Hydrophobic Details

8/10/2019 Hydrophobic Details

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Drop Shape Analysis : Drop shape analysis (DSA) is an image analysis method for determining thecontact angle from the shadow image of a sessile drop and the surface tension or interfacial tensionfrom the shadow image of a pendant drop Background: A drop is metered onto a solid sample (sessile drop) or is located at the tip of a needle(pendant drop). An image of the drop is recorded with the help of a camera and transferred to thedrop shape analysis software. A contour recognition is initially carried out based on a grey-scaleanalysis of the image. In the second step, a geometrical model describing the drop shape is fitted tothe contour. Contact angle: The contact angle is given by the angle between the calculated drop shape functionand the sample surface, the projection of which in the drop image is referred to as the baseline

Sessile drop with fitted contour (shown in green)

Several methods for determining the contact angle with different measuring ranges andapplications are available when modelling the drop shape:

Surface TensionThe surface tension is always calculated with the help of aYoung-Laplace fit to the contour of a pendant dropdetermined by image analysis. The image scale and thedifference in density between drop phase and bulk phasemust be known for this (for more information, see underpendant drop) .
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Measuring methods

A contact angle goniometer is used to measure the contact angle.

The static sessile drop method

The sessile drop method is measured by a contact angle goniometer using an opticalsubsystem to capture the profile of a pure liquid on a solid substrate. The angle formedbetween the liquid/solid interface and the liquid/vapor interface is the contact angle. Oldersystems used a microscope optical system with a back light. Current-generation systemsemploy high resolution cameras and software to capture and analyze the contact angle.Angles measured in such a way are often quite close to advancing contact angles.Equilibrium contact angles can be obtained through the application of well definedvibrations.

The dynamic sessile drop method

The dynamic sessile drop is similar to the static sessile drop but requires the drop to bemodified. A common type of dynamic sessile drop study determines the largest contactangle possible without increasing its solid/liquid interfacial area by adding volumedynamically. This maximum angle is the advancing angle. Volume is removed to produce thesmallest possible angle, the receding angle. The difference between the advancing andreceding angle is the contact angle hysteresis.

The dynamic sessile drop method

Dynamic Wilhelmy method

A method for calculating average advancing and receding contact angles on solids of uniformgeometry. Both sides of the solid must have the same properties. Wetting force on the solidis measured as the solid is immersed in or withdrawn from a liquid of known surface tension.Also in that case it is possible to measure the equilibrium contact angle by applying a verycontrolled vibration. That methodology, called VIECA, can be implemented in a quite simpleway on every Wilhelmy balance. [7]

Single-fiber Wilhelmy method

Dynamic Wilhelmy method applied to single fibers to measure advancing and recedingcontact angles.

Washburn's equation capillary rise method

Enables measurement of average contact angle and sorption speed for powders and otherporous materials. Change of weight as a function of time is measured .[8]
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Dynamic contact angleThe dynamic contact angle is the contact angle which occurs in the course of wetting (advancingangle) or de-wetting (receding angle) of a solid.

Background

The advancing angle in particular is used for investigating solid surfaces, as in doing so thecontact angle is measured at a surface which is repeatedly being re-wetted and also at severalpositions which are very close to one another. This minimises time effects, such as evaporation,and averages out local inhomogeneities.

The receding angle which occurs on de-wetting is usually smaller than the advancing angle. Thisdifference, referred to as hysteresis, is used as a measure of the roughness of the solid.

One criterion for deciding between dynamic and static contact angle is the technical wettingprocess observed. Dynamic processes, such as coating, are better modelled by means ofdynamic measurements. Static contact angles are often more meaningful for assessing quasi-static processes, e.g. bonding in semiconductor technology.

The speed chosen for dynamic measurements must not be too great in order to avoid falsifyingthe measurement by the introduction of mechanical energy. However, on the other hand, it mustnot be too small in order to rule out the time effects described above.

Methods

1) Increasing and reducing the volume of a sessile drop during drop shape analysis2) Measurement of the Wilhelmy contact angle by measuring the force on a vertically

immersed plate

3) Shape analysis of a drop which moves over the inclined surface of a solid
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Advancing angleThe advancing angle between a liquid and a solid is the contact angle which is produced in thecourse of the wetting process, in contrast to the static contact angle with which the contact areabetween liquid and solid is not changed from the outside during the measurement. Theadvancing angle - like the receding angle - is described as the dynamic contact angle .

Background

The advancing angle is frequently used for investigating solid surfaces, as in doing so the contactangle is measured at a surface which is freshly wetted and also at several positions which arevery close to one another. This minimises time effects, such as evaporation, and averages outlocal inhomogeneities.

Methods

Increasing the volume of a sessile drop during drop shape analysis . The volume of thedrop is increased by hand or with a motorised piston. At the same time, images arerecorded and evaluated. In practice, a drop of about 3-5 L is formed on the surface ofthe solid with the help of a syringe needle and then slowly enlarged. In doing so, theinterface migrates outwards.

At the beginning, the contact angle measured is not independent of the drop size, as thecontact with the needle affects the drop shape. Only after this stage can the advancing

angle be measured sensibly. Instruments: DSA100 , DSA30 , DSA25

Measurement of the advancing angle by increasing the volume

Measurement of the Wilhelmy contact angle by measuring the force on a verticallysuspended plate which is immersed and withdrawn during the measurement. Thereceding angle is measured at the same time using this method. Instruments: K100

Shape analysis of a drop which moves over the inclined surface of a solid. Instruments: DSA100 or DSA30 with tilting table
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Young-Laplace fitThe Young-Laplace-fit is a Method for determining the contact angle in drop shape analysis . The drop shape is analysed based on the shape of an ideal sessile drop , the surface curvature

of which results only from the force equilibrium between surface tension and weight.

Background

According to the Young-Laplace equation, with a curved liquid surface, there is a relationshipbetween the radii of curvature r 1 and r 2, the surface tension and the Laplace pressure p:

With a sessile drop under the effect of gravity, the curvature is affected by the hydrostatic pressure resulting from the weight of the liquid. At the same time, the radii of curvature of thesurface change as a function of height. With the Young-Laplace fit, parameters of an equationsystem which models the shape of the sessile drop are determined by means of numericalanalysis. The point of intersection of the modelled contour with the baseline (three-phase

point ) is determined in order to measure the contact angle.

In the case of a symmetrical, undistorted drop on a level, smooth surface, the Young-Lapace

fit gives the best correspondence between the theoretical drop shape and the real drop shape.In addition, the surface tension of the liquid can be calculated if the scale of the video imageof the drop and the density of the liquid are known.

As the shape of the drop deviates from the elliptical form for large contact angles andvolumes, in these cases the Young-Laplace fit is preferred to the conic section method . TheYoung-Laplace fit is not suitable for asymmetrical drops (e.g. when measuring the dynamiccontact angle ) or for samples which are not level (e.g. for measurements on a tilting table).

Three-phase pointIn the two-dimensional projection of a three-phase system, a three-phase point is a point which iscommon to the three phase boundary lines. In the frequently considered case of a sessile drop ona solid surface in air when measuring the contact angle, this is the transition point between theliquid/solid, liquid/gaseous and solid/gaseous boundary lines.
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Static contact angle The static contact angle is the contact angle with which the contact area between liquid and solid is notchanged from the outside during the measurement, in contrast to the dynamic contact angle which is

produced in the course of wetting (advancing angle ) or de-wetting (receding angle ).

Background

A pure liquid in the saturated vapor phase forms the Young contact angle on an ideal, chemically andtopologically homogeneous solid as described by Young's equation . In this case, the static and dynamiccontact angles would be equal.

With real solids, liquids and ambient conditions, the contact angle can vary as a function of time andlocation. Inhomogeneities and changes of wetting over time can therefore be detected by means of thestatic contact angle.

If, on the other hand, the effect of location and time is to be minimized, then the advancing angle is usuallymeasured.

A further criterion for deciding between static and dynamic contact angle is the technical wetting processobserved. Dynamic processes, such as coating, are better modelled by means of dynamic measurements.Static contact angles are often more meaningful for assessing quasi-static processes, e.g. bonding insemiconductor technology.

Measuring methods

Drop shape analysis on sessile drops : Analysis of the image of a sessile drop deposited on a solid.

Instruments: DSA100 , DSA30 , DSA25 , MobileDrop

Captive bubble method : Analysis of the image of an air bubble in a liquid beneath a solid.

Instruments: DSA100 , DSA30 , DSA25

Top-view distance method : The curvature of the drop surface associated with the contact angle is

measured using the distance between light spots which are reflected on the top of a drop surface.

Instrument: TVA100
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Roll-off angleThe roll-off angle is the angle of inclination of a surface at which a drop rolls off it. As a rule, it isused to characterize ultrahydrophobic surfaces with very high contact angles (>>90) where thedrop is approximately spherical. With smaller contact angles, although a drop can also movefrom the surface, it is usually initially deformed and then slides over the surface.

The roll-off angle is an empirical variable which is highly dependent on the particular measuringconditions, such as drop size and tilt speed.

Biomimetic things in nature


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1. Anti-fogging propertyThe steam fogs a mirror and a glass easily, because moisture in the air is cooled down andnumerous waterdrops form on the surface of the usual materials. However, utilizing the super-hydrophilic coating, the water cannot exist in the shape of a drop, but spreads flatly on thesuper-hydrophil ic surface. Figure 4. shows the dependence of the anti -fogging abil i ty on thecontact angle of water.

Recently, the opposite way to remove waterdrops from the surface of material has beenproposed. For example, super-hydrophobic coating is applied for windshield to remove thewaterdrops easily. However, super-hydrophobic surface cannot stop fogging of the windshieldunless the waterdrops is removed with wind power or vibration. On the contrary, the super-hydrophil ic surface never fogs on condit ion of no wind nor vibration

Ap pl yi ng the su per -hyd ro phi l ic phot oc ata lys t for a su rf ace of a mi rr or or a glas s, the y re ma inthe clear surface without fogging semipermanently. Figure 5. shows the difference of thefogging with steam between normal glass and the photocatalyst coated glass.

Though the normal glass fogs with steam, the super-hydrophilic glass remains transparent. Inthis way, the super-hydrophilic technology easily realizes the anti-fogging glass products andmirrors with low costs.

2. Self-cleaning by a rainfall

Usually, it is very difficult to remove oil from resins with water unless using detergent. While,utilizing the super-hydrophilic coating, oil can be easily removed from the surface, because the


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surface has so much stronger affinity with water than oil. Therefore, the oil on the super-hydrophilic surface is easily removed by soaking the material in water, as shown on Fig. 6.

In this way, oil can be easily removed only by washing with water without detergent.

In urban area, almost dirty marks on the exterior wall are mainly caused by the exhaust of

automobiles and factories, which are hydrophobic and oily. Utilizing the super-hydrophiliccoating, these hydrophobic dirty marks can be self-cleaned with a rainfall, and you can keepthe wall clean semipermanently.

Fig. 7 shows the difference of self-cleaning ability by a rainfall between the super-hydrophilic


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coated and normal silicone coated cement plates.

I t is obvious that the original plates remarkably get dir ty, while the super-hydrophil ic coatedplates remain almost clean. By the accelerated weathering test, the super-hydrophilic coatingcan keep the self-cleaning ability more than 3000 hours, which means the coating hasdurability of more than 10 years in the usual environment.Silicone sealant is used to seal the gap of aluminum panels and windows of a building.Because some components of the sealant are washed away with a rainfall, they soil the exteriorwalls. The super-hydrophilic coating also can be applied to keep off the hydrophobic dirtymarks caused by silicone sealant from the walls, as shown on Fig. 8.


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3. Other applications In addit ion to the above-mentioned applications, the super-hydrophil ici ty can be applied formany other products utilizing secondary properties. For example, the super-hydrophilic surfacedries quickly utilizing the property of that the water cannot flatly spreads on it. This effect canbe applied to preventing dewdrops forming inside a windowpane and a greenhouse, for thepurpose of protecting vegetables from rotting by dewdrops. Besides, bubbles hardly stick onthe super-hydrophil ic surface in the water. This effect also should have many applications.

Suggested application of super-hydrophil ic technology are shown in Table 1.The basic patents for the super-hydrophilic technology have been resistered on March13,1998.The worldwide patent applications and resistrations are in progress.The total numberof patents is 350.

As the in te nsi ty of UV li ght in a ro om is not st ro ng en oug h to re ac h the su per -h yd ro ph il ic i ty, weare now improving the material that reaches super-hydrophilicity with less UV light, in addition,we are examining the efficient lighting to the photocatalyst.

Table 1 Suggested applicat ions of super-hydrophil ic technology Division Function Ap plic ati on

Materials for aroad

Cleaning easiness Tunnel l ighting,Tunnel wall,

Clear soundproof wall Self Cleaning by arainfall

Traffic sign, Lightning,Soundproofed wall, Guardrail,Decorative laminated panel andReflector on a read

An ti-f og gi ngproperty Road mirror

Materials for ahouse

Cleaning easiness Parts of a Kitchen, a Bathroomand Interior furnishings

Self Cleaning by arainfall

Exterior tiles, Siding boards,Window, Sash, Screen door,Gate door, Roof, Sun parlor,Handrail of a verandah

An ti-f og gi ngproperty

Mirror of a Bathroom and aDresser


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Ac ce lera ted dry ing Toilet, Window, Bathroom

Materials for atal l building

Self Cleaning by arainfal

Window, Sash, Curtain wall,Painted steel plate, Aluminumpanel, Tile, Building stone,Crystallized glass, Glass film

Materials for astore

Cleaning easiness Showcase Self Cleaning by arainfall

Signboard , Fingerpost, Showwindow, The exterior of a store

An ti-f og gi ngproperty Refrigerated showcase

Materials foragriculture

Self Cleaning by arainfall, Preventingdewdrops forming

Plastic and Glass greenhouse

Materials for anelectric andelectronicinstrument

Cleaning easiness Computer display Self Cleaning by arainfall

Upper glass of a solarcell , Insulator

Preventingdewdrops forming

Heat exchanger of an airconditioner,High-voltage cable

Materials forvehicles

Self Cleaning by arainfall

Painting and Coating of vehicles ,The outside of windows,Headlights


The inside of windows, Glassfilm,Helmet visor

Preventingdewdrops forming

Sideview mirror, Rearview mirrorand Windshield of a motorcycle,Sidemirror film

Materials for anopticalinstrument

An ti-f og gi ngproperty Optical lens

Materials formedicalinstruments andsupplies

Bio-compatibil i ty Contact lens, Catheter

Daily necessitiesand Consumerproducts

Cleaning easiness Tableware, Kitchenware Self Cleaning by arainfall Spray of anti-fouling coat


Spray of anti-fogging coat , Anti-fogging fi lm

Paint Al l pro per t iesmentioned avobe Paint,Coat


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I t is obvious that the original plates remarkably get dir ty, while the super-hydrophil ic coatedplates remain almost clean. By the accelerated weathering test, the super-hydrophilic coatingcan keep the self-cleaning ability more than 3000 hours, which means the coating hasdurability of more than 10 years in the usual environment.Silicone sealant is used to seal the gap of aluminum panels and windows of a building.Because some components of the sealant are washed away with a rainfall, they soil the exteriorwalls. The super-hydrophilic coating also can be applied to keep off the hydrophobic dirtymarks caused by silicone sealant from the walls, as shown on Fig. 8.


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Documents

Hydrophobic Details