delayed freezing of water droplet on silver nanocolumnar thin film

8/12/2019 Delayed freezing of water droplet on silver nanocolumnar thin film

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Delayed freezing of water droplet on silver nanocolumnar thin filmDhruv P. Singhand Jitendra P. SinghCitation:Appl. Phys. Lett. 102, 243112 (2013); doi: 10.1063/1.4811751View online: http://dx.doi.org/10.1063/1.4811751View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i24Published by theAmerican Institute of Physics.Additional information on Appl Phys LettJournal Homepage: http://apl.aip.org/Journal Information: http://apl.aip.org/about/about_the_journalTop downloads: http://apl.aip.org/features/most_downloadedInformation for Authors: http://apl.aip.org/authors

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deposited on the Ag samples and images of the droplets were

captured at 25 frames per second. The contact angles of Ag

samples were measured by analyzing the images of water

droplets with the ImageJ software (National Institute of

Health, USA). It was observed during the freezing experi-

ment that, without any precaution, the cold sample surface

was covered with frost within few seconds. This phenom-

enon of the ice formation on cold sample surface simply

wipes out the effect of nanostructured porous interface to the

deposited water droplets. Therefore, it is very crucial to

avoid frost during the experiment. To stop the frost forma-

tion, the sample stage was placed inside a close transparent

box and before start the cooling, dry nitrogen gas was intro-

duced in the box to reduce the humidity of inside air.

SEM images of the conventional and nanocolumnar thin

film Ag samples are shown in Fig.2. The average length and

diameter of nanocolumns were found to be about 1.6 lm and120nm, respectively. To understand the effect of nanoco-

lumnar thin film on the freezing process of a water droplet,

samples of conventional and nanocolumnar Ag thin films

were simultaneously put on the sample stage. Initially the

temperature of sample stage was set to 10 C and after

depositing the water droplets on both samples, cooling was

started to reduce the surface temperature below the freezing

point. The snapshots of water droplets sitting over the sam-

ples at different time instants are shown in Fig. 3. Readers

should also refer to a supplementary video that shows the

freezing of water droplets on nanocolumnar and conven-

tional Ag thin film samples as shown in Fig. 3 (the movie is

played fast-forwarded at 4 times). Interestingly, it wasobserved that the droplets on both samples remain in liquid

state even when the temperature crossed the freezing point.

In this stage, the water attains a super cooled state and

remains unfreeze. When the temperature decreases to about

4

C the transparency of droplet on conventional filmappears to decrease gradually, indicating the start of ice

nucleation. The counting of time was started from the instant

when the water droplet begins to freeze. The droplet on con-

ventional film surface freezes completely with formation of

a small sharp-pointed protrusion on the top in the next 31.2 s.

This observed appearance of pointed protrusion on top can

be considered as the combining effect of expansionof the ice

and surface tension of water inside the droplet.10,21 Here it is

interesting to notice that even up to this instant, the droplet

on nanocolumnar sample remains unchanged. The ice forma-

tion in this droplet was started after the time interval of about

62.4 s and completed in the next 37.2 s. This observed time

gap of 62.4 s between the start of ice formation in the drop-lets clearly indicates that the evolution of nanocolumns

induce a significant delay in the start of freezing of the water

droplet on Ag surface.

In this case, the water droplet was first deposited on the

Ag samples and then the cooling was started. In the next set

of experiments, another possible condition of the freezing of

water droplet on a solid surface was investigated. We

observed the effect of nanocolumnar film when water droplet

was deposited on a pre-cooled Ag sample surface. In this

experiment, both the conventional and nanocolumnar thin

film samples were kept on the cooling stage and temperature

was set to 10 C. The water droplets were deposited only

when the temperature of sample surface reached to 10 C.

FIG. 1. Schematic of the experimental apparatus used for observing the

freezing behavior of static sessile water droplets.

FIG. 2. SEM images of Ag samples, (a) conventional thin film (a 0) and(b) nanocolumnar thin film (a 85).

FIG. 3. Two water droplets deposited on Ag nanocolumnar film (droplet 1)

and conventional Ag thin film (droplet 2). (a) The droplets were just depos-

ited on both the samples at temperature 10 C. After depositing the droplets,

cooling of both the samples was started. On reaching to sub-zero tempera-

ture (4 C), (b) freezing of droplet 2 starts, time is set to 0 s (c) completefreezing of droplet 2 in 31.2s (d) freezing of droplet 1 starts at 62.4 s

(e) complete freezing of droplet 1 at 99.6 s (enhanced online) [URL:http://

dx.doi.org/10.1063/1.4811751.1 ].

243112-2 D. P. Singh and J. P. Singh Appl. Phys. Lett.102, 243112 (2013)

http://dx.doi.org/10.1063/1.4811751.1http://dx.doi.org/10.1063/1.4811751.1http://dx.doi.org/10.1063/1.4811751.1http://dx.doi.org/10.1063/1.4811751.1


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The snapshots of droplets at different time instants starting

from the time when deposited on cooled sample surfaces to

the complete freezing are shown in Fig. 4. In this case, the

freezing of droplets begins immediately after depositing on

the cooled sample surface. The ice formation starts near the

liquid-solid interface, and a clear upward movement of the

ice front with time was observed. The position of ice fronts

within the water droplets on both the samples is indicated by

the dotted red lines in Fig. 4. Comparing to the conventionalthin film, a clear lack in the progress of ice front with time

can be observed for the nanocolumnar sample. This differ-

ence in the position of ice fronts at different time instants

suggests for the variation in freezing process on the two sam-

ples. Here we consider the freezing time as the time taken by

a water droplet after deposited on cooled sample surface to

the complete freezing with pointed top. Then the freezing

time on conventional and nanocolumnar film samples comes

out to be about 33.8 and 71.1 s, respectively. It suggests that

the freezing of water droplet on the nanocolumnar surface

takes more than double the time of conventional Ag thin

film.

In case of a sessile droplet on cold surface, the thermal

exchange occurs mainly at the liquid-solid interface.9,12,22 It

suggests that the heat transfer or cooling rate in a droplet

will be low on the sample offering a small interface area.

The interface area on conventional Ag thin film of flat sur-

face will be the circular contact area of droplet at the base,

and it can be calculated simply by measuring the base radius

(1.1 mm) of water droplet. The value of interface area on

conventional Ag film comes out to be about 3.8 mm2. On the

other hand, the standing nanocolumnar growth makes the

film surface very rough and porous so actual interface area

cannot be calculated directly without knowing the exact wet-

ting behavior of Ag nanocolumnar film. To understand thewetting behavior, the contact angles of both the conventional

as well as nanocolumnar Ag film samples were measured at

room temperature (25 C). The contact angle was found to

increase from 92 for the conventional film to 135 for nano-

columnar Ag sample. The observed increase in contact angle

on nanocolumnar surface can be explained using the Cassie-

Baxter model.2326 In the model, for a nanostructured porous

surface, the water droplet is considered to be sitting upon a

composite surface of the solid tops and the air gaps, alternat-

ing between a liquidsolid interface and a liquidair inter-face. So, the replacement of solid surface by air reduces the

availability of effective surface energies resulting in the less

force acting to drag the water to spread over the surface and

finally leads to the increase in contact angle. For Ag nanoco-

lumnar film, air can exist in the vicinity of the columns to

make it a composite (Ag-air) surface. The similar increments

in contact angle on nanocolumnar thin films have also been

reported in previous studies.2628 Thus, in case of Ag nano-

columnar film, instead of a continuous flat surface, the water

droplet sits on Ag-air composite surface. The thermal con-

ductivity of air (0.024 W m1 K1) is almost negligible com-

pared to Ag (406W m1 K1). Therefore, only the solid

surface area of Ag nanocolumns in contact with the water

droplet can be considered as an effective interface area for

the thermal exchange. Schematic of the water droplet on Ag

nanocolumnar surface showing the effective interface (liq-

uid-solid) area and the liquid-air interface area is given in

Fig. 5. The area fraction of the liquid-solid interface is

defined as solid fraction25,26,29,30 (f) that can be calculated

using the Cassie-Baxter equation24,25

f 1 cos h0=1 cos h; (1)

where, h0 is the contact angle on nanostructured surface and

h is the contact angle on conventional surface. Taking h as

the contact angle (92) measured on conventional Ag thinfilm, the f for nanocolumnar sample of contact angle (h0)

135 comes out to be 0.3. Now, once the solid fraction f is

known, the effective interface area can be calculated by sim-

ply multiplyingfwith the total interface (liquid-solid and liq-

uid-air) area. If the water wets only the top of the silver

nanocolumns, then the droplet base area can be considered

as total interface area. Measuring the base radius (0.56 mm),

the droplet base area on nanocolumnar sample was found to

be 0.98 mm2. Considering the values of droplet base area and

the calculated solid fraction, the effective interface area

comes out to be 0.3 mm2

for the nanocolumnar surface which

is about one tenth of the value obtained for the conventional

thin film sample (3.8 mm2). This large reduction in the effec-

tive interface area on nanocolumnar surface, which yields

high thermal exchange, can be considered as a deciding fac-

tor for the observed variation in freezing process of a sessile

water droplet.

FIG. 4. Water droplet freezing process on the two Ag samples kept at tem-

perature of10 C. The dotted red line indicates the position of ice front atdifferent time instants. FIG. 5. Schematic of a water droplet sitting on Ag nanocolumnar thin film.




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In conclusion, the present work shows the effect of

nanocolumnar morphology of Ag thin film on the freezing

process of a static sessile water droplet. A series of observa-

tions of the water droplet freezing processes were carried out

under the two different possible surface temperature condi-

tions: cooling of samples was started after depositing the

droplets and second, the droplets were deposited on pre-

cooled samples (10 C). Taking the conventional Ag thin

film as reference surface, the experimental results showedthat the nanocolumnar thin film significantly delays as well

as slows down the freezing process of water droplet. The

observed delay in freezing on silver nanocolumns is

explained in terms of reduction in effective interface area

within the framework of Cassie-Baxter model.

D.P.S. kindly acknowledges CSIR, India, for the senior

research fellowship. This research was supported by the fi-

nancial grant (Grant No. RP02444) from DST, India.

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delayed freezing of water droplet on silver nanocolumnar thin film

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