The Wall Street Journal - 14.03.2020 - 15.03.2020

C4| Saturday/Sunday, March 14 - 15, 2020 **** THE WALL STREET JOURNAL.

Why Do

Raindrops

Make Bubbles?

EVERYDAY PHYSICS

HELEN CZERSKI

REVIEW

PHOTOS BY MICHAEL M. PHILLIPS/THE WALL STREET JOURNAL

SPRING SHOWERS

have arrived in London,

but I don’t really mind.

I find the splat of rain-

drops falling on pud-

dles a very comforting sound, one

that I associate with childhood and

the urge to immediately go and jump

in all the puddles I can find. As a

physicist who studies bubbles, I’ve

got a scientific interest in these pud-

dles too, because they sometimes

have bubbles floating on the top.

Raindrops are small—slightly less

than a 10th of an inch in diameter, in

a typical shower—but they often

generate bubbles that can be half an

inch across. However carefully you

watch, you’ll never see the physics at

work, since the whole process takes

around two hundredths of a second.

It’s a hidden part of our world that

happens right in the open.

Since the 1960s, scientists have

spent a surprising amount of time

studying water drops hitting pools of

water, and the list of ways in which

a drop can produce a bubble keeps

getting longer and more astonishing.

For example, when it rains on deep

water, the drop impact briefly cre-

ates a hole in the water’s surface,

and a small bubble may pinch off

from the bottom as the hole closes.

Every new bubble formed in this

way produces a faint sound, like a

bell being struck. The tone is only

just within the human hearing range,

about two octaves above the highest

note on a piano. But scientists have

experimented with using it to mea-

sure rainfall rate.

Bigger drops with a lower impact

speed generate bigger bubbles, and

these make a tone that’s well within

the human hearing range. (The Japa-

nese have a garden musical instru-

ment that uses and amplifies this

sound, called asuikinkutsu.)Really

tiny bubbles are created when a drop

falls slowly and traps air between it-

self and the pool.

The humble puddle is also the

theater for an even more elegant

process. The splash caused by an im-

pacting drop can create a circular

wall around the impact point. This is

called a crown, because the top of

the wall quite often breaks up into

droplets, making it look just like a

toy crown. The critical parameter is

the puddle depth. If it’s too shallow,

you get a splash but no bubble; if it’s

too deep, you might get a submerged

small bubble.

But when things are just right, the

splash rebounds off the bottom of

the puddle and the circular wall

reaches up even higher than usual.

The surface of the water acts as if

it’s an elastic sheet, so that as the

elongated crown travels upward, it

also bends inward to form a dome.

The edges collide in the middle but

stay intact, leaving a hemisphere of

air trapped underneath a watery

shell and creating a nice big bubble.

In a typical outdoor puddle there is

likely to be a bit of organic material

that will stick to the bubble’s sur-

face, helping the bubble to last long

enough for us to see it.

I find this process amazing. It’s so

delicate and requires such a specific

set of physical processes to happen,

yet it’s so common that we’ve all

seen the outcome. You and I could

watch puddles for hours and see

bubbles appear as if by magic. But

every bubble is a reminder that the

human eye can’t see everything

that’s going on and that nature be-

comes more beautiful as we under-

stand more about how it works.

It takes 0.02 seconds for a bubble to

form when a drop falls on water.

GETTY IMAGES

M

auritania, a vast

Muslim country

on the edge of

West Africa, has

adopted a unique

strategy to protect itself against an

Islamist militant threat that is get-

ting bigger and closer: It has turned

more than a quarter of its territory

into a no-go zone.

In 2018, Mauritania handed its

military control over 108,000

square miles of the Sahara, an

area roughly the size of Nevada.

Late last month, the Mauritanian

army gave The Wall Street Journal

unprecedented access to the exclu-

sion zone. Inside, troops have

great latitude to arrest or kill any-

one suspected of smuggling arms,

trafficking drugs or importing Isla-

mist violence. “If you’re in the mil-

itary zone and you don’t have per-

mission, you’re a suspected

terrorist,” said army Maj. Sidi Mo-

hamed Hedeid.

It was a move born of a fear that

bad things are coming Mauritania’s

way. The country straddles the Sa-

hara, and in the south, the Sahel, a

belt of semiarid land that stretches

just below the great desert. A wave

of Islamist militancy is overwhelm-

ing the countries of the Sahel. Affil-

iates of al Qaeda and Islamic State

killed more than 4,000 people in

Burkina Faso, Niger and Mali last

year in some 800 attacks.

Deep inside Mauritania’s exclu-

sion zone is a military outpost

called Lemgheity, or “covered well.”

The name comes from a water

source that travelers braving the

Sahara could find there. No village

or trading post is nearby. The mili-

tary stations its troops here be-

cause it’s a crossroads for nefarious

actors coming from Western Sa-

hara, Algeria and Mali.

Just over the Mali border, hun-

dreds of square miles of desert are

controlled by fighters from Jama’at

Nasr al-Islam wal Muslimin, or

JNIM, an umbrella group formed by

three al Qaeda allies. Along ancient

caravan routes, the lines blur

among militants, arms traffickers,

camel herders and drug mules. At

two wells long used by travelers

and herders, al Qaeda fight-

ers now meet and control

who gets to drink, the Mau-

ritanians say.

“As long as our neigh-

bors aren’t in charge of

their territory, we have to

take extra steps to protect

ours,” said Col. Siyed El

Asry, deputy commander of

the military zone, after in-

specting his troops at

Lemgheity.

From Mali, a rocky

spine called El Hank, or

“jaw,” extends more than

250 miles southwest

across Mauritania; its

boulders of dark gray

shale have survived 500

million years of erosion

around the escarpment. Before

GPS navigation, travelers used El

Hank as their guidepost through

the emptiness of the desert.

The Mauritanian army splits

Lemgheity’s forces between two

adjacent positions atop El Hank.

Stone guard towers offer clear

fields of fire at likely approaches

from Algeria, Mali and Western

Sahara. Two lines of rocks mark

an airstrip on the otherwise fea-

BYMICHAELM.PHILLIPS

to persuade prisoners that vio-

lence is against their religion.

In addition to training comman-

dos, the U.S. has donated two

Cessna surveillance aircraft that

can transmit live footage of enemy

positions. Washington applauds

Mauritania’s anti-militant zeal,

while also criticizing the country’s

human-rights record. The State De-

partment has suspended some mil-

itary aid because of Mauritania’s

failure to crack down on human

trafficking and hereditary slavery.

In the capital, Nouakchott, ru-

mors persist that the government

secretly forged a nonaggression

pact with the militants that ex-

plains the lack of successful recent

attacks, an allegation that the

army hotly denies. “There is no

agreement with al Qaeda,” said

Maj. Hedeid. “They’re our enemy.

We’re killing them.”

The military believes the no-go

zone is the source of its success.

In the first two months of this

year, troops twice stopped sus-

pects in the desert. In mid-Febru-

ary, one of Mauritania’s Western

allies intercepted signals and

tipped off the Mauritanian army

that two suspicious pickups had

entered from Western Sahara and

were heading east through the

zone. Another truck that crossed

into Mauritania from Mali was

headed west to meet them.

A Mauritanian Super

Tucano ground-attack

fighter plane, bought from

Brazil, scrambled from an

air base on the edge of

the desert. “We watched

them second by second,”

said Col. Noueh. At the

rendezvous point, military

photos show, the Super

Tucano strafed one of the

trucks, which then ex-

ploded into orange flame,

releasing a rooster’s tail

of oily black smoke that

rose like a beacon into the

open sky. Ground troops

swarmed the charred re-

mains, seized the other

trucks and a pile of ma-

chine guns and rifles, and

arrested six suspects, sev-

eral in military-style camouflage

pants and one with a leg wound.

The soldiers treated the injured

man, tied blindfolds around the

prisoners’ heads and lined them

up next to the contraband for pho-

tos. (The army has yet to release

information about their nationali-

ties or intentions.)

On the last day of February,

with the Saharan sun at its hot-

test, the troops at the Lemgheity

outpost retreated to a stone build-

ing, plopped down on cushions on

a deep-red floral rug and tore into

a shared plate of stewed mutton

and soft white bread. They slurped

shot glasses of sugary tea.

On a television hooked up to a

satellite dish, the men watched as

Taliban and U.S. negotiators signed

an accord intended to end the 18-

year conflict in Afghanistan, which

began with al Qaeda’s hijacking of

four American jetliners on 9/11. It

was a surreal moment in the ever-

shifting war against Islamist ex-

tremists, combatants on one front

watching those on another make

peace. “I’d like the same thing here

in the Sahel,” said Maj. Hedeid.

tureless desert floor below, where

searing heat distorts the air and

creates a mirage of shimmering

pools of water. A Mauritania flag,

green and red with a yellow star-

and-crescent, flutters in front of a

tiny stone-and-mud mosque.

In 2005, theoutpost was the

scene of the bloodiest militant at-

tack in Mauritania in recent years.

Some 150 fighters poured over the

border from Mali, and at dawn on a

June day, seized the high ground on

El Hank. The militants surprised

Lemgheity’s poorly trained, poorly

armed 50-man garrison. Seventeen

soldiers were killed and were bur-

ied in a mass grave downhill from

the outpost they died failing to de-

fend. Today, soldiers pray there,

holding their hands palms-upward

in the traditional display of faith.

With each subsequent attack af-

ter 2005, militants ventured fur-

ther out of the desert and toward

towns and cities. Three troops

died in a 2007 attack. The follow-

ing year, militants captured and

decapitated 12 soldiers. In 2011,

militants believed to be from al

Qaeda in the Islamic Maghreb

crossed into Mauritania’s south

and kidnapped a gendarme.

But the 2011 attack was the last

that Mauritania has suffered. In

2009, the military formed an elite

Special Intervention Group, which

has since received advanced infan-

try and logistical training from

U.S. Marines. The commandos now

man seven outposts in the exclu-

sion zone; the military won’t dis-

close the size of the force there.

Mobile units spend up to two

weeks at a time roaming the hin-

terlands in desert-tan Toyota Land

Cruiser pickup trucks hunting for

infiltrators.

It’s not a free-fire zone. The

army must have reasons to suspect

that the pickup trucks they spot

crossing the desert are carrying

terrorists or smugglers. But officers

have wide discretion to arrest or

kill suspects without military-zone

travel permits. “We scrutinize tar-

gets very closely,” said Col. Ahmed

Salem Noueh, the army’s chief of

planning. “Sometimes the enemy

comes as a trader. Sometimes he

comes as a herder.”

Commandos also dig wells and

provide medicine for desert dwell-

ers along the Mali border, accord-

ing to the Mauritanians. “The ter-

rorists would like to win the

hearts of the population,” said

Maj. Hedeid. “We want to beat

them to it.” Mauritanian officials

say that they hold jailhouse coun-

seling sessions with captured mili-

tants, in which Islamic scholars try

Mauritaniahasclosedoffawideswathofthe

SaharatoblockthepathofIslamistmilitants.

Insidea

No-GoZone

ForTerror

A deadly

2011 attack

was the last

that the

country has

suffered.

Mauritanian mobile units

practice tactics in the Sahara.

A Mauritanian commando

mans a guard tower

at the Lemgheity outpost.

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