. 4.45(a) is the number of atoms of the largest fragment. In this figure, we can clearly classified this is a coalescence case. But there two stages of vaporized rate and the vaporized effect is clearly stronger than typical coalescence case. Because the droplet be stretched, the surface of droplet is bigger than a coalescence droplet. The Fig. 4.45(b) is the vibrational temperature of atoms of the largest fragment, we find out the temperature rapidly increased when the droplet pair occur impact. After the droplet impact the temperature rapidly fall off during 7~10ps.

The Fig. 4.45(c) is the rotational energy of the largest fragment, we find out the rotational energy rapidly increased, because the initial relative translation contributed to rotational energy. During the process the rotational energy decreased function of time, because the largest fragment be stretched. The energy never falls off rapidly, and value is ten times of coalescence case, it show clear that the droplet is rotating with out of shape. And the Fig. 4.45(d) is the angular momentum of the largest fragment in different directions.

The collision behavior of Fig. 4.25 (b=0.625, V=1000 m/s) is be classified in stretching separation regime. In this case, the droplet pair disrupted into 2 droplets and one “satellite” droplets followed short a narrow tail. The Fig. 4.46(a) is the number of atoms of the largest fragment. At the first, the number of atoms became double of one droplet, then the droplet separation in two main droplets. In this case, there is no 2^nd separation of largest fragment, because there only one satellite with the other fragment.

At final stage the number of fragment is decreasing by evaporation effect. The Fig.

4.46(b) is the vibrational temperature of atoms of the largest fragment, we find out the temperature rapidly increased when the droplet pair occur impact. After the droplet impact the temperature rapidly fall off during 7~10ps, then the temperature raise up until ~50ps, then the temperature rapidly fall off again when separation occurred. The Fig. 4.46(c) is the rotational energy of the largest fragment, we find out the rotational energy rapidly increased, because the initial relative translation contributed to rotational energy. During 15~90ps the rotational energy decreased function of time, because the largest fragment be stretching, then the energy rapidly fall off when separation occur. And the Fig. 4.46(d) is the angular momentum of the largest fragment in different directions.

The Fig. 4.27~Fig. 4.28 is the snapshots of the head-on (b=0) droplet pair collision under high pressurized ambient (~0.55 atm) with very low initial velocity.

These are classified in droplet pair bounce regime. In Fig. 4.27, the relative velocity between droplet is 10 m/sec. Form Fig. 4.27(a) to Fig. 4.27(f), we observed the droplet pair to approach each other with very low velocity, and when the distance reach a value the droplet pair stop to approach. At the same time, the vaporized effect occurs .violently and rapidly. The shortest distance between droplet pair is named

“gas gap”, in previously e.g., [Pan and Law, 2004 and Murad and Law, 1999]. The Fig. 4.28 is the same with Fig. 4.27 with different relative velocity (30 m/sec). The Fig. 4.49 and Fig. 4.50 are the atoms distribution in X direction at various time (time=

25, 250, 500, 750, 1000 and 1250ps) which is mapping Fig. 4.27 and Fig. 4.28. we can find out the time step while the droplet is most close is 750ps. In this analysis, we can to estimate the value of “gas gap”. The gas gap of bouncing of droplet pair with 10 m/sec is 4.086 nm. While the relative velocity is 30m/s, the value of gas gap 2.724nm. Therefore, the value of gas gap depends on the magnitude of relative velocity or the magnitude of kinetic energy. We estimated the value of gas gap equal the value of previous studies [Pan and Law, 2004 and Murad and Law, 1999] the value of gas gap is 3~4 nm. The Fig. 4.48(a) is the number of atoms of the largest fragment. During the processes the number of fragment is decreasing by evaporation effect. The Fig. 4.48(b) is the vibrational temperature of atoms of the largest fragment.

The Fig. 4.48(c) is the rotational energy of the largest fragment, we find out the

rotational energy is varing in very narrow range. And the Fig. 4.48(d) is the angular momentum of the largest fragment in different directions. We can find out the angular momentum occur very strong vibration. Because, the largest droplet impact on the gas gap.

The Fig. 4.29~Fig. 4.30 is the snapshots of the head-on (b=0) droplet pair collision under high pressurized ambient (~0.55 atm) but the temperature of ambient is 324k, which different with Fig. 4.27~Fig. 4.28 (~216 k). These are classified in droplet pair bounce regime. In Fig. 4.29, the relative velocity between droplet is 10 m/sec. Form Fig. 4.29(a) to Fig. 4.29(f), we observed the droplet pair to approach

each other with very low velocity, and when the distance reach a value the droplet pair stop to approach. At the same time, the vaporized effect occurs violently and rapidly. The Fig. 4.30 is the same with Fig. 4.29 with different relative velocity (30m/sec). The Fig. 4.51 and Fig. 4.52 are the atoms distribution in X direction at various time (time= 25, 250, 500, 750, 1000 and 1250ps). We can find out the time step while the droplet is most close is 750ps. The gas gap of bouncing of droplet pair with 10m/sec is 2.724nm. While the relative velocity is 30m/s, the value of gas gap 4.086 nm. Therefore, the value of gas gap depends on the magnitude of relative velocity or the magnitude of kinetic energy. We can find out the gas gap distributions is opposite with Fig. 4.27 and Fig. 4.29. That’s because, the higher temperature has

the lower number density value of gas gap and the vaporized atoms is move fast at higher temperature.