The Fabrication of PDMS- based Functional Surface

The Fabrication of PDMS-based Functional Surface

Mimicing the Namib Desert Beetle Back for

Roll uping Water Vapor in the Air

Abstraction—This paper nowadayss?functional surface

designed for roll uping H2O vapour, which is combined of PDMS

and the micro degree hydrophilic metal metal ball. To copy

biological construction, the metallic balls are designed as hydrophilic

portion and PDMS substrate as hydrophobic construction. The ball is

used as nucleation centres to absorb H2O vapour in the air to

signifier droplet. Then, with the bead bit by bit turning up, it will

fall off from the ball by gravitation. The hydrophobic belongings of

PDMS could advance toppling of bead more swimmingly without

adhesion. This device is fabricated by micromachining procedure,

where utilizing PMMA as contrary cast and depositing fluorocarbon

movies on the PDMS to foster heighten the hydrophobic belongings.

The trial consequences prove that this thought and H2O aggregation can be

achieved evidently.

Keywords—Desert Beetle ; hydrophilic and hydrophobic portion ;

H2O capturing

I. Introduction

With bit by bit turning of planetary populations, people is

confronting the deficit of limited H2O resources, particularly in

African and Asiatic country. Furthermore, the air pollution will do

this state of affairs even worse. The H2O resources have become an

of import factor which impacts the development and advancement

of country’s economic system. Collecting H2O vapour from the air is a

executable solution in dry country. Water vapour will distill into

droplets on the cold surface, which could animate us manufacturing

the faster heat exchange surfaces. However, non merely excess

energy demand to be consumed to bring forth the temperature

differences, but besides efficiency of condensation is low [ 1 ] . At

nowadays, the most widely applied method in desert states is

utilizing H2O adsorbent to capture H2O vapour with some energy

ingestion in high temperature [ 2 ] . Another method is

inspired by spider. Many dews suspended along spider web

were shaped after raining. This leads people to weave big

web to roll up H2O vapour. And this method has been widely

used in drouth and dazed states [ 3 ] [ 4 ] . Although it is easy

to fabricate H2O roll uping web, the existent state of affairs is

hardly satisfactory and the rate of H2O aggregation can merely

make up to about 2 % in the mist due to the midst gauze wires

and great mesh country [ 5 ] . Actually, there is a good demo in the

nature, Stenocara is a beetling life in desert, shown inFig.1,

which has ability to roll up H2O by catching the mist in the

moist air to determine large droplets on its bumpy back surface. And

the large droplet can flux into Stenocara’ oral cavity when the

beetle’s organic structure sits at 23 grade to the land [ 7 ] . SEM

detecting proved that those macroscopic bumps have

highly smooth surface and present strong hydrophilic

belongings which can assist capture vapour. The other country

comprising of smaller bumps arranged in hexangular array in

micro degree has strong hydrophobic belongings.

In this paper, we present a biomimick construction of this beetle

for roll uping H2O vapour. The surface is combined of PDMS

and the micro degree hydrophilic metal metal ball which is

fabricated by micromachining procedure. First we discuss of this

design in inside informations, so the fiction procedure, following with

proving apparatus and analysis of consequences.

II. THEORETICAL ANALYSIS

The metallic balls are chosen as vapour capturing points with

roughened PDMS lodging fluorocarbon movies chosen as

hydrophobic portion [ 8 ] . The mark surface we will fabricate is

showed inFig.2. In this subdivision, detailed analysis will be

nowadays to explicate why abovementioned stuffs have been

selected as the constituents of our surface and the whole

procedure of H2O collection. The whole procedure can be divided

into two stairss. The first 1 is Vapor Capturing. In this measure,

the hydrophilic metallic parts play a critical function in pulling

H2O molecular. As we know, a great trade of disorderly

Hydrophobic country

Hydrophobic

bump

Fig.1 Morphology of Stenocara’s back [ 6 ]

978-1-4799-4726-3/14/ $ 31.00 © 2014 IEEE 417

Proceedings of the 9th IEEE International

Conference on Nano/Micro Engineered and Molecular Systems

April 13-16, 2014, Hawaii, USA

thermic motion of infinite H2O molecules is traveling on

inside the ambiance. When there is a surface bing, the

H2O molecular have a certain chances to clash with the

surface in certain temperature.

A sort of stuff could bring forth a possible field with a

fixed strength near the surface. If the field have stronger

strength to pin down gas molecular in the air, the stuff have

higher surface energy. On the status that the surface

composed of metallic oxide have strong ability to organize

electromagnetic attractive interaction to pin down the H2O molecule,

it will determine a monolayer of H2O molecular after a period of

clip. Therefore, we can use BET Theory as to explicate this

procedure.Fig.3shows us the how the soaking up which is

represented with “V/Vm” of one sort of gas molecular alterations

with the fluctuation of the ratio of vapor force per unit area to its saturated

vapor force per unit area, called Adsorption Isotherm [ 9 ] .

Voltmrepresents soaking up when the surface is wholly

covered by gas molecular. The whole procedure can be separated

into two parts. Before making to the point “A” , the whole

surface hasn’t been wholly covered by the gas molecular.

The BET theoretical account could be replaced with Langmuir theoretical account [ 11 ] .

The procedure could be expressed in Langmuir Equation as

Formula 1.1 showed.Phosphorusrepresents vapor force per unit area. In the

2nd portion, the stuff surface will wholly be covered by

a monolayer of gas molecular when the vapour force per unit area could

maintain on increasing in a certain scope. When the whole procedure

enters the 3rd portion. The material surface and gas molecular

monolayer will pull more and more molecular. If the gas is

H2O vapour, it will turn a H2O droplet. The 3rd portion can be

expressed with Formula 1.2. Under normal fortunes, the

ratio of vapor force per unit area P to saturated vapor force per unit area P0 is

normally 5 % -35 % [ 9 ] . And this ratio is besides called Relatively

Humidity ( RH ) .

The figure of beds of attracted H2O molecular could

range to eternity when the RH in certain temperature is in stopping point

to 100 % theoretically. In the clip of H2O vapour in the air

trapped by multilayer, there ever exists some H2O

molecular get awaying from surface. In the terminal, the two procedures

will make dynamic equilibrium. Otherwise, due to the finite

country of stuff surface, the H2O multilayer must organize a small

droplet. If the RH doesn’t range to impregnation point, the outmost

H2O molecular will get away from surface in malice of the high

ability to absorb H2O molecular in the high energy surface.

Therefore, the droplets can be formed. If the RH has reached to

impregnation, which is 100 % , the interface between H2O

molecular multilayer and the vapour in the air will make to

dynamic equilibrium. For the clip being, the presence of H2O

droplets could be maintained on the status that the droplets

hold formed antecedently. Or the procedure of droplets organizing

won’t have an consequence on the fluctuation of concentrated RH. If RH is

less than 100 % and the visual aspect of droplets takes topographic point,

there must be fog bing in the air. It is the fog, non high RH,

that makes small droplets turn bigger and bigger. Therefore, if

the H2O multilayer grow continually to organize droplets. Some

environmental status must be satisfied. Two of the most

of import factor are the generating of fog and air current. All the

inside informations will be introduced in Experimental Test portion. The following

measure of the whole water-collecting procedure is droplets turn overing

down. When the droplets have grown large plenty that the

interaction between the bottom beds of the droplets and

material surface is non strong plenty to keep them, they will

turn over down. And the hydrophobic portion plays an of import function in

this procedure. On history of the low surface energy, the H2O

vapour won’t condense into droplets in malice of some H2O

molecular attached to the surface in the H2O capturing procedure.

After the depositing of C4F8 movie, the chapped surface can

be extremely hydrophobic. When the surface sits at a certain Angle

with the land, the droplets will turn over down the surface in

spherical form due to the hydrophobic belongings of stuff

surface.

III. Fabrication Procedure

The PDMS which is equipped with reinforced

hydrophobic belongings by surface roughening and depositing

C4F8 movie could advance toppling of droplets more swimmingly

without adhesion [ 8 ] . Some metal balls each of which is

deposited a bed of Titanium with the thickness of 1?m and

has low surface energy to capture H2O vapour in the moist air

are used to determine vapour capturing points. The spherical bumps

are made of metallic balls. Actually, if the metallic portion of a

ball is exposed in a big country, the spherical droplet will non fall

Fig.2 ( a ) Basic construction of functional surface ( B ) Water collection of

functional surface

( a ) ( B )

V/Vm

Point “A”

P/P0 ( RH )

100 %

5 % -35 %

Fig.3 Absorption with the fluctuation of RH based on BET theoretical account [ 10 ]

1.1

1.2

Formula 1 The quantitative look of soaking up

418

off the surface due to the great electrostatic force and Van Der

Waals interaction. The great mass of each ball is fixed in the

PDMS organic structure with a little portion exposed to the air. This design is

accomplished by the application of a series of Micro

engineerings as showed in theFig.4.

MMA stuff has many first-class public presentations, such as

low-cost, easy processing, chemical stableness, etc. First, the

sand blaring procedure is applied to do the PMMA surface

roughened so as to beef up the raggedness of the PDMS.

Then, some holes which are used to repair the metallic balls and

do a little portion exposed to the air are drilled through the

PMMA. Till so, we have gained the PMMA cast. Pouring

the PDMS solution to cover all the chapped surface of

PMMA and metallic balls after puting the metallic balls in the

holes opening. We will obtain the concluding biomimetic stuff

after heat-treated to guarantee the completely hardening of

PDMS in the temperature of 65oˆZ? . After skining off from the

PMMA cast, the semi-finished biomimetic stuff will be

deposited a bed of C4F8 movie executed by Ion Coupled

Plasma equipment. Then, the same PMMA which is besides

deposited C4F8 bed on the chapped surface to protect the

C4F8 movie on the PDMS from being lost will once more be put on

the PDMS surface with each ball in the corresponding hole

gap. C4F8 bed deposited on the open portion of metallic

balls could be clean up by processing of O2 plasma. In the terminal,

the concluding biomimetic stuff is obtained. And the contrary

theoretical account and sample are showed inFig.5.It turns out that the

fabricated biomimetic surfaces have highly little sum of

PDMS residues necessarily by using PMMA cast because

of the PDMS fluxing into the spread resulted from the defective

lucifer between the holes and balls. This sort of staying will

hamper H2O molecules attaching to the hydrophilic points in

the procedure of the water-capturing belongings trial. These residues

can be eliminated manually. The diameter of metallic balls is

3mm and the size of the holes of PMMA contrary cast is about

1mm uniformly.

IV. EXPERIMENT AND ANALYSISE

In virtuousness of the elephantine temperature differences between

daylight and dark in Namib Desert, fogs that originate offshore

from the hit of the cold Benguela Current and warm air

from the Hadley Cell make a fog belt that often envelops

parts of the desert. Coastal parts can see more than

180 yearss of thick fog a twelvemonth and much of the life rhythm of

animate beings like Stenocara in the Namib relies on these fogs as the

chief beginning of H2O [ 12 ] . Here, the environment similar with

desert where Stenocara lives is imitated. First, wet air was

flowed toward the fabricated biomimetic surface by an air fan.

The increasing of the RH and fog generating is executed by an

air humidifier. When the increasing of the fog generated by the

humidifier is equal to that of fluxing to the ambiance, RH

will keep changeless about over a long period of

clip. RH is in the scope of 60 % -65 % with the temperature in

the scope of 16 oˆZ? -16.5 oˆZ? .For the intent of fasting the

condensation procedure, an air fan is for speed uping the speed

of the mixture of dry air and vapour. The faster the speed is,

the faster the droplets grow. The samples are laid up on a base

with its angle sitting at 23°to the land merely like Stenocara’s

organic structure demoing when roll uping vapour. All the experimental

devices are put in a plastic box which can semi-isolate inner

environment from outside ambiance and aid keep

changeless RH and temperature. To foreground the consequence of

capturing and turn overing down, several controlled trails are made

to exemplify belongings of different parts of our design. In general,

metal has excellent hydrophilic belongings because of the

being of natural oxide bed. In this paper, the Ti has

been chosen to fabricate our vapour capturing parts. After

being opposed to the ambiance for a long period of clip, the

hydrophilic Ti dioxide would look on the surface of

Ti. First, the vapour capturing belongings of a Si

wafer deposited a thin bed of Ti was verified to do

clear that the metallic balls truly has ability to pull H2O

molecular.

It can be found out that a big amount of nucleating points

come to look on the wafer surface evidently. These points

act as karyon of condensation to pull more and more H2O

molecular to do it turn bigger and bigger as shown inFig.6.

( a ) ( B )

( degree Celsius ) ( vitamin D )

( vitamin E ) ( degree Fahrenheit )

( g )

( H ) ( I )

Fig.4 Micro-fabrication procedure based on PMMA contrary cast

( a ) prepared PMMA home base ( B ) Sand blasting ( degree Celsius ) punching through the

whole home base ( vitamin D ) Metallic balls placing ( vitamin E ) PDMS pouring ( degree Fahrenheit ) PDMS skining

( g ) C4F8 lodging on the PDMS and change by reversal cast ( H ) O2 plasma

barrage ( I ) Final sample

( a ) ( B )

Fig.5 ( a ) PMMA contrary cast and ( B ) concluding sample

419

Therefore, the H2O capturing portion could be made out of

Titanium so. The hydrophobic portion in our design has

highly hapless public presentation to capture vapour as we have

analyzed in the Theoretical Analysis. The contact angle of

roughened PDMS lodging fluorocarbon is larger than that of

ordinary PDMS, as showed inFig. 7 ( a ). Vapor capturing

public presentation have been showed inFig. 8 ( B ).

Finally, the public presentation of mark surface comprised of

the above two sorts of stuffs is tested. The proving consequence

shows us that the great beads of H2O signifier in the underside of the

hydrophilic metallic balls and some of them have rolled down

off the surface. In the interim, on the hydrophobic portion

distill some H2O droplets. There are two grounds which

can explicate this state of affairs. The first 1 is that the procedures of

surface roughening and fluorocarbon polymer lodging aren’t

really accurate so that the contact angle of hydrophilic portion is non

large plenty. The other ground is that the metallic balls are

arranged closely. And turbulency would bring forth among the

ball to alter way of vapor gesture and increase the action

clip of vapour and balls. This construction will do the form of

droplets near to sphere and advance the dew turn overing down the

surface. A sort of state of affairs should be prevented that if the

PDMS in the occasion linking metallic ball and other

PDMS portion may be peeled off in the procedure of acquiring rid of

PDMS covering the balls so that the breaking away of the droplets

from the balls encounter elephantine opposition. Final trial consequence is

showed inFig.8. There is a great H2O droplet on every

metallic ball be givening to turn over down.

V. SUMMARY

Mimicing bumps on the dorsum of the Stenocara in two parts,

hydrophilic portion to capture vapour in moist air and the other portion

hold super-hydrophobic belongings which help the big plenty

droplet turn over down. Metal normally has first-class hydrophilic

belongings on history of the being of metallic oxide which

could assist determine hydrogen bond between H2O molecular and

oxide. The BET surface assimilation theory is besides applied to analyse the

surface assimilation procedure. PDMS is chose as stuff to manufacture the

hydrophobic portion due to the features of flexible and

easy processed. In the trial, our design has satisfactory H2O

capturing public presentation similar with desert beetle’s map.

Recognition

This work is supported by the National Natural Science

Foundation of China ( Grand No. 91023045 and No. 61176103 ) ,

and Doctoral Program Fund ( No.20110001110103 ) .

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Fig.6 Water capturing consequence of Titanium

( a ) ( B )

Fig. 7 ( a ) Hydrophobic belongings and ( B ) vapour soaking up of chapped

PDMS lodging C4F8

Water droplet

Fig. 8 Concluding trial consequence of H2O capturing belongings of mark surface

420

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