Droplet-droplet collision under low pressurized ambient

Under this condition, we find out the stretching coalescence for the first time.

But unfortunately, we do not capture any droplet pair bouncing under this condition.

The Fig. 4.10~Fig. 4.12 is the snapshots of the head-on (b=0) droplet pair collision under low pressurized ambient with different initial velocity (V= 1250, 1375 and 1500 m/s). In Fig. 4.10, (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. 4.11, (V=1375 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 bigger droplets. In Fig. 4.12, (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 fragmented droplets size is still bigger than the droplets of simulation of the same condition under vacuum. Because, there are less atoms be vaporized under low pressurized (~0.055 atm) ambient

The Fig. 4.19~Fig. 4.22 is the snapshots of the non-head-on droplet pair collision under low pressurized ambient (~0.055 atm) with different initial velocity and different impact parameter.

The collision behavior of Fig. 4.19 (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. 4.40(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. 4.40(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. 4.46(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. And the Fig. 4.40(d) is the angular momentum of the largest fragment in different directions. In this case, the Fig. 4.40 is almost the same with Fig. 4.37. Because the relative velocity is too small, in this case we can’t find out the pressurized ambient effect.

The collision behavior of Fig.4.20 (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. 4.41(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 stage 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. 4.41(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. 4.41(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. 4.41(d) is the angular momentum of the largest fragment in different directions.

The collision behavior of Fig. 4.21 (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 several “satellite” droplets follow long a narrow tail. The Fig. 4.42(a) is the number of atoms of the largest fragment. At the first, the number of atoms is became double of one droplet, then the droplet separation in two main droplet, when the time at near 100ps the largest fragment to meet the 2^nd separation, because the trail breakup. At final stage the number of fragment is decreasing by evaporation effect. The Fig.

4.42(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. 4.42(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 15~100ps 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. 4.42(d) is the angular momentum of the largest fragment in different directions. Compared Fig. 4.40(a) and Fig. 4.42(a), we find out a interest phenomenon. During 80~120ps, while fragment size decreased by to meet 2^nd separation the rotational energy is increased by out of shape of to meet 2^nd separation.

The collision behavior of Fig. 4.22 (b=0.25, V=1375 m/s) is be classified in shattering separation regime. In this case, the droplet pair to meet huge impact kinetic energy, the droplet fragments into over 5 droplets. The Fig. 4.43(a) is the number of atoms of the largest fragment. At the first, the number of atoms is became double of one droplet, then the largest fragment to meet twice serious fragmentation, during the process the number of fragment is decreased serious than other cases. In the shattering case, we can find out the fragment times is more than shattering case under vacuum.

The Fig. 4.43(b) is the vibrational temperature of atoms of the largest fragment, we find out the temperature rapidly increased when the droplet pair occur impact, and the peak value of the temperature is higher than others under the same condition. Then the temperature fall off rapidly when separation occurred at 90ps. The Fig. 4.43(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 twice when fragmentations

occur. And the Fig. 4.43(d) is the angular momentum of the largest fragment in different directions.