Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 7.7.1 - Drop Impact, Spreading, Splashing and Penetration into Electrospun Nanofiber Mats
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Permanent Link: http://ufdc.ufl.edu/UF00102023/00194
 Material Information
Title: 7.7.1 - Drop Impact, Spreading, Splashing and Penetration into Electrospun Nanofiber Mats Collision, Agglomeration and Breakup
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Lembach, A.N.
Tan, H.-B.
Roisman, I.V.
Gambaryan-Roisman, T.
Zhang, Y.
Tropea, C.
Yarin, A.L.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: nanofiber
electrospinning
porous
drop impact
splashing
capillary
hydrophobic
pinning
 Notes
Abstract: Experiments were conducted to study the splashing threshold and penetration of a drop impacting on electrospun polymer nanofiber mats. The nanofiber cross-sectional diameters were of the order of several hundred nanometers, the pore sizes in the mats of about several micrometer. PAN (Polyacrylonitrile) a partially wettable Polymer was used to electrospin nanofiber mats. In addition, nanocomposite nanofibers were used, which contained CB (carbon black) nanoparticles embedded in the fibers. The CB nanoparticles strongly enhanced roughness of the nanofiber surfaces. The experiments revealed that drop impact on nanotextured surfaces of nanofiber mats produce spreading similar to the one on impermeable surfaces. However, at the end of the spreading stage the contact line is pinned and drop receding is prevented. At higher impact velocities, prompt splashing events with formation of tiny drops were observed. It was shown that the well-known splash parameter Kd can be used as an acceptable scaling for splash, however the threshold value for the coated surfaces is higher than that for dry flat substrates. In addition, water spreading after drop impact inside nanofiber mats was quantified.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
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Bibliographic ID: UF00102023
Volume ID: VID00194
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: 771-Lembach-ICMF2010.pdf

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7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Drop Impact, Spreading, Splashing and Penetration into Electrospun Nanofiber Mats


A.N. Lembach1'2, H.-B. Tan', I.V. Roisman1'2, T Gambaryan-Roisman1'3, Y Zhang4, C.Tropeal'2
A.L. Yarin1'4


'Center of Smart Interfaces, Technische Universitat Darmstadt, Petersenstr. 32, 64287 Darmstadt, Germany
2Institute of Fluid Mechanics and Aerodynamics, Technische Universitat Darmstadt
3Institute of Technical Thermodynamics, Technische Universitait Darmstadt
4Department of Mechanical and Industrial Engineering, University of Illinois at Chicago,
842 W. Taylor St., Chicago IL 60607-7022, U.S.A.
lembach @csi.tu-darmstadt.de


Keywords: nanofiber, electrospinning, porous, drop impact, splashing, capillary, hydrophobic, pinning,




Abstract

Experiments were conducted to study the splashing threshold and penetration of a drop impacting on electrospun polymer
nanofiber mats. The nanofiber cross-sectional diameters were of the order of several hundred nanometers, the pore sizes in the
mats of about several micrometer. PAN (Polyacrylonitrile) a partially wettable Polymer was used to electrospin nanofiber mats.
In addition, nanocomposite nanofibers were used, which contained CB (carbon black) nanoparticles embedded in the fibers.
The CB nanoparticles strongly enhanced roughness of the nanofiber surfaces. The experiments revealed that drop impact on
nanotextured surfaces of nanofiber mats produce spreading similar to the one on impermeable surfaces. However, at the end of
the spreading stage the contact line is pinned and drop receding is prevented. At higher impact velocities, prompt splashing
events with formation of tiny drops were observed. It was shown that the well-known splash parameter Kd can be used as an
acceptable scaling for splash, however the threshold value for the coated surfaces is higher than that for dry flat substrates. In
addition, water spreading after drop impact inside nanofiber mats was quantified.


Introduction

Electrospun nanofiber mats are porous permeable materials
composed of individual non-woven polymer nanofibers
(with diameter of about several hundred nanometers) which
are randomly orientated in the mat plane. The size of
interfiber pores is of the order of several micrometer
(Reneker et al. 2007, Yarin et al. 2007, Greiner and Wendorf
2007, Reneker and Yarin 2008). The electrospun nanofiber
mats are usually produced from polymers which are either
partially wettable or non-wettable. The pores in the mats are
filled with air. This makes them typically poorly wettable by
water. Moreover, due to the presence of texture of several
sizes (roughness or nanopores, or sometimes even beads on
individual nanofibers, nanofiber diameters and the pore
sizes), electrospun nanofiber mats can behave essentially as
superhydrophobic materials, and drops deposited on them
softly remain practically spherical at their surface (Jiang et
al. 2004). Coating of nanofibers with non-wettable materials
further enhances their superhydrophobicity when water
drops are softly deposited on them (Ma et al. 2005).
Drop impacts on nanofiber mats attracted attention only
recently and demonstrated several non-trivial outcomes
(Srikar et al. 2009, Han and Steckl 2009). In general, drop
impacts on impermeable surfaces involve such phenomena
as spreading, receding, splashing and bouncing (Yarin 2006).
However, drop impacts on partially wettable PAN.


nanofiber mats demonstrated that receding, splashing and
bouncing were practically eliminated (Srikar et al. 2009).
Drop spreading after impact was similar to that on
impermeable surfaces, but drop contact line had been
pinned at the end of the spreading stage, which led to a
significant enhancement of cooling of hot nanofiber-coated
surfaces in drop cooling (Srikar et al. 2009). On the other
hand, mats with fibers coated by Teflon demonstrated
pronounced receding and bouncing of impacting water
drops (Han and Steckl 2009).


Figure 1: Electron Microscope Image of nanofiber mat and
a drop deposited softly on partially wettable PAN nanofiber
mat.








The present work is aimed at detailed elucidation of the
physical phenomena taking place after water drops impact
on surfaces coated with nanofiber mats. In particular, we are
interested in the effect of wettability and roughness of
nanofiber materials on the impact outcome, as well as in the
effect of the impact conditions on such phenomena as
pinning of contact line, receding motion and splashing (if
any).
Nanofibers were electrospun from PAN (Polyacrylonitrile, a
partially wettable polymer with water contact angle on a
cast sample of about 30-40), PCL (Polycaprolactone, a
non-wettable polymer, with water contact angle on a cast
sample over 900), or from PCL containing CB (carbon black
nanoparticles), which tends to increase roughness of
individual nanofibers (Tiwari et al. 2008). The
electrospinning setup is described elsewhere (Reneker et al.
2007). Circular nanofiber mats of diameter of about several
centimeters, thickness of the order of several hundred
micrometers with porosity of the order of 90% were
produced (Reneker et al. 2007). Figure depicts a scanning
electron microscope image of a PAN nanofiber mat used in
the experiments. This image provides information on
three-dimensional topology of nanofiber mats. Nanofiber
mats were electrospun on copper discs attached to a
grounded electrode. The discs with nanofiber mats on their
top were used as targets in drop impact experiments. Copper
discs with flat or convex tops were used, which made the
corresponding nanofiber mats on top either plane or
rounded, respectively.


Target


Target fi
Liquid film


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

on all nanofiber mat surfaces irrespective of their chemical
structure and roughness, it is a result of their porous
structure. A rolling off angle for water drops on nanofiber
mats simply does not exist. Small drops softly deposited on
the nanofibers looked spherical, they almost did not change
the shape when flipped upside down, and never detached
from the surface. The latter might be beneficial for spray
cooling through nanofiber mats as in Srikar et al. 2009,
since such cooling is possible at any surface irrespective its
orientation with respect to gravity. Similar observations of
drop sticking on flipped-over surfaces were reported for
some other nano-textured materials in Liu and Gua (2007).

Over the surface

Experiments with drop impacts on nanofiber mats were
performed to observe the impact phenomena using a high
speed video system. Two configurations of the experimental
setup used in our experiments are shown in fig. 2 In the first
configuration (fig.2a) a drop produced by a drop generator
accelerates by gravity and the impacts onto a horizontal
target. A laboratory syringe used as a drop generator allows
producing drops of 2 3 mm in diameter whereas a
commercial piezo electric micro drop device (Microdrop)
generates drops of approximately 100 gm in diameter.
The second configuration (fig. 2b) is designed to investigate
impact of drops in the intermediate diameter range (0.4 to
1.4 mm). In this configuration an impact of a primary drop
onto a horizontal wetted impactor generates several
secondary droplets. Some of these secondary droplets then
collide with the coated surface of the vertically oriented
target. The outcome of these collisions has been captured by
the high-speed video system and then analyzed.


0


Figure 2: Schematic of two variants of the experimental
setup. (a) Syringe drop generator used for direct impacts of
2-3 mm drops. (b) Syringe drop generator producing
primary drops. Their impact on the horizontal liquid film
produces corona splashing, which generates secondary tiny
droplets of about 0.4 1.4 mm diameter. These secondary
droplets then impact onto a vertical target covered by
nanofiber mat.

Advancing and receding contact angles of water on the
nanofiber mat surfaces were measured optically. All three
samples show a large advancing contact angle: 1030 for
PAN, 1080 for PCL and 1000 for PCL+CB. In the case of
non-wettable PCL (with or without CB), the advancing
contact angle on nanofibers is close to the one on a cast
sample. On the other hand, on PAN nanofibers the contact
angle is significantly larger than that on a cast sample,
which is explained by the fact that air entrapped in the pores
(about 90% porosity) facilitates hydrophobicity, even
though the nanofibers are made of a relatively wettable
material. The receding contact angle was approaching zero,
indicating a very large hysteresis. Since this effect happens


Figure 3: Different modes of drop impact on PAN
nanofiber mat. (a) deposition, (b) fingering without splash,
(c) receding splash and (d) advancing splash. Time span is
1.5 msec.









A drop softly deposited on the nanofiber mat surface is
almost spherical (fig.l) demonstrating a seemingly super
hydrophobic state. However, after several minutes this drop
spreads out and then is partially sucked into the nanofiber
mat. After about 20 min the drop completely disappears
from the nanofiber mat surface. This phenomenon is
described in more detail in the following "Under the
surface" section. A totally different outcome was observed
when a water drop of 2 mm diameter impacted onto the
same nanofiber mat at a speed of 2 m/s. As fig.3a) show, the
drop first spreads on the nanofiber mat surface as on a dry
rigid completely wettable substrate and then remains pinned
in the spread-out configuration and does not recede. Pinning
of spread-out drops after the spreading stage of impact was
used in Srikar et al. (2009) to facilitate spray cooling of hot
surfaces coated with nanofiber mats a paradoxical
situation where cooling was intensified by putting an
insulation-like "fur coat" on a hot surface.


140
Deposition, no fingers
120o Advancing splash <9 A 9






3000 4000 5000 6000
Re


40
S Fingering
20 Receding splash
00 *
80

(b)
3000 4000 5000 6000
Re


Figure 4: Observed outcomes of drop impact at various
parameters near splashing threshold.

In fig. 3 it is visible that the drop, depending on its velocity
and diameter, can develop different spreading modes
ranging from deposition-like to splash-like ones. All the
types of drop behavior shown in fig. 3 deposition, fingering,
advancing and receding breakup, are familiar from drop
impacts on impermeable surfaces (Yarin 2006). However,
there are striking distinctions characteristic of drop impacts
on nanofiber mats. Namely, a corona splash, similar to that
of drops impacting on thin films of water, was never
observed, neither was bouncing.
It is impossible to determine definite thresholds for impact
conditions corresponding to specific impact outcomes. In
fact, a range of impact conditions can be identified at which
all the modes can occur with some probability for the same
impact conditions. It is explained by the fact that splash, as
a result of instability, is initiated by initial disturbances of
the drop which cannot be controlled in the experiment. In
fig. 4 the results of the observations of various impact
outcome are shown for the threshold region. In an attempt to
generalize the results, the diagram is shown for two
independent dimensionless parameters: the Reynolds
number, Re =rDVo m and the well-known splashing
parameter Kd = [D3V, r3 /(ms2)]1/4, where p, g and a
denote liquid density, viscosity and surface tension, D and
Vo the drop diameter and impact velocity. Figure 5
represents a cumulative plot of the outcomes of drop impact
on a PAN nanofiber mat with a thickness of about 100 min,
porosity of about 90% and roughness R, of 8 uim. The data
were acquired using the two variants of the experimental
setup depicted in fig. 2 The range of drop diameters in the
experiments was from 0.1 to 3.5 mm and for the range of


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

the impact velocities was from 0.4 to 3.6 m/s.
The dimensionless number Kd allows to delineate rather
well the domain corresponding to drop deposition without
fingers and advancing splash (see fig. 3a). However, as is
shown in fig. 3b, in a certain parameter domain various
impacts can produce various outcomes with a certain
probability. The corresponding estimate of the probability of
various outcomes as a function of the Kd number is shown
in fig. 5.


100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
20


Adv Splash


e ij v Fingers

40 60 80 100 120 140 160 180
K


Figure 5: Probability distribution of different impact
outcomes as a function Kd number.

According to Mundo et al. (1995), the drop splashing
threshold for a flat smooth substrate corresponds to Kd =
57.7. In the case of drop impact onto a nanofiber mat the
threshold value of Kd separating deposition without fingers
and advancing splash is higher, approximately Kd = 87,
which indicates that the nanofiber coating of the target
surface prevents splash.
The receding splashes, in which secondary drops are formed
in receding fingers, were mostly seen in conjunction with
the advancing splash and seldom observed as a sole splash
phenomenon on nanofiber mat surfaces. The rarity of the
receding splash outcomes can be explained by the properties
of the nanofiber mat surface. Namely, a large contact angle
hysteresis characteristic of nanofiber mat surfaces renders
liquid on the surface practically immobile and requires a
large amount of energy for drop separation from a finger.
However, if a drop possesses the required critical energy, it
will likely be separated at the advancing splash stage.
Another interesting observation is that occasionally, a drop
can eject a secondary tiny drop upon impact that is later
intercepted by a moving finger originating from the primary
drop. In such cases, no secondary drops are ultimately lost,
which means that such events do not meet our definition of
splash. Cumulative masses of the splashing tiny droplets in
the advancing splash were about 1-2% of the primary drop
mass.

Under the surface

In an additional series of experiments the phenomena,
which occur inside nanofiber mats were studied. Namely, an
experiment was performed to measure the rate of water
spreading inside nanofiber mats (parallel to the underlying
substrate surface). The experimental setup is shown
schematically in fig. 6. Measurements were done in
atmosphere with controlled humidity of the surrounding air.
In all the experiments, drops of about 2 mm were dripped








from about 10 cm height onto nanofiber mat surface to
ensure similar initial conditions and impact parameters
below the splashing limit. For observations in this
experiment, a digital video camera with the frame rate of 30
frames per second was used.






I


Conditi oned 0

Figure 6: Setup with conditioned air to control humidity.

The refraction indexes of the nanofibers and water are such
that if a certain moisture level is reached in the nanofiber
mat, it becomes transparent and the underlying darker
copper surface becomes visible through the nanofiber mat.
This allows observation of slow water spreading inside
nanofiber mats.







Figure 7: Water spreading inside a PAN nanofiber mat. a)
Images with wetted spot configurations at different time
moments (t=0 s corresponds to the moment when water in
the drop deposited onto nanofiber mat has come to rest). b)
Contours of the wetted spot inside the nanofiber mat at
different time moments. The images are taken here for a
time interval below 6 s when the wettability driven
spreading in the mat is far from being over. The whole
process lasts several minutes. c) Close-to-circular contours
of a wetted spot.

The captured images were used to characterize the growing
dark area visible through the transparent nanofiber mat
spots. The wetted nanofiber mat areas are rather fractal-like
looking, therefore their areas were determined using
pixel-by-pixel counting. It is emphasized that fig. 7b) shows
the worst case of the fractal-like spreading, and many
images of the spreading process were quite circular, like fig.
7 c). One measured pixel had the size corresponding to
about 10 gm. The accuracy is increased by averaging the
successive video images. This was also necessary to
decrease the "noise" in the measurement. The dark
(transparent) spots were observed and measured under
different angles to show that optical artifacts do not
influence the results.
The darker areas did not occur on the targets coated with
nanocomposite nanofiber mats with the embedded CB
nanoparticles, the two-layer (PCL over PAN), and some
PCL samples. Water spreading in such mats could not be
detected with our method. Also, in the case where PCL or
PAN nanofiber mats were too thick, our method was


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

inapplicable. Some areas stayed non-transparent throughout
the whole spreading process, even though they were already
surrounded by transparent areas, which corresponds to large
differences in moister concentration in nanofiber mats. In
such cases the wet region inside nanofiber mats was rather
three-dimensional than two-dimensional. With two-layer
nanofiber mats (PCL over PAN) even for very thin mat
thicknesses, spreading could not be observed. The drop
above the surface was not distinguishable from pure PCL.
Only in the case of two-dimensional propagation of
moisture, measurements could be done with PAN. The
situation for non-wettable PCL is not that clear. There were
PCL nanofiber mats that show water suction similar to PAN
nanofiber mats. However, there also were PCL nanofiber
mats, which did not intake any water. This two-fold nature
of water-PCL affinity might be related to the possible
absorption of water at the carbonyl sites of the ester groups
in PCL via hydrogen bonding, as was shown in Peng et al.
(2003) and Dror et al. (2007).
According to Lykov (1966), wettability driven water
spreading in porous nanofiber mats can be described by a
diffusion-like equation, which in the axisymmetric
isothermal case takes the form
9u 10 f o u\
= or'r' (1)

where u is the dimensionless moisture content inside
nanofiber mat incorporating both liquid water and its vapor,
r represents the radial direction with the origin at the drop
impact center, t is time, and am is the moisture transfer
coefficient.
Solutions of eq. (1) are subjected to the following initial and
boundary conditions

u p (r), t = ; u -0, r--oo. (2)
The corresponding general solution of the problem (1)-(2) is
well-known (Carslaw and Jaeger, 1959). For a drop impact
such that (p(r)=l for rD/2, where D
is the drop diameter, the solution yields
D/2
u(r, t) = 2- e --- e T* Io ( d 2
2amt 2amt
o (3)
where Io denotes the modified Bessel function of the first
kind of order zero. The integral in eq. (3) has to be
evaluated numerically.


Figure 8: (a) Moisture distribution computed from eq. (5),
(b) The dimensionless moisture front position as a function
of the dimensionless time. The moisture front is defined as a
position for which u = uf = 0.1.

Figure 8a) shows the moisture distribution in the nanofiber
mat computed from eq. (3) for different time moments in
terms of the dimensionless time t = 4tam ID2. It is seen
from the figure that the initially very sharp gradient of the









moisture content near the radial position r=D/2 decreases
due to the moisture diffusion.
A visible front of contrast between the wetted and
practically non-wetted part of the mat corresponds to rf (t)

found from eq. (3) for u = uf, where uf is a sufficiently
low (0.1-0.2) level of moisture in the mat, which is still
recognizable by a camera used for the observations (the
approach similar to that of Wu and Peng, 2009). Figure 8b)
shows the dimensionless position of the front
(Rf =2rf/D) as a function of the square root of the
dimensionless time computed from the numerical solution
of eq. (3) by setting uf =0.1 as a definition of the moisture

front. Note that for 7>- 0.8 the moisture content
everywhere becomes so low that the concept of the moisture
front is not applicable any more. Figure 8b) shows that
initially the position of the moisture front changes
approximately linearly with 4 (see the fitting line). This
linear dependence can be fitted by the equation

Rf = 1.492y/r + 1.03, or rf = 1.492va + const (4)
which can be compared to the experimental data. The
numerical value of the factor in eq. (4) may depend on the
choice of the threshold value of uf .
At large time values (at >> D2) eq. (4) can be expanded
in power series based on small parameter e = 2 /(amt).
Then, the approximate expression for u is

D2 D4 + 8D2r2
1 = + O(E3)
16amt 512a t2 (5)
The maximal value of Rf and the time instant at which the
maximum of Rf is reached are expressed as


Rf max = D -1
L 16Uf


1/2 D2
8 max 32amuf


i \ I
time (see) time (sec) time (sec)
Figure 9: Wetted area in PAN nanofiber mat versus time.
The locus of minimum values is plotted in doted lines, as
well as the locus of the maximum values. The average
values for 10 measurements at a fixed humidity level are
plotted in solid lines. (a) 7% humidity, (b) 81% humidity
and (c) several humidity values (7%, 18%, 31%, 44%, 56%,
68% and 81%) together.

The experimental data for the wetted area in a -100 gm
thick PAN nanofiber mat is plotted in fig. 9 versus time for a
range of humidity levels in the surrounding air. Three
distinct stages of water-nanofiber mat interaction can be
distinguished. The first one, during the first few
milliseconds after impact is the fast inertia-dominated
spreading. Due to the short duration, the first stage is not


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

recognizable in the graphs in fig. 9. That is the reason that
the curves in fig. 9 raise in a burst-like manner at the very
first moments. The subsequent stage of the wettability
driven wetting of the nanofiber mats in fig. 9 produces
gradually raising lines. The spreading-wetted spot then
reaches a plateau value. After that, the method used could
not detect any additional spreading inside the nanofiber mat
(which might be still present), and the observations were
ceased. It is emphasized that a subsequent drying of the
wetted spot does not proceed as a receding dewetting, but
rather as a simultaneous "bleaching" of the whole wetted
spot until the detection threshold is reached.
The plots in fig. 9 show that for low humidity values the
differences between maximum values of the wetted area
were insignificant, while they strongly depend on the
humidity level. For high humidity values, the differences
between the time needed for the observable maximum
spreading for the three curves in the right middle frame in
fig. 9 were large, while in the left frame in fig. 9 for low
humidity the differences in time were negligible. At the
intermediate spreading stage slopes of all curves are very
close to each other.
Using eq. (4) to fit the wetted area data in figs. 9 we find the
value of the moisture transfer coefficient
am = 8'10 cm2/ s. It is interesting, that despite the fact
that humidity affects the value of the maximum visible
radius of the wetted spot, it almost does not influence the
rate of spreading. It can be assumed that the moisture
spreading stage is nearly unaffected by evaporation into
surrounding air, whereas evaporation starts to play an
important role at the later stages, corresponding to the
maximum wetted spot.

320
300-
280-* -
260-
240- .*
220--
S200- ,
18 180 .
160
140 ,
8 10 12 14 16 18 20 22
t (sec)
Figure 10: Maximum spreading area as a function of the
time to maximum spreading.

The maximum wettability driven spreading observed and
the time needed to achieve the maximum spreading are
plotted in fig. 10 for the whole humidity range. Each point
on this graph corresponds to different humidity levels. The
linear dependence of R, fma on t. is in agreement
with the predictions (6).

Conclusions

It was demonstrated that drop impacts on electrospun
nanofiber mats almost instantaneously result in spread-out
wetted spots over the surface, which practically neither
recede or bounce. At the following stage wettability driven
sideway water spreading inside partially wettable nanofiber
mats begins which can last minutes, and in some cases the









wetted area inside the mat looks similar to fractals. Prompt,
advancing and receding splashes play secondary role, with
mass losses due to them on the scale of 1-2%. Receding
fingering is practically completely suppressed. In summary,
drop impacts on nanofiber mats demonstrate a number of
novel unexpected phenomena, which make hydrodynamics
and heat transfer on nanofiber-coated surfaces fascinating
and potentially beneficial for applications.

Acknowledgements

This work was partially supported by the Center for Smart
Interfaces, TU Darmstadt. The participation of H.-B. Tan
was made possible through an awarding scholarship from
DAAD through the RISE program. YZhang and A.L.Yarin
are grateful for partial support of their work by National
Science Foundation through Grant NIRT CBET-0609062.
T.Gambaryan-Roisman wishes to acknowledge the financial
support of DFG (German Science Foundation) through the
Emmy Noether-Program.

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7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

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