1
00:00:10,880 --> 00:00:13,940
PROFESSOR: So the transfer
of respiratory pathogens

2
00:00:13,940 --> 00:00:16,970
is primarily accomplished
through droplets

3
00:00:16,970 --> 00:00:19,880
that are emitted by
an infected person

4
00:00:19,880 --> 00:00:24,740
and then either breathed
in or ending up on surfaces

5
00:00:24,740 --> 00:00:28,880
and touched and incorporated
into the body in some other way

6
00:00:28,880 --> 00:00:31,590
by a susceptible person.

7
00:00:31,590 --> 00:00:35,450
So let's begin by talking
about the formation of droplets

8
00:00:35,450 --> 00:00:37,860
during respiration.

9
00:00:37,860 --> 00:00:44,600
So these droplets can
form in different parts

10
00:00:44,600 --> 00:00:47,220
of the respiratory tract.

11
00:00:47,220 --> 00:00:49,700
So the respiratory tract
refers to the whole system

12
00:00:49,700 --> 00:00:52,760
of your breathing
apparatus in your body.

13
00:00:52,760 --> 00:00:55,200
So that includes, of
course, your lungs,

14
00:00:55,200 --> 00:00:59,390
which involves a
network of passages

15
00:00:59,390 --> 00:01:02,210
going from the large bronchus
down to the bronchioles

16
00:01:02,210 --> 00:01:08,240
and ultimately to the alveolar
sacs where the air is exchanged

17
00:01:08,240 --> 00:01:10,730
or oxygen gets into the
blood and carbon dioxide

18
00:01:10,730 --> 00:01:13,670
is picked up, and
then you exhale.

19
00:01:13,670 --> 00:01:18,080
In the upper respiratory
tract, we have of course

20
00:01:18,080 --> 00:01:21,470
the mouth and the nose
and the larynx, the voice

21
00:01:21,470 --> 00:01:24,530
box where sounds are made.

22
00:01:24,530 --> 00:01:26,300
The nasopharynx is
sort of the region

23
00:01:26,300 --> 00:01:30,470
behind the mouth
and the nose where

24
00:01:30,470 --> 00:01:32,900
the passages are connected.

25
00:01:32,900 --> 00:01:35,270
And in all of those
different regions

26
00:01:35,270 --> 00:01:37,640
of the respiratory tract,
when we breathe in,

27
00:01:37,640 --> 00:01:40,280
air is coming through
in one direction,

28
00:01:40,280 --> 00:01:42,979
and of course when we
exhale, it's coming back out.

29
00:01:42,979 --> 00:01:45,000
And there is a lot of
fluid in the lungs.

30
00:01:45,000 --> 00:01:49,880
So the airways are
lined typically

31
00:01:49,880 --> 00:01:53,210
with surfactant film
and mucus, which

32
00:01:53,210 --> 00:01:57,039
is a thick substance
we're all familiar with.

33
00:01:57,039 --> 00:02:00,110
It can vary in composition
but generally has

34
00:02:00,110 --> 00:02:03,420
some large macromolecules
and in particular proteins

35
00:02:03,420 --> 00:02:04,970
that are called
mucins, which give it

36
00:02:04,970 --> 00:02:06,860
it's sort of thick consistency.

37
00:02:06,860 --> 00:02:10,160
There are also ions such as
sodium and chloride, which

38
00:02:10,160 --> 00:02:12,050
are dissolved in the liquid.

39
00:02:12,050 --> 00:02:16,880
And even liquid such
as saliva in your mouth

40
00:02:16,880 --> 00:02:22,370
have a similar composition but
less of the sort of thick mucin

41
00:02:22,370 --> 00:02:25,370
proteins that I mentioned
compared to the deeper

42
00:02:25,370 --> 00:02:26,540
parts the respiratory tract.

43
00:02:26,540 --> 00:02:27,950
Of course, when
someone gets sick,

44
00:02:27,950 --> 00:02:31,730
also there can be more of
the sort of mucus and phlegm

45
00:02:31,730 --> 00:02:37,190
that's generated to help the
body deal with the pathogen.

46
00:02:37,190 --> 00:02:38,930
So all those liquids
and fluids are

47
00:02:38,930 --> 00:02:42,000
present in different parts
of the respiratory tract.

48
00:02:42,000 --> 00:02:45,950
And so there are a number of
mechanisms which are still

49
00:02:45,950 --> 00:02:49,550
subject to scientific
research and debate by which

50
00:02:49,550 --> 00:02:53,150
droplets are created
and ultimately emitted

51
00:02:53,150 --> 00:02:55,280
when a person is breathing out.

52
00:02:55,280 --> 00:02:59,210
So let's begin by thinking
about such processes

53
00:02:59,210 --> 00:03:01,880
in the upper respiratory tract.

54
00:03:09,980 --> 00:03:12,100
So in the upper
respiratory tract,

55
00:03:12,100 --> 00:03:16,900
we can imagine, first of
all, that the passages have

56
00:03:16,900 --> 00:03:18,430
a little bit larger spacing.

57
00:03:18,430 --> 00:03:21,070
So for example, your mouth
might be open by centimeters

58
00:03:21,070 --> 00:03:22,480
or millimeters.

59
00:03:22,480 --> 00:03:24,700
If you go into
your nose, there's

60
00:03:24,700 --> 00:03:28,240
of course various hairs and
smaller structures which

61
00:03:28,240 --> 00:03:33,250
are often covered with mucus
and liquids, which as the air

62
00:03:33,250 --> 00:03:39,370
is passing by, could be leading
to some breakup of droplets.

63
00:03:39,370 --> 00:03:42,670
And then of course also in
the voicebox and other areas

64
00:03:42,670 --> 00:03:44,840
of the upper respiratory tract.

65
00:03:44,840 --> 00:03:53,640
So the main mechanism here
for generating droplets

66
00:03:53,640 --> 00:03:58,650
would be the breakup of
viscoelastic filaments

67
00:03:58,650 --> 00:04:00,300
in a fluid flow.

68
00:04:00,300 --> 00:04:07,010
So another word for
breakup is fragmentation

69
00:04:07,010 --> 00:04:15,060
of viscoelastic filaments.

70
00:04:15,060 --> 00:04:20,829
And by that, I mean that the
mucus especially is a fluid.

71
00:04:20,829 --> 00:04:23,640
So it has a viscosity, a
resistance to shear flow.

72
00:04:23,640 --> 00:04:25,320
But it also can have
some elasticity.

73
00:04:25,320 --> 00:04:27,320
If you pull on it, it can
pull back a little bit

74
00:04:27,320 --> 00:04:29,730
because there are these
macromolecules present.

75
00:04:29,730 --> 00:04:32,610
So in general, we have a
somewhat complicated reality

76
00:04:32,610 --> 00:04:35,900
of that liquid or that fluid.

77
00:04:35,900 --> 00:04:38,000
And a filament
refers to the fact

78
00:04:38,000 --> 00:04:40,430
that those droplets
can be stretched out,

79
00:04:40,430 --> 00:04:43,110
and as the air is then
blowing past those filaments,

80
00:04:43,110 --> 00:04:44,420
it can start to break up.

81
00:04:44,420 --> 00:04:46,260
So this is our basic mechanism.

82
00:04:46,260 --> 00:04:50,450
And this is mainly
going to be happening

83
00:04:50,450 --> 00:04:56,450
while a person is exhaling, at
least in terms of emissions.

84
00:04:56,450 --> 00:04:57,920
It's also possible
when you Inhale,

85
00:04:57,920 --> 00:04:59,710
there'll be some of
those droplets created.

86
00:04:59,710 --> 00:05:03,200
They go into your lungs or
get deposited on the surfaces

87
00:05:03,200 --> 00:05:05,690
and then manage to somehow
come back out again.

88
00:05:05,690 --> 00:05:08,570
But certainly during exhaling,
you would imagine more--

89
00:05:08,570 --> 00:05:12,480
or you could see actually that
more droplets are created.

90
00:05:12,480 --> 00:05:16,200
So if we think of
some examples of that,

91
00:05:16,200 --> 00:05:20,850
we might have, for example,
when I'm speaking or breathing

92
00:05:20,850 --> 00:05:23,930
and my mouth is a
little bit open,

93
00:05:23,930 --> 00:05:30,770
if I imagine drawing kind
of let's say a person's lips

94
00:05:30,770 --> 00:05:34,080
and mouth might look
something like this.

95
00:05:34,080 --> 00:05:36,810
So I'm kind of
exaggerating here,

96
00:05:36,810 --> 00:05:39,320
but of course there's
saliva present,

97
00:05:39,320 --> 00:05:43,310
and there may be little
filaments that form.

98
00:05:43,310 --> 00:05:45,830
Of course, we can see this.

99
00:05:45,830 --> 00:05:50,000
And then as we're inhaling, and
especially as we're exhaling,

100
00:05:50,000 --> 00:05:54,860
then these filaments will kind
of bend and they can break,

101
00:05:54,860 --> 00:05:57,110
and some of them
will be emitted.

102
00:05:57,110 --> 00:06:01,070
And in fact, these have
been recently visualized

103
00:06:01,070 --> 00:06:04,220
in great detail.

104
00:06:04,220 --> 00:06:06,710
And anyway, so
that's one mechanism.

105
00:06:06,710 --> 00:06:08,780
So it's these
filaments of saliva

106
00:06:08,780 --> 00:06:15,450
in this case could
be forming around--

107
00:06:15,450 --> 00:06:17,790
I'll just mention
this picture might

108
00:06:17,790 --> 00:06:19,380
be, for example, the mouth.

109
00:06:19,380 --> 00:06:22,080
We could also look at
the act of speaking.

110
00:06:22,080 --> 00:06:26,430
We will discuss in detail
later in this course

111
00:06:26,430 --> 00:06:29,100
that the emissions of
infectious droplets

112
00:06:29,100 --> 00:06:32,130
is very strongly correlated
with vocalization.

113
00:06:32,130 --> 00:06:34,860
If you're speaking,
there's many more emissions

114
00:06:34,860 --> 00:06:37,300
than when you're just
simply breathing,

115
00:06:37,300 --> 00:06:39,480
and when you're speaking
in a louder volume

116
00:06:39,480 --> 00:06:42,600
or when you're singing,
that rate of emission

117
00:06:42,600 --> 00:06:44,850
goes up very significantly.

118
00:06:44,850 --> 00:06:46,980
So there's clearly emissions
related to the vocal--

119
00:06:49,620 --> 00:06:55,140
the voicebox and to the
vocal folds in the glottis,

120
00:06:55,140 --> 00:06:57,500
which is basically the voicebox.

121
00:06:57,500 --> 00:07:04,950
So what that looks like is if
you take kind of a side view,

122
00:07:04,950 --> 00:07:06,470
there are these--

123
00:07:06,470 --> 00:07:07,890
as a cross-sectional
view, there's

124
00:07:07,890 --> 00:07:10,400
these folds where the
air is flowing through,

125
00:07:10,400 --> 00:07:13,290
let's say, in this direction.

126
00:07:13,290 --> 00:07:16,290
And these are kind
of waving together.

127
00:07:16,290 --> 00:07:18,090
They're vibrating where
the frequency could

128
00:07:18,090 --> 00:07:19,860
be, for example,
100 hertz depending

129
00:07:19,860 --> 00:07:24,360
on the tone of your speech
and the type of vowels

130
00:07:24,360 --> 00:07:27,720
you're making or other sounds.

131
00:07:27,720 --> 00:07:31,130
And again, what we
have is that some here

132
00:07:31,130 --> 00:07:35,420
are saying this might
be in the glottis.

133
00:07:35,420 --> 00:07:38,780
This could be the vocal folds.

134
00:07:38,780 --> 00:07:43,270
And this is basically
the voicebox,

135
00:07:43,270 --> 00:07:45,490
is more colloquial term.

136
00:07:45,490 --> 00:07:47,770
And as the air is
flowing through there,

137
00:07:47,770 --> 00:07:49,159
this part is vibrating.

138
00:07:49,159 --> 00:07:52,450
So there's some kind
of maybe motion.

139
00:07:52,450 --> 00:07:54,280
I'll just kind of
indicate like this

140
00:07:54,280 --> 00:07:57,850
just that this is kind
of shaking and vibrating

141
00:07:57,850 --> 00:07:59,440
and coming together.

142
00:07:59,440 --> 00:08:01,810
And of course,
there's also mucus

143
00:08:01,810 --> 00:08:05,240
and other liquids that are
here lining all these things.

144
00:08:05,240 --> 00:08:07,600
And when those folds
come close together,

145
00:08:07,600 --> 00:08:11,230
they touch each other, and they
can pull apart and again form

146
00:08:11,230 --> 00:08:14,470
these filaments that can
break up and generate

147
00:08:14,470 --> 00:08:17,950
droplets that will be
emitted of different sizes.

148
00:08:17,950 --> 00:08:21,350
Now, one thing to notice
is the length scale,

149
00:08:21,350 --> 00:08:24,520
so the mouth when it's opening
might have a length scale

150
00:08:24,520 --> 00:08:26,980
obviously on the order of maybe
centimeters but more likely

151
00:08:26,980 --> 00:08:29,180
millimeters in the regions
where there could actually

152
00:08:29,180 --> 00:08:31,150
be emissions of droplets.

153
00:08:31,150 --> 00:08:34,120
If we look at the vocal
cords, that scale is also

154
00:08:34,120 --> 00:08:36,460
going to be millimeters, but
when the vocal cords really

155
00:08:36,460 --> 00:08:37,990
come together and
pull apart, we might

156
00:08:37,990 --> 00:08:39,820
be looking at scales that
are much smaller than that.

157
00:08:39,820 --> 00:08:41,770
So some of these filaments
that are breaking up

158
00:08:41,770 --> 00:08:44,080
could be significantly
smaller, and so

159
00:08:44,080 --> 00:08:47,760
vocalization may lead to
droplets that are quite a bit

160
00:08:47,760 --> 00:08:48,260
smaller.

161
00:08:48,260 --> 00:08:50,550
In fact, in the case of the
mouth, as I just mentioned,

162
00:08:50,550 --> 00:08:54,130
the sort of length scale might
be of order of millimeters

163
00:08:54,130 --> 00:08:56,890
for the filaments
that are breaking up.

164
00:08:56,890 --> 00:08:59,680
And the size of the
droplets R might

165
00:08:59,680 --> 00:09:06,140
be on the order of 10 to
100 microns or even bigger,

166
00:09:06,140 --> 00:09:06,640
actually.

167
00:09:06,640 --> 00:09:09,880
In fact, it can even
go up to-- well,

168
00:09:09,880 --> 00:09:12,500
maybe not quite millimeters, but
in the case of, let's say when

169
00:09:12,500 --> 00:09:14,960
you're coughing or spitting,
certainly you are spitting out

170
00:09:14,960 --> 00:09:16,630
millimeters, but
it could be even--

171
00:09:16,630 --> 00:09:18,250
maybe I'll put even
here 1 millimeter

172
00:09:18,250 --> 00:09:21,040
as sort of a kind of upper
bound on the types of droplets

173
00:09:21,040 --> 00:09:22,840
that you could be emitting.

174
00:09:22,840 --> 00:09:25,930
In the case of the voicebox, our
length scale's a bit smaller.

175
00:09:25,930 --> 00:09:28,930
It might be on the order
more like of 100 microns

176
00:09:28,930 --> 00:09:31,430
for these filaments
that are breaking up.

177
00:09:31,430 --> 00:09:33,920
And the radius of droplets
that you're going to form

178
00:09:33,920 --> 00:09:35,510
are going to be
smaller, and they

179
00:09:35,510 --> 00:09:38,510
might be ranging
more in the 1 to 10

180
00:09:38,510 --> 00:09:41,510
micron range or
possibly larger, again,

181
00:09:41,510 --> 00:09:42,560
depending on the details.

182
00:09:42,560 --> 00:09:44,750
If you're coughing and
there's a lot of mucus here,

183
00:09:44,750 --> 00:09:49,260
certainly you could get maybe
larger than that as well.

184
00:09:49,260 --> 00:09:52,610
So breakup of filaments
is a primary mechanism

185
00:09:52,610 --> 00:09:56,760
of drop formation, especially
in the upper respiratory tract.

186
00:09:56,760 --> 00:09:58,970
Now what about in the
lower respiratory tract?

187
00:09:58,970 --> 00:10:00,680
So that's really
referring to your lungs.

188
00:10:11,050 --> 00:10:14,890
So in the lower
respiratory tract,

189
00:10:14,890 --> 00:10:21,060
there is significant
evidence and also at least

190
00:10:21,060 --> 00:10:23,010
qualitative theories
and to some extent

191
00:10:23,010 --> 00:10:27,880
quantitative theories showing
that the main mechanism is not

192
00:10:27,880 --> 00:10:31,850
so much the breakup of
filaments in a flow,

193
00:10:31,850 --> 00:10:39,550
but rather the bursting
of filaments of mucus

194
00:10:39,550 --> 00:10:42,760
but in much smaller
domains, where it's not

195
00:10:42,760 --> 00:10:45,310
so much that the fluid is
whipping by and breaking apart

196
00:10:45,310 --> 00:10:47,350
the droplets, but it's simply
breaking up due to surface

197
00:10:47,350 --> 00:10:47,890
tension.

198
00:10:47,890 --> 00:10:50,830
Just that it's this
instability kind

199
00:10:50,830 --> 00:10:53,490
of like in a dripping faucet
or a stream of liquid when you

200
00:10:53,490 --> 00:10:55,660
start to stretch it out and
let surface tension act,

201
00:10:55,660 --> 00:10:57,160
it kind of squeezes
down, eventually

202
00:10:57,160 --> 00:10:58,190
wants to make droplets.

203
00:10:58,190 --> 00:11:00,130
So that's kind of the
rupture of a film.

204
00:11:00,130 --> 00:11:03,590
Under surface tension, it's
more likely to be the mechanism.

205
00:11:03,590 --> 00:11:09,700
And so this is kind of
maybe more generally

206
00:11:09,700 --> 00:11:18,690
can be thought of as an
elastocapillarity instability

207
00:11:18,690 --> 00:11:32,130
of mucosal films,
specifically in the deepest

208
00:11:32,130 --> 00:11:35,940
part of the lungs and
in the smallest passages

209
00:11:35,940 --> 00:11:43,170
during during inhaling
in the bronchioles

210
00:11:43,170 --> 00:11:45,450
and also, to some
extent, in the alveola.

211
00:11:51,740 --> 00:11:58,450
During inhaling, that's when
the breakup is happening,

212
00:11:58,450 --> 00:12:00,400
and then any droplets
that are creating,

213
00:12:00,400 --> 00:12:03,670
some may deposit on the walls
of the respiratory tract,

214
00:12:03,670 --> 00:12:06,490
but some fraction of them
will be swept back out again.

215
00:12:06,490 --> 00:12:08,650
So let me explain this a
little bit more detail.

216
00:12:08,650 --> 00:12:12,400
I should also mention
this mechanism is also

217
00:12:12,400 --> 00:12:21,620
referred to as the bronchial
film burst hypothesis.

218
00:12:24,280 --> 00:12:27,820
And I say it's a hypothesis
because despite the fact

219
00:12:27,820 --> 00:12:30,310
that there's been a lot
of study of the droplets

220
00:12:30,310 --> 00:12:33,190
that are produced by
different forms of respiration

221
00:12:33,190 --> 00:12:35,920
and some theoretical
modeling, it's

222
00:12:35,920 --> 00:12:39,580
difficult to actually observe
this process occurring

223
00:12:39,580 --> 00:12:40,910
in the body.

224
00:12:40,910 --> 00:12:42,430
And so it's still--

225
00:12:42,430 --> 00:12:45,560
it's a hypothesis that
people are still studying.

226
00:12:45,560 --> 00:12:47,350
So what we're
thinking of here is

227
00:12:47,350 --> 00:12:54,760
if we zoom in to a bronchiole,
which is a passage that looks

228
00:12:54,760 --> 00:12:57,880
maybe something like this,
it's like basically it's

229
00:12:57,880 --> 00:12:59,470
a flexible tube.

230
00:12:59,470 --> 00:13:01,660
And the smallest
ones of these now

231
00:13:01,660 --> 00:13:05,540
are getting down to the
scale of 100 microns or so.

232
00:13:05,540 --> 00:13:07,750
So it is kind of like a
typical length scale here of,

233
00:13:07,750 --> 00:13:13,170
let's say for the radius,
might be 100 microns or less.

234
00:13:13,170 --> 00:13:17,160
And these of course are
lined with mucus as well.

235
00:13:17,160 --> 00:13:19,620
And in some places,
there's a bridge.

236
00:13:19,620 --> 00:13:28,780
So it's kind of like there's
almost like bubbles of air

237
00:13:28,780 --> 00:13:30,490
with sort of little
bridges of mucus.

238
00:13:30,490 --> 00:13:32,470
In fact, you may actually
have even some places

239
00:13:32,470 --> 00:13:36,440
where the passage after exhale
has completely collapsed.

240
00:13:36,440 --> 00:13:38,290
And so maybe some parts
of it are touching.

241
00:13:38,290 --> 00:13:40,150
Others are not touching.

242
00:13:40,150 --> 00:13:46,510
But there's kind of these little
bridges of liquid or films,

243
00:13:46,510 --> 00:13:51,070
bronchial films that are kind of
extending across at least part

244
00:13:51,070 --> 00:13:53,620
or even all of those channels.

245
00:13:53,620 --> 00:13:56,470
Now, imagine we start
in this situation,

246
00:13:56,470 --> 00:13:57,700
and we start inhaling.

247
00:13:57,700 --> 00:14:00,490
And let's just say this is
the direction of inhaling.

248
00:14:00,490 --> 00:14:02,310
Let's imagine that
the alveola, which

249
00:14:02,310 --> 00:14:05,970
is kind of on the end
of this tube, and so

250
00:14:05,970 --> 00:14:08,560
let's see what happens
if we start inhaling.

251
00:14:12,990 --> 00:14:16,120
So for inhaling, then
the air is flowing in.

252
00:14:16,120 --> 00:14:17,580
And so the first
thing that happens

253
00:14:17,580 --> 00:14:21,240
is that these bubbles are
going to start essentially--

254
00:14:21,240 --> 00:14:23,850
this film is essentially
going to be pushed.

255
00:14:23,850 --> 00:14:27,180
As that liquid is being pushed,
we have some flows occurring.

256
00:14:27,180 --> 00:14:28,950
There's some recirculation
flows in there.

257
00:14:28,950 --> 00:14:32,430
Also, there's interaction with
the elastic or stretchy walls,

258
00:14:32,430 --> 00:14:35,980
which are soft,
of the bronchiole,

259
00:14:35,980 --> 00:14:37,710
and so it can expand.

260
00:14:37,710 --> 00:14:40,200
So if you go to
the next step, you

261
00:14:40,200 --> 00:14:45,590
may find as you continue
inhaling that now this tube has

262
00:14:45,590 --> 00:14:46,460
expanded a bit.

263
00:14:46,460 --> 00:14:47,840
So it might look more like this.

264
00:14:51,360 --> 00:14:57,290
And then this, to
some extent, this film

265
00:14:57,290 --> 00:14:58,790
would start to get stretched.

266
00:15:05,690 --> 00:15:09,590
And then at some point, as
this thing is trying to open,

267
00:15:09,590 --> 00:15:13,190
and also it's under some
flow, but it's going to burst.

268
00:15:13,190 --> 00:15:15,350
And this bursting again
is not quite the same

269
00:15:15,350 --> 00:15:17,970
as this situation because
the flows are much slower.

270
00:15:17,970 --> 00:15:21,500
So here, these flows are often
at so-called high Reynolds

271
00:15:21,500 --> 00:15:24,170
number, as we'll talk
about later in this class.

272
00:15:24,170 --> 00:15:27,940
High Reynolds number refers
to the tendency of the flow

273
00:15:27,940 --> 00:15:31,120
to become unstable and for
inertial effects and momentum

274
00:15:31,120 --> 00:15:32,330
of fluid to become important.

275
00:15:32,330 --> 00:15:35,370
At the scale of the mouth or the
nose or even the vocal cords,

276
00:15:35,370 --> 00:15:37,460
there can be significant
inertial effects and very

277
00:15:37,460 --> 00:15:39,680
complex flows.

278
00:15:39,680 --> 00:15:41,180
On the other hand,
when we get down

279
00:15:41,180 --> 00:15:43,040
to the smallest
channels in the lungs,

280
00:15:43,040 --> 00:15:44,420
and especially
when we reach kind

281
00:15:44,420 --> 00:15:47,030
of the dead end, these
sort of the alveolas, which

282
00:15:47,030 --> 00:15:49,610
is basically a bunch
of little sacs,

283
00:15:49,610 --> 00:15:55,200
they're kind of at the end here,
then it's kind of a dead end.

284
00:15:55,200 --> 00:15:58,930
There can't be any like very
fast flow through that system.

285
00:15:58,930 --> 00:16:01,840
And so it's actually a low
Reynolds number situation.

286
00:16:01,840 --> 00:16:06,390
So we're not talking about
turbulent flows or sprays

287
00:16:06,390 --> 00:16:07,950
of liquid at high
Reynolds number.

288
00:16:07,950 --> 00:16:10,080
Instead, we're
talking about films

289
00:16:10,080 --> 00:16:12,630
that are getting stretched out,
and then they simply break up

290
00:16:12,630 --> 00:16:15,330
under the effect of
capilarity, which

291
00:16:15,330 --> 00:16:16,580
refers to surface tension.

292
00:16:16,580 --> 00:16:18,210
So basically, when
you expose a surface

293
00:16:18,210 --> 00:16:19,790
and stretch out a
liquid film, it just

294
00:16:19,790 --> 00:16:22,470
tends to break up into
little droplets, basically

295
00:16:22,470 --> 00:16:25,000
in order to minimize its energy.

296
00:16:25,000 --> 00:16:27,360
So what we'll see
here is that maybe one

297
00:16:27,360 --> 00:16:28,860
of these films over
here has already

298
00:16:28,860 --> 00:16:32,370
burst and will lead
to some droplets that

299
00:16:32,370 --> 00:16:34,120
are being created.

300
00:16:34,120 --> 00:16:42,650
So this bursting of the film
is what leads to the droplets.

301
00:16:42,650 --> 00:16:44,630
And when you're
inhaling, those get swept

302
00:16:44,630 --> 00:16:46,010
a little further downstream.

303
00:16:46,010 --> 00:16:49,070
Some of them may deposit on the
walls and go back into the film

304
00:16:49,070 --> 00:16:51,680
and coalesce into the
film, but others will

305
00:16:51,680 --> 00:16:53,540
remain suspended in the air.

306
00:16:53,540 --> 00:16:56,750
And now when you
exhale, you start

307
00:16:56,750 --> 00:16:58,190
pushing back the other way.

308
00:16:58,190 --> 00:17:04,500
The tube is more
open now, and we

309
00:17:04,500 --> 00:17:07,440
may have a situation
like this where there's

310
00:17:07,440 --> 00:17:09,410
no more sort of
spanning films left,

311
00:17:09,410 --> 00:17:11,250
but there's some fraction
of these droplets.

312
00:17:11,250 --> 00:17:14,640
A few of them may have deposited
and coalesced on the surfaces,

313
00:17:14,640 --> 00:17:17,069
but they're going to start
getting blown out the other way

314
00:17:17,069 --> 00:17:17,569
now.

315
00:17:21,420 --> 00:17:23,730
And so these are the droplet
emissions right here.

316
00:17:29,520 --> 00:17:31,150
I'll just say it'll
eventually do that.

317
00:17:31,150 --> 00:17:33,000
So some fraction of these
will make it all the way out.

318
00:17:33,000 --> 00:17:34,850
Of course, those droplets
can deposit anywhere

319
00:17:34,850 --> 00:17:35,900
in the respiratory tract.

320
00:17:35,900 --> 00:17:38,040
In fact, some of them, if you're
breathing through your nose,

321
00:17:38,040 --> 00:17:40,080
may end up getting caught
in your nose, actually.

322
00:17:40,080 --> 00:17:42,000
And so there's an
exchange of fluids

323
00:17:42,000 --> 00:17:44,250
between the different parts
of the respiratory tract,

324
00:17:44,250 --> 00:17:46,620
but some fraction of those
droplets will get out.

325
00:17:46,620 --> 00:17:49,230
And then ultimately, when
you're finished exhaling,

326
00:17:49,230 --> 00:17:50,800
now the pressure is released.

327
00:17:50,800 --> 00:17:56,760
And this tube kind of relaxes
back to its original state

328
00:17:56,760 --> 00:17:59,730
where there's some mucus here
and there's some places maybe

329
00:17:59,730 --> 00:18:05,310
where it's closed and there's
these possibly spanning films

330
00:18:05,310 --> 00:18:07,860
in some places where
it's almost touching.

331
00:18:07,860 --> 00:18:10,650
So these are some of
the basic processes

332
00:18:10,650 --> 00:18:13,650
by which droplets are emitted.

333
00:18:13,650 --> 00:18:16,380
As you can see by the range
of different processes

334
00:18:16,380 --> 00:18:20,970
that are possible in
the human physiology

335
00:18:20,970 --> 00:18:22,350
that we've just
described, we can

336
00:18:22,350 --> 00:18:24,030
see there's a range
of droplet sizes

337
00:18:24,030 --> 00:18:27,420
that will depend on the
respiratory activity.

338
00:18:27,420 --> 00:18:30,230
Are you breathing lightly
because you're sleeping?

339
00:18:30,230 --> 00:18:32,040
Are you breathing
heavily at high speeds

340
00:18:32,040 --> 00:18:33,600
because you're exercising?

341
00:18:33,600 --> 00:18:35,670
Are you vocalizing
and generating

342
00:18:35,670 --> 00:18:38,730
droplets in a different
way in the larynx?

343
00:18:38,730 --> 00:18:40,680
All those activities
play a role.

344
00:18:40,680 --> 00:18:43,540
And also there are variations
between individuals.

345
00:18:43,540 --> 00:18:46,980
And finally, if a person is
sick and all these fluids

346
00:18:46,980 --> 00:18:50,940
I've sketched here as mucus
contain pathogens such as virus

347
00:18:50,940 --> 00:18:54,600
or bacteria, then of course
the degree of infection,

348
00:18:54,600 --> 00:18:57,780
the viral load or the
total amount of pathogen,

349
00:18:57,780 --> 00:19:00,600
the total amount of
bacteria plays a role

350
00:19:00,600 --> 00:19:02,880
as well in sort
of how infectious

351
00:19:02,880 --> 00:19:04,980
the emissions are
from breathing.

352
00:19:04,980 --> 00:19:06,870
But these are some of
the basic principles.

353
00:19:06,870 --> 00:19:10,200
And now we'll move on to ask,
what happens to those droplets

354
00:19:10,200 --> 00:19:15,230
after they leave the mouth
of the infected person?