1
00:00:15,709 --> 00:00:13,910
with the advent of space technology new

2
00:00:18,080 --> 00:00:15,719
opportunities are emerging for

3
00:00:21,080 --> 00:00:18,090
scientific research with exciting

4
00:00:23,060 --> 00:00:21,090
applications in many fields to take

5
00:00:26,000 --> 00:00:23,070
advantage of this new era in space

6
00:00:27,800 --> 00:00:26,010
exploration basic research into the

7
00:00:30,220 --> 00:00:27,810
fundamental characteristics of the

8
00:00:32,889 --> 00:00:30,230
environment of space such as long-term

9
00:00:37,130 --> 00:00:32,899
microgravity and ultra-high vacuum

10
00:00:39,170 --> 00:00:37,140
should first be undertaken the purpose

11
00:00:41,330 --> 00:00:39,180
of the research presented in this video

12
00:00:44,000 --> 00:00:41,340
is to understand more clearly the

13
00:00:46,850 --> 00:00:44,010
effects of surface tension on fluid

14
00:00:49,760 --> 00:00:46,860
convection the predominance of surface

15
00:00:52,459 --> 00:00:49,770
tension effects over buoyancy effects is

16
00:00:55,160 --> 00:00:52,469
a unique characteristic of microgravity

17
00:00:58,099 --> 00:00:55,170
environments and is critically important

18
00:01:00,349 --> 00:00:58,109
to many microgravity applications such

19
00:01:04,039 --> 00:01:00,359
as material processing and fluid

20
00:01:06,590 --> 00:01:04,049
management the fluid system chosen for

21
00:01:09,380 --> 00:01:06,600
this study the liquid sessile droplet

22
00:01:11,840 --> 00:01:09,390
has been selected because among many

23
00:01:14,600 --> 00:01:11,850
fluid systems most affected by surface

24
00:01:16,640 --> 00:01:14,610
tension liquid droplets are particularly

25
00:01:20,660 --> 00:01:16,650
important to many industrial and

26
00:01:22,969 --> 00:01:20,670
biological processes evaporation induced

27
00:01:25,520 --> 00:01:22,979
surface tension effects on liquid

28
00:01:28,730 --> 00:01:25,530
droplets and their associated transport

29
00:01:31,149 --> 00:01:28,740
process can significantly enhance heat

30
00:01:33,679 --> 00:01:31,159
and mass transfer

31
00:01:36,710 --> 00:01:33,689
they are also critical to a wide range

32
00:01:39,260 --> 00:01:36,720
of technologies such as single crystal

33
00:01:41,990 --> 00:01:39,270
growth the spray drying and cooling of

34
00:01:44,649 --> 00:01:42,000
metal and the advanced droplet radiators

35
00:01:47,690 --> 00:01:44,659
of the space stations power systems

36
00:01:50,359 --> 00:01:47,700
while quantitative research on surface

37
00:01:51,830 --> 00:01:50,369
tension induced flow is pivotal to our

38
00:01:54,469 --> 00:01:51,840
understanding of the microgravity

39
00:01:57,800 --> 00:01:54,479
environment it is difficult to achieve

40
00:01:59,929 --> 00:01:57,810
on earth not only do surface tension

41
00:02:02,569 --> 00:01:59,939
driven flow phenomena take place solely

42
00:02:04,429 --> 00:02:02,579
in small-scale systems under normal

43
00:02:07,550 --> 00:02:04,439
gravity surface tension properties

44
00:02:09,699 --> 00:02:07,560
themselves are very easily disturbed by

45
00:02:13,190 --> 00:02:09,709
experimental observation

46
00:02:15,559 --> 00:02:13,200
thus computational simulations

47
00:02:18,140 --> 00:02:15,569
offer an attractive way to investigate

48
00:02:21,110 --> 00:02:18,150
internal convection within liquid

49
00:02:23,360 --> 00:02:21,120
droplets during this video a

50
00:02:25,880 --> 00:02:23,370
cross-sectional representation of the

51
00:02:29,089 --> 00:02:25,890
hemispherical liquid droplet under ideal

52
00:02:32,990 --> 00:02:29,099
conditions is used to show internal

53
00:02:35,300 --> 00:02:33,000
fluid motion the droplet is placed on an

54
00:02:37,910 --> 00:02:35,310
unheated plate with evaporation

55
00:02:40,009 --> 00:02:37,920
occurring along the free surface the

56
00:02:43,280 --> 00:02:40,019
black dots within the liquid droplet are

57
00:02:46,490 --> 00:02:43,290
fluid tracers used to visualize fluid

58
00:02:50,210 --> 00:02:46,500
flow because the processes that occur

59
00:02:52,339 --> 00:02:50,220
are assumed symmetric with respect to

60
00:02:54,770 --> 00:02:52,349
the centerline we will be describing

61
00:02:58,699 --> 00:02:54,780
only the flow field in the right half of

62
00:03:01,250 --> 00:02:58,709
the droplet two types of evaporation

63
00:03:03,920 --> 00:03:01,260
induced convection are presented one

64
00:03:06,470 --> 00:03:03,930
under normal gravity where buoyancy is

65
00:03:09,199 --> 00:03:06,480
the most significant force the other

66
00:03:11,930 --> 00:03:09,209
under microgravity where buoyancy force

67
00:03:15,500 --> 00:03:11,940
subsides and surface tension effects

68
00:03:17,140 --> 00:03:15,510
predominate the patterns of flow that

69
00:03:20,050 --> 00:03:17,150
described them were obtained

70
00:03:23,210 --> 00:03:20,060
computationally using an Adi scheme

71
00:03:25,879 --> 00:03:23,220
while a runga cotta scheme tracks the

72
00:03:28,160 --> 00:03:25,889
motion of the fluid tracers the

73
00:03:34,430 --> 00:03:28,170
calculations required about one hour of

74
00:03:36,559 --> 00:03:34,440
CPU time on the Cray XMP 24in buoyancy

75
00:03:39,410 --> 00:03:36,569
induced convection otherwise known as

76
00:03:43,210 --> 00:03:39,420
Rayleigh convection gravity is the

77
00:03:46,039 --> 00:03:43,220
dominant force in stratified fluids

78
00:03:49,190 --> 00:03:46,049
stability exists when heavier fluids

79
00:03:51,140 --> 00:03:49,200
live beneath lighter fluids that is why

80
00:03:53,660 --> 00:03:51,150
when a heavier fluid lies above a

81
00:03:56,210 --> 00:03:53,670
lighter one the heavier fluid will tend

82
00:03:59,000 --> 00:03:56,220
to descend when the criteria for

83
00:04:03,220 --> 00:03:59,010
stability are exceeded the result will

84
00:04:06,440 --> 00:04:03,230
be fluid motion induced by gravity we

85
00:04:08,750 --> 00:04:06,450
begin with a liquid droplet having a

86
00:04:11,240 --> 00:04:08,760
uniform temperature shown in orange

87
00:04:14,150 --> 00:04:11,250
which is the same as the plate upon

88
00:04:16,759 --> 00:04:14,160
which it rests as evaporative cooling

89
00:04:19,150 --> 00:04:16,769
occurs along the free surface of the

90
00:04:22,009 --> 00:04:19,160
drop the temperature of that surface

91
00:04:26,600 --> 00:04:22,019
decreases which makes the fluid there

92
00:04:29,450 --> 00:04:26,610
heavier as evaporation proceeds the

93
00:04:32,029 --> 00:04:29,460
temperature continues to decrease and a

94
00:04:35,209 --> 00:04:32,039
yellow rim representing the colder fluid

95
00:04:37,490 --> 00:04:35,219
at the surface broadens eventually as

96
00:04:41,240 --> 00:04:37,500
the fluid turns even colder the rim

97
00:04:44,240 --> 00:04:41,250
becomes green and then blue as this

98
00:04:46,580 --> 00:04:44,250
increasingly heavier fluid descends the

99
00:04:49,760 --> 00:04:46,590
relatively warmer fluid of the interior

100
00:04:53,209 --> 00:04:49,770
ascends setting up a clockwise fluid

101
00:04:56,450 --> 00:04:53,219
convection because buoyancy force is a

102
00:04:59,209 --> 00:04:56,460
body force it induces a global motion

103
00:05:02,689 --> 00:04:59,219
and convection occurs almost everywhere

104
00:05:05,089 --> 00:05:02,699
within the liquid drop after an

105
00:05:08,330 --> 00:05:05,099
initially rapid flow the rate of

106
00:05:10,010 --> 00:05:08,340
convection gradually slows down as the

107
00:05:12,920 --> 00:05:10,020
temperature field becomes vertically

108
00:05:15,409 --> 00:05:12,930
more stable the clockwise fluid motion

109
00:05:18,350 --> 00:05:15,419
in the upper layer is reduced and it

110
00:05:26,390 --> 00:05:18,360
almost still predominantly lateral

111
00:05:28,790 --> 00:05:26,400
motion results eventually however as

112
00:05:30,730 --> 00:05:28,800
evaporative cooling continues to reduce

113
00:05:33,680 --> 00:05:30,740
the temperature at the surface a

114
00:05:37,159 --> 00:05:33,690
coupling between internal convection and

115
00:05:38,439 --> 00:05:37,169
that cooling induces to thermal

116
00:05:41,450 --> 00:05:38,449
stratifications

117
00:05:44,200 --> 00:05:41,460
the first stratification moves clockwise

118
00:05:47,659 --> 00:05:44,210
at the lower edge of the droplet surface

119
00:05:49,999 --> 00:05:47,669
the second moves counterclockwise and

120
00:05:52,790 --> 00:05:50,009
begins at the top of the surface as

121
00:05:56,059 --> 00:05:52,800
another vortex of cooler fluid moves

122
00:05:58,339 --> 00:05:56,069
downward into the interior demonstrating

123
00:06:03,159 --> 00:05:58,349
that evaporation induced buoyancy

124
00:06:06,709 --> 00:06:03,169
convection is a dynamic unsteady process

125
00:06:09,110 --> 00:06:06,719
unlike buoyancy dominant flow surface

126
00:06:12,469 --> 00:06:09,120
tension dominant flow has not been well

127
00:06:15,010 --> 00:06:12,479
studied however we do know that it is

128
00:06:19,579 --> 00:06:15,020
partly a function of temperature and

129
00:06:21,649 --> 00:06:19,589
concentration the present study focuses

130
00:06:24,800 --> 00:06:21,659
on the effect of temperature on surface

131
00:06:28,129 --> 00:06:24,810
tension the warmer the fluid the weaker

132
00:06:30,379 --> 00:06:28,139
the surface tension and conversely the

133
00:06:34,129 --> 00:06:30,389
colder the fluid the stronger the

134
00:06:36,290 --> 00:06:34,139
surface tension again we begin with a

135
00:06:38,540 --> 00:06:36,300
liquid droplet having a uniform

136
00:06:40,310 --> 00:06:38,550
temperature which is the same as the

137
00:06:44,120 --> 00:06:40,320
plate upon which it rests

138
00:06:46,610 --> 00:06:44,130
as evaporative cooling occurs along the

139
00:06:49,310 --> 00:06:46,620
droplets free surface the temperature

140
00:06:52,100 --> 00:06:49,320
there decreases shown here by the

141
00:06:55,360 --> 00:06:52,110
broadening yellow rim and it's gradual

142
00:06:57,800 --> 00:06:55,370
transformation from yellow to green

143
00:07:00,200 --> 00:06:57,810
because the temperature at the bottom of

144
00:07:02,690 --> 00:07:00,210
the plate is still warm a temperature

145
00:07:05,060 --> 00:07:02,700
gradient begins to form where the plate

146
00:07:08,090 --> 00:07:05,070
meets the lower edge of the relatively

147
00:07:10,910 --> 00:07:08,100
colder droplet the surface tension is

148
00:07:13,540 --> 00:07:10,920
weakest near the warm plate but stronger

149
00:07:15,950 --> 00:07:13,550
at the colder surface as a result

150
00:07:18,560 --> 00:07:15,960
surface tension is able to pull the

151
00:07:21,500 --> 00:07:18,570
fluid upwards that is away from the

152
00:07:26,090 --> 00:07:21,510
plate and fluid viscosity transfers

153
00:07:28,190 --> 00:07:26,100
momentum into the interior conservation

154
00:07:31,820 --> 00:07:28,200
of mass dictates that a circular

155
00:07:34,640 --> 00:07:31,830
counterclockwise motion or vortex wraps

156
00:07:38,780 --> 00:07:34,650
cold fluid into the interior of the drop

157
00:07:41,750 --> 00:07:38,790
to form a cold pocket the end result is

158
00:07:44,300 --> 00:07:41,760
a self propelling process with a maximum

159
00:07:47,390 --> 00:07:44,310
velocity and the largest temperature

160
00:07:49,370 --> 00:07:47,400
gradient coupled in the cold pocket to

161
00:07:52,310 --> 00:07:49,380
maintain the surface tension induced

162
00:07:55,780 --> 00:07:52,320
fluid flow which pushes the vortex all

163
00:07:59,150 --> 00:07:55,790
the way up the droplets free surface

164
00:08:00,940 --> 00:07:59,160
let's compare our computational results

165
00:08:04,840 --> 00:08:00,950
with what we observe experimentally

166
00:08:08,120 --> 00:08:04,850
using laser shadowgraph

167
00:08:10,820 --> 00:08:08,130
traces of aluminum particles allow us to

168
00:08:14,660 --> 00:08:10,830
visualize evaporation induced convective

169
00:08:17,030 --> 00:08:14,670
patterns inside a chloroform drop a few

170
00:08:21,470 --> 00:08:17,040
seconds after it was placed on an

171
00:08:24,950 --> 00:08:21,480
isothermal plate for regions of flow can

172
00:08:28,100 --> 00:08:24,960
be identified region 1 located at the

173
00:08:31,130 --> 00:08:28,110
top of the drop is stagnant region 2

174
00:08:35,180 --> 00:08:31,140
consists of weak clockwise convection

175
00:08:37,300 --> 00:08:35,190
currents driven by buoyancy force region

176
00:08:41,540 --> 00:08:37,310
3 is composed of a few layers of

177
00:08:44,290 --> 00:08:41,550
hexagonal flow cells finally region 4

178
00:08:46,910 --> 00:08:44,300
has relatively stronger counterclockwise

179
00:08:49,520 --> 00:08:46,920
convection currents which according to

180
00:08:53,330 --> 00:08:49,530
our computational results are driven by

181
00:08:54,020 --> 00:08:53,340
surface tension since the liquid drop is

182
00:08:56,990 --> 00:08:54,030
a small

183
00:08:59,090 --> 00:08:57,000
system surface tension is expected to

184
00:09:01,820 --> 00:08:59,100
play a dominant role in the evaporation

185
00:09:05,150 --> 00:09:01,830
process and this is also what we observe

186
00:09:08,210 --> 00:09:05,160
experimentally as evaporation proceeds

187
00:09:10,160 --> 00:09:08,220
the weak buoyancy induced flow region

188
00:09:13,120 --> 00:09:10,170
shrinks while a counter clockwise

189
00:09:17,090 --> 00:09:13,130
surface tension flow in Region four

190
00:09:19,340 --> 00:09:17,100
displaces Region three upwards this is

191
00:09:21,110 --> 00:09:19,350
exactly the result we saw earlier in the

192
00:09:24,350 --> 00:09:21,120
graphical presentation of surface

193
00:09:26,990 --> 00:09:24,360
tension dominant convection thus the

194
00:09:30,430 --> 00:09:27,000
computational results qualitatively

195
00:09:33,590 --> 00:09:30,440
match the experimental results well

196
00:09:36,740 --> 00:09:33,600
however the observations also reveal a

197
00:09:38,870 --> 00:09:36,750
drawback of the experimental method not

198
00:09:41,180 --> 00:09:38,880
only is the stagnant nature of region

199
00:09:43,880 --> 00:09:41,190
one exaggerated by the particles of

200
00:09:46,160 --> 00:09:43,890
aluminum that accumulate there but the

201
00:09:49,180 --> 00:09:46,170
accumulation eventually destroys the

202
00:09:51,650 --> 00:09:49,190
surface tension properties altogether

203
00:09:54,620 --> 00:09:51,660
let's review the graphical presentation

204
00:09:57,530 --> 00:09:54,630
of buoyancy dominant and thermo

205
00:10:00,260 --> 00:09:57,540
capillary dominant convection using

206
00:10:02,330 --> 00:10:00,270
numerical methods the elapsed time

207
00:10:04,430 --> 00:10:02,340
during this segment of the video will

208
00:10:08,180 --> 00:10:04,440
more accurately reflect real time

209
00:10:10,850 --> 00:10:08,190
processes in buoyancy dominant

210
00:10:14,030 --> 00:10:10,860
convection gravity is the dominant force

211
00:10:16,540 --> 00:10:14,040
as evaporation occurs the temperature at

212
00:10:19,580 --> 00:10:16,550
the droplet surface decreases a

213
00:10:22,060 --> 00:10:19,590
clockwise convection results as the

214
00:10:25,180 --> 00:10:22,070
increasingly colder and heavier fluid

215
00:10:28,190 --> 00:10:25,190
descends and the relatively warmer fluid

216
00:10:30,800 --> 00:10:28,200
ascends eventually as the rate of

217
00:10:33,740 --> 00:10:30,810
convection slows down the clockwise

218
00:10:36,470 --> 00:10:33,750
fluid motion is reduced and lateral

219
00:10:42,860 --> 00:10:36,480
motion predominates until a second

220
00:10:45,080 --> 00:10:42,870
counter clockwise motion begins when we

221
00:10:47,210 --> 00:10:45,090
observe the surface tension dominant

222
00:10:49,880 --> 00:10:47,220
flow characteristics of microgravity

223
00:10:52,850 --> 00:10:49,890
environments a different pattern of

224
00:10:54,920 --> 00:10:52,860
convection occurs the resulting fluid

225
00:10:57,230 --> 00:10:54,930
motion is generated by the surface

226
00:11:00,140 --> 00:10:57,240
tension gradient along the droplet

227
00:11:03,590 --> 00:11:00,150
surface which pulls the fluid upward in

228
00:11:06,200 --> 00:11:03,600
a circular counterclockwise motion the

229
00:11:07,940 --> 00:11:06,210
end result is a self propelling process

230
00:11:10,490 --> 00:11:07,950
with velocity and ten

231
00:11:14,750 --> 00:11:10,500
coupled to maintain the surface tension

232
00:11:16,940 --> 00:11:14,760
induced flow in this video we have seen

233
00:11:20,060 --> 00:11:16,950
a direct simulation of natural

234
00:11:22,970 --> 00:11:20,070
convection inside liquid droplets caused

235
00:11:24,920 --> 00:11:22,980
by evaporative cooling of the two

236
00:11:27,470 --> 00:11:24,930
mechanisms that induce convection

237
00:11:30,200 --> 00:11:27,480
the first was gravitational buoyancy

238
00:11:33,050 --> 00:11:30,210
force and the resulting fluid transport

239
00:11:35,510 --> 00:11:33,060
the well studied rayleigh natural

240
00:11:37,310 --> 00:11:35,520
convection the other mechanism which is

241
00:11:39,560 --> 00:11:37,320
less familiar to the thermal fluid

242
00:11:42,860 --> 00:11:39,570
society but more relevant to future

243
00:11:45,320 --> 00:11:42,870
space venture is surface tension induced

244
00:11:48,350 --> 00:11:45,330
by a thermal gradient and the resulting

245
00:11:51,680 --> 00:11:48,360
fluid motion the so called Marangoni or

246
00:11:53,420 --> 00:11:51,690
thermo capillary convection the

247
00:11:55,340 --> 00:11:53,430
fundamental difference is between these

248
00:11:59,210 --> 00:11:55,350
two convection patterns can be briefly

249
00:12:02,300 --> 00:11:59,220
summarized as follows gravitational

250
00:12:05,150 --> 00:12:02,310
buoyancy force is a body force thus the

251
00:12:07,340 --> 00:12:05,160
induced fluid dynamics is global and the

252
00:12:10,940 --> 00:12:07,350
resulting temperature field in the fluid

253
00:12:13,460 --> 00:12:10,950
is more homogeneous li mixed thermo

254
00:12:16,130 --> 00:12:13,470
capillary force is a surface force and

255
00:12:18,290 --> 00:12:16,140
here both the induced fluid motion and

256
00:12:23,210 --> 00:12:18,300
the thermal stratification is more

257
00:12:26,090 --> 00:12:23,220
intensified along the free surface under

258
00:12:27,980 --> 00:12:26,100
conditions of decreasing gravity the

259
00:12:30,070 --> 00:12:27,990
pattern of fluid flow will gradually

260
00:12:33,410 --> 00:12:30,080
become less dominated by buoyancy

261
00:12:35,320 --> 00:12:33,420
convection eventually when the force of

262
00:12:37,940 --> 00:12:35,330
gravity has been reduced significantly

263
00:12:42,980 --> 00:12:37,950
the flow pattern will be dominated by

264
00:12:44,840 --> 00:12:42,990
surface tension this research video has

265
00:12:47,240 --> 00:12:44,850
illustrated the clear differences

266
00:12:50,150 --> 00:12:47,250
between two mechanisms of fluid

267
00:12:53,750 --> 00:12:50,160
transport thermo capillary convection

268
00:12:56,330 --> 00:12:53,760
and buoyancy dominant convection in the

269
00:12:58,730 --> 00:12:56,340
future the investigator will expand his

270
00:13:00,980 --> 00:12:58,740
studies of fluid flow to explore the

271
00:13:03,860 --> 00:13:00,990
effects of surface tension on double

272
00:13:07,490 --> 00:13:03,870
diffusion and two phase related problems

273
00:13:09,140 --> 00:13:07,500
such as crystal growth he will also

274
00:13:11,330 --> 00:13:09,150
explore the potential of thermo

275
00:13:13,610 --> 00:13:11,340
capillary convection for the droplet

276
00:13:16,960 --> 00:13:13,620
radiators involved in the Space

277
00:13:19,400 --> 00:13:16,970
Station's power system such

278
00:13:21,740 --> 00:13:19,410
computational studies are laying the

279
00:13:24,070 --> 00:13:21,750
groundwork that will enable NASA

280
00:13:27,140 --> 00:13:24,080
and its partners in academia and

281
00:13:30,080 --> 00:13:27,150
industry to design the expensive

282
00:13:34,340 --> 00:13:30,090
experimental ventures in outer space of

283
00:13:36,740 --> 00:13:34,350
the coming decade together numerical and

284
00:13:38,900 --> 00:13:36,750
experimental studies we'll be able to

285
00:13:41,270 --> 00:13:38,910
meet the challenge of exploiting the

286
00:13:43,790 --> 00:13:41,280
revolutionary capabilities the space

287
00:13:46,550 --> 00:13:43,800
station offers thus helping to fulfill