Applied Surface Science 467–468 (2019) 979–991

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Applied Surface Science

journal homepage: www.elsevier.com/locate/apsusc

Full Length Article

Fabrication of a superhydrophobic surface using a fused deposition T

modeling (FDM) 3D printer with poly lactic acid (PLA) filament and dip
coating with silica nanoparticles
Kyong-Min Lee, Hani Park, Jihun Kim, Doo-Man Chun
⁎

School of Mechanical Engineering, University of Ulsan, Ulsan, South Korea

A RT ICLE INFO ABSTRACT

Keywords: Fused deposition modeling (FDM) 3D printers are widely used for rapid prototyping and customized products
Superhydrophobic 3D printing because they are inexpensive and open source 3D printers. 3D printers can be used to produce complex designs,
Dip coating and there are almost no limits in terms of product shape. Various difierent materials for 3D printers have been
Silica nanoparticles widely studied to improve the functionality of 3D-printed parts. One such functional material is super-
Methyl ethyl ketone
hydrophobic 3D printable material. However, the material preparation is not easy, and relatively expensive
material must be used for the whole product even though the material for superhydrophobicity is required only
for the surface of the product. In this study, a commercially available FDM 3D printer without any modifications
and a widely used polylactic acid (PLA) filament material were used to make patterned surface structures, and a
hydrophobic coating with nanoscale structures was realized by a dip coating process using hydrophobic silica
nanoparticles and methyl ethyl ketone. The wettability change in the 3D printed part before and after the dip
coating process was evaluated by means of static contact angle and sliding angle measurements. The efiects of
the 3D printed grid and line patterned surface structures were also studied for the superhydrophobic surface. In
addition, several complex three-dimensional structures were demonstrated. This process can be easily applied
using FDM 3D printers and PLA filament and can be used for many applications such as liquid position control
and low water adhesion.

1. Introduction surfaces, which have a contact angle between water and the solid sur-
face greater than 150° and a sliding angle less than 10°. Super-
Polymer 3D printing technology has been widely studied, and there hydrophobic surfaces can achieve unique functionalities such as self-
are many 3D printers and materials in the market. Among polymer 3D cleaning, anti-icing, corrosion protection, water-oil separation, drag
printers, fused deposition modeling (FDM) 3D printers have become reduction, water collection, and liquid transportation [7–12]. Low
popular due to their simple system configuration and open source surface energy and micro or nanoscale surface structures are generally
policy. FDM 3D printing uses a thermoplastic material. The thermo- required to make a superhydrophobic surface, as summarized in Fig. 1.
plastic material is fed through a moving heated printer head, and the When the surface has micro or nanoscale structures, it can create two
molten material is forced out through the nozzle of the printer head. difierent types of wetting, including the Wenzel state (full wetting) and
The material is deposited on the growing workpiece, and the printer the Cassie-Baxter state (partial wetting) [13,14]. Superhydrophobic
head is moved to create the printed shape. The head usually moves in surfaces are commonly obtained via partial wetting. Hydrophobic ma-
layers, moving in two dimensions to deposit one horizontal plane at a terials or coatings have been used to achieve low surface energy, and
time before moving upward slightly to make a new layer. A wide micro or nanoscale surface structures have been fabricated by various
variety of materials have been developed for FDM 3D printing such as processes such as laser surface ablation and mechanical machining
acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), high-im- [15–19]. In addition, even if the surface has high surface energy, re-
pact polystyrene (HIPS), thermoplastic polyurethane (TPU), and ali- entrant surface structures can create a superhydrophobic surface
phatic polyamides (nylon) to improve the functionality of 3D printers [20–23]. Wang et al. developed superhydrophobic 3D printing by in-
[1–6]. One possible use of 3D printers is to produce superhydrophobic corporating a vinyl-terminated initiator into a UV curable resin.

⁎
Corresponding author.
E-mail address: dmchun@ulsan.ac.kr (D.-M. Chun).

https://doi.org/10.1016/j.apsusc.2018.10.205
Received 30 May 2018; Received in revised form 30 September 2018; Accepted 25 October 2018
Available online 26 October 2018
0169-4332/ © 2018 Elsevier B.V. All rights reserved.
K.-M. Lee et al. Applied Surface Science 467–468 (2019) 979–991

coating can be easily applied with commercial FDM 3D printers and

PLA filament and can be used for many applications such as liquid
position control and low water adhesion.

2. Experimental

2.1. Sample preparation

The samples were fabricated using a FDM 3D printer (DP200,

Sindoh, Korea) without any modification or additional equipment. A
widely used FDM 3D printing material, poly(lactic acid) (PLA,
3DP200PWH, Sindoh, Korea), was used as the filament in the 3D
printer. PLA is hydrophilic material, so additional hydrophobic coating
and micro-nanoscale structures are required for superhydrophobic
surface [27,28]. After sample printing, a dip coating process was ad-
ditionally carried out to change the surface wettability and surface
morphology of the 3D printed samples. Hydrophobic fumed silica na-
noparticles (SiO2, Konasil, OCI Co Ltd, Korea) and MEK (2-Butanone,
Fig. 1. Efiects of surface morphology and surface energy on wettability. 99.7%, SAMCHUN, Korea) were used for dip coating to create a low
surface energy and additional nano-microscale structure on the surface
Superhydrophobic 3D structures were realized using this material [24]. of the 3D printed sample by melting the surface of the PLA using MEK
Lv et al. developed a 3D printing approach using hydrophobic nanosi- and attaching hydrophobic silica nanoparticles to the surface. The
lica-filled polydimethylsiloxane (PDMS) ink to make a super- compositions of MEK and silica nanoparticles were 95 wt% and 5 wt%,
hydrophobic PDMS membrane for oil-water separation [25]. These respectively, as in previous studies [10,29–32]. Dip coating with the 3D
approaches utilized low surface energy materials and 3D printed rough printed sample was carried out for 1 min, and then the dip coated 3D
surfaces. However, the material preparation is not easy, and the newly printed samples were dried at room temperature for more than 12 h.
developed material can change the mechanical properties of the whole The dried dip coated 3D printed samples were immersed in ethanol and
part. Furthermore, relatively expensive material must be used for the cleaned using an ultrasonic cleaner (NXP-1002, KODO Technical Re-
whole structure even though the material required for super- search Co, Ltd, Korea) for 10 min. The 3D printed and dip coated
hydrophobicity is only needed on the surface of the product. Xing et al. samples were evaluated after drying at room temperature for 1 h. The
reported the 3D printed superhydrophobic PLA membrane prepared by sample preparation is schematically summarized in Fig. 2.
FDM printing of PLA, chemical etching with acetone for 6 h, and
coating of polystyrene nanosphere with dopamine added Tris-bufier
solution for 6 h. This research reported superhydropohobic membrane 2.2. Surface analysis
structure for oil-water separation, but the sample preparation required
relatively long process time and various chemical treatments. In addi- Scanning electron microscopy (SEM, JSM-73600, JEOL, Japan) and
tion, this study showed only two dimensional membrane parts [26]. confocal microscopy (VK-X200 series, Keyence, Japan) were used to
In this study, a commercially available FDM 3D printer and widely analyze the morphologies of the samples. The microscale surface
used polylactic acid (PLA) filament material were used for making in- structure of the 3D printed samples was observed by confocal micro-
expensive 3D printed rough surface structures, and a hydrophobic scopy, and the nanoscale structures created by coating with hydro-
coating was realized by a dip coating process using hydrophobic silica phobic silica nanoparticles were observed by SEM. The water droplet
nanoparticles and methyl ethyl ketone (MEK) to minimize the change of contact angle (CA) and the sliding angle (SA) were measured using a
the original mechanical properties of the structural part and to treat contact angle meter (Smart drop, Femtofab Co. Ltd., Korea). For CA and
only the surface of the 3D printed part. The wettability change of the 3D SA measurements, five specimens were used for each condition, and the
printed part before and after the dip coating process was evaluated with volume of the water droplets was 11 Kl, because smaller volumes could
static contact angle and sliding angle. The efiects of the 3D printed grid not be placed on the superhydrophobic surface (Video S1). For SA, a
and line patterned surface structures were also studied. In addition, tilting speed of 1.6°/s was used. In addition, the type of wettability, i.e.,
several complex three-dimensional structures were demonstrated. The the Wenzel state (full wetting) or the Cassie-Baxter state (partial wet-
results confirmed that the proposed process with 3D printing and dip ting), was observed from the optical images.

Video S1. Interactions between 10 µl and 11 µl water droplets on a dispensing needle and a superhydrophobic surface fabricated by 3D printing and dip coating.

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Fig. 2. Schematic image of sample preparation.

Fig. 3. Images of the plate samples: (a) CAD image, (b) model image in 3D with difierent orientations, and (c) schematic image of 3D printing induced patterns on the
samples.

Fig. 4. Images of the 3D printed plate samples with difierent orientation planes (XY plane and XZ plane): (a) optical images and (b) confocal microscopy images of
the patterned surfaces.

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Fig. 5. SEM images of the XY sample at difierent magnifications: (a) 3D printed surface, (b) after dipping in pure MEK and (c) after dip coating with MEK and
hydrophobic silica nanoparticles.

Fig. 6. SEM images of the XZ sample at difierent magnifications: (a) 3D printed surface, (b) after dipping in pure MEK and (c) after dip coating with MEK and
hydrophobic silica nanoparticles.

2.3. Sample design for 3D printing layer formation. The structure of the surface is an important parameter
for generating superhydrophobic surfaces. In this study, the efiects of
FDM 3D printing is a layer-by-layer process. Each layer can be the 3D printing induced patterns in the XY plane and Z direction, and
fabricated by moving in two dimensions to deposit one horizontal plane wettability were evaluated to study the design for manufacturing (DfM)
using the tool path of the 3D printer head, and then a new layer can be in FDM 3D printers. For sample preparation, the initial plate design
fabricated by moving the 3D printer head slightly upward. Therefore, (30 mm - 30 mm - 3 mm) without any additional patterns on the
FDM 3D printers generally produce two difierent microscale pattern surface was created by computer aided design (CAD), as shown in
structures, along the XY plane in the tool path and in the Z direction by Fig. 3(a). Two difierent plate samples with difierent orientations were

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Fig. 7. Wettability results of 3D printed plate samples: (a) contact angles of XY and XZ 3D printed surface, after dipping in pure MEK and after silica coating, (b)
sliding angles of XY and XZ 3D printed surface, after dipping in pure MEK and after silica coating, (c) contact angles of dip coated XY and XZ samples after 5 months,
and (d) sliding angles of dip coated XY and XZ samples after 5 months.

fabricated along the XY plane and XZ plane to evaluate the efiects of the patterns were maintained after dip coating, and the microscale struc-
3D printing induced patterns from the tool path (XY sample) and layers ture formed by MEK dipping and nanoscale structures formed by hy-
(XZ sample) as shown in Fig. 3(b) and (c). drophobic silica nanoparticles were clearly observed after dip coating.

3. Results 3.2. Wettability of 3D printed plate samples

3.1. Surface morphology Generally, wettability can be measured with water droplet CA and
SA. With grid or pillar structures, the wettability shows isotropic be-
The 3D printing patterns induced by tool path (XY sample) and havior; that is, CA and SA are almost the same regardless of measure-
layers (XZ sample) were evaluated by optical images and confocal mi- ment direction [7,9,16,33–35]. However, the wettability of line pat-
croscopy, as shown in Fig. 4. The patterns were macroscopic and could terned surfaces can show anisotropic behaviors, wherein two difierent
be observed with the normal eye. The peak-to-peak distances of XY values are observed based on the direction, either perpendicular to the
samples and XZ samples were about 400 µm and 200 µm, respectively. line patterns or parallel to the line patterns [10,36–38]. In this research,
These values are the same as the adjacent tool path distance and layer 3D printing induced microscale line patterns, so CA and SA were
thickness, respectively. The average peak-to-valley heights of the pat- measured along the two difierent directions, perpendicular to the line
terns on XY samples and XZ samples with 10 repeated measurements patterns and parallel to the line patterns. The wettabilities on XY and
were about 33 µm and 40 µm, respectively. The heights of the patterns XZ samples were evaluated before and after dip coating, as shown in
on the XZ samples were slightly higher than the ones on the XY samples. Fig. 7(a) and (b). CA and SA were first measured on XY and XZ samples
After dip coating, the nanoscale structures produced by hydrophobic before dip coating. The CA difierences (,CA) of all samples were large
silica nanoparticles were observed from SEM images. Figs. 5 and 6 show enough (,CA > 10°) to be anisotropic [39–43], and water droplet
the SEM images of the XY sample and XZ sample with difierent mag- sliding was not observed even with 180° tilting. Among the samples, XZ
nifications before and after dip coating. The tool path patterns and layer samples showed a relatively larger difierence than XY samples. After

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Fig. 8. Optical images of a water droplet on (a) XY sample before dip coating, (b) XZ sample before dip coating, (c) XY sample after dip coating, and (d) XZ sample
after dip coating.

Fig. 9. User-defined designs of (a) line patterns and (b) grid patterns.

dip coating, all the CAs of XY and XZ samples became higher than 150°. structures were newly formed after coating, and the surface energy may
The ,CAs of all samples were less than 10°, and they showed almost decrease with hydrophobic silica nanoparticles (PLA is hydrophilic).
isotropic wetting behavior. The wetting state was clearly changed from Both CAs along the two difierent directions increased, and the ,CA
the Wenzel state (full wetting) to the Cassie-Baxter state (partial wet- decreased as sliding of a water droplet was achieved. These trends are
ting) by the dip coating of hydrophobic silica nanoparticles, as shown in in good agreement with the findings of Lee et al. and can be explained
Fig. 8. After dip coating, air trapped below the water droplet was based on the surface roughness and surface free energy [10,38–40].
clearly observed, and the shape of water droplet became isotropic. The To evaluate the stability of the dip coated 3D printing surfaces, the
efiect of microscale structure formed by MEK and nanoscale silica na- samples were stored in ambient conditions, and the CA and SA were re-
noparticles became dominant after dip coating, and the efiect of mi- measured after 5 months, as shown in Fig. 7(c) and (d). The CAs and
croscale line patterns became very small. The SA of the XY samples SAs clearly were not changed, and the wettability of the dip coated 3D
could not be observed, so the XY samples showed no sliding angles even printing samples was maintained.
though the CA of XY samples were higher than 150°. The surface of the
XY samples showed the petal efiect [23,44,45]. However, SAs were
observed for the XZ samples, and the surface showed sliding water 3.3. Wettability of user defined patterns in 3D printing
droplets with less than 16° tilting angle. A water droplet on a surface
with patterns tends to slide more easily along the direction parallel to The dip coated 3D printing plates showed good hydrophobic be-
the patterns. This result may indicate a strong pinning efiect along the havior with CAs higher than 150°, but the SAs were not smaller than
direction perpendicular to the line patterns [10,46–49]. Nanoscale 10°. Lower SAs are generally required for water droplet position control
and low water adhesion. When the flat PLA surfaces were fabricated

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Fig. 10. Images of CAD designs with user-defined (a) line patterns with 1.0 mm step size and (b) grid patterns with 1.0 mm step size. 3D printed samples with user-
defined (c) line patterns with 1.0 mm step size and (d) grid patterns with 1.0 mm step size.

using a hot compression molding process, the CA was 73.8°, which pattern. As shown in Fig. 11(c) and (d), CA and SA were re-measured
changed to 114.1° after dip coating. These results showed that using after 5 months to evaluate the stability of the dip coated 3D printing
only dip coating could not achieve higher CA than 150°, so the 3D line patterns. The line patterns showed similar results for CA and SA
printing induced microscale structures are also very important for high after 5 months, and the stability was good enough to maintain its ori-
CA. Therefore, additional user-defined surface structures created by 3D ginal wettability.
printing are necessary to achieve high CA and low SA, especially for XY The CA and SA results of the grid patterns after dip coating are
planes, which showed no SA even with 3D printing induced line pat- shown in Fig. 12(a) and (b). The CAs of 0.8, 1.0, 1.2, and 1.4 mm step
terns after dip coating. Because there are almost no design limitations sizes were 157°, 158°, 155°, and 151° in Direction 1 and 157°, 158°,
for 3D printing, additional user-defined line patterns and grid patterns 154°, and 151° in Direction 2, respectively. The SAs of 0.8, 1.0, and
on XY planes were evaluated. Fig. 9 shows the line patterns and grid 1.2 mm step sizes were measured to be 7°, 13°, and 23° in Direction 1
patterns. The step size, i.e., the distance between two adjacent 3D and 6°, 13°, and 23° in Direction 2, respectively. The SA results at a
printed structures, was set to 0.8, 1.0, 1.2, and 1.4 mm. The width of the 1.4 mm step size could not be measured. All CAs and SAs showed clear
3D printed structures was 0.8 mm, and the height of the 3D printed isotropic behavior. This result shows that CA decreases and SA in-
patterns was 3 mm. The sample size was 30 mm - 30 mm. Fig. 10 creases as the step size increases because the interval between grid
shows the CAD images and 3D printed samples with user-defined line patterns becomes wider and water droplets more easily enter the grid
patterns and grid patterns. pockets as the step size increases. The results of the grid patterns were
In the line pattern, the CA and SA were measured from two difierent also re-measured about 5 months after dip coating to evaluate their
directions, perpendicular to the line patterns and parallel to the line stability. As shown in Fig. 12(c) and (d), the results after 5 months were
patterns, as shown in Fig. 9(a). As shown in Fig. 11(a) and (b), the CAs similar to the results just after dip coating. The results indicated that,
of 0.8, 1.0, 1.2, and 1.4 mm step sizes were 157°, 153°, 155°, and 147° even if the same 3D printing was used on the XY plane, it is possible to
along the direction perpendicular to the line patterns and 150°, 143°, produce a superhydrophobic surface (CA > 150° and SA < 10°) with
132°, and 113° along the parallel direction of the line patterns after dip good stability by using additional user-defined grid patterns with a
coating, respectively. The SAs of 0.8, 1.0, 1.2, and 1.4 mm step sizes 0.8 mm step size.
were 25°, 23°, no sliding, and again no sliding along the direction With user-defined patterns, the wetting state was clearly changed
perpendicular to the line patterns, respectively. Along the direction from the Wenzel state (full wetting) to the Cassie-Baxter state (partial
parallel to the line patterns, the values were 16°, 13°, 10°, and 21°, wetting) by the dip coating of hydrophobic silica nanoparticles, as
respectively. When step size increased, the difierences in CA and SA shown in Fig. 13. After dip coating, air trapped below the water droplet
increased. With 1.2 and 1.4 mm of step sizes, the SA could not be was clearly observed, and the shape of the water droplet became iso-
measured along the direction perpendicular to the line patterns because tropic. The efiect of microscale structure formed by MEK and nanoscale
the large step size acted as a barrier to water droplet motion. The above silica nanoparticles became dominant after dip coating, and the efiect
results show that CA and SA were clearly anisotropic because of the line of microscale user-defined pattern became small relative to that of the

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Fig. 11. Wettability results of 3D printed samples with user-defined line patterns after dip coating: (a) contact angles, (b) sliding angles, (c) contact angles after
5 months, and (d) sliding angles after 5 months.

water droplet. However, isotropic and anisotropic wetting can be con- accuracy of the FDM 3D printer, the repeatability of superhydrophobic
trolled based on the user-defined pattern designs such as lines and surface fabrication was confirmed with five difierent samples for each
grids. condition. This result indicated that the proposed fabrication method
can be used for fabrication of a superhydrophobic surface even with a
commercial FDM 3D printer having relatively low dimensional accu-
4. Discussion racy.
Superhydrophobic surfaces with CAs higher than 150° and SAs
Superhydrophobic surfaces were fabricated using a FDM 3D printer smaller than 10° were realized with user-defined patterned structures.
with a PLA filament material and dip coating with hydrophobic silica The line patterned surfaces showed anisotropic superhydrophobic sur-
nanoparticles. When the 3D printing induced patterns of tool path and faces, and the grid patterned surfaces showed isotropic super-
layer were evaluated, only the surface with layer patterns showed a CA hydrophobic surfaces. Fig. 14 shows demonstrations of isotropic su-
higher than 150° and an SA around 10° after dip coating. A super- perhydrophobic surfaces. Fig. 14(a) shows the low water adhesion of
hydrophobic surface was achieved without layer patterns using an ad- superhydrophobic half sphere bowl with porous holes, which could
ditional user-defined grid pattern. The flat PLA surfaces after dip store water with air venting (Video S2). Fig. 14(b) shows a maze with a
coating showed only a 114.1° CA, so the micro or macroscale surface water droplet that could be controlled by tilting (or gravity). This de-
patterns were essential for superhydrophobic surfaces. Therefore, the monstration showed the feasibility of a freeform superhydrophobic
mechanism for superhydrophobic surface formation was suggested to surface for children's toys and science studies (Video S3). Fig. 15 shows
be a combination of low surface energy caused by hydrophobic silica demonstrations of anisotropic superhydrophobic surfaces. The sliding
nanoparticles on hydrophilic PLA and nano/microscale (or nano/mac- speed of the water droplet can be controlled using a product with an-
roscale) hierarchical surface structures with nanoparticles and 3D isotropic wetting, as shown in Fig. 15(a) (Video S4), and the motion of
printing patterns. A FDM 3D printer without any modification or ad- the water droplet can be controlled as shown in Fig. 15(b) (Video S5).
ditional equipment was used in this research, and the resolution of the Various difierent functional superhydrophobic 3D products for liquid
3D printer (including layer and tool paths) was several hundred mi- position control and low water adhesion can be realized using the
crometers. In addition, the dimensional error between the CAD and real proposed process.
3D printed part was sometimes large enough to be observed by the
naked eye. Even though there was a limitation in the dimensional

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Video S2. Superhydrophobic 3D printed half sphere with porous structure.

Video S3. A maze with a superhydrophobic surface.

Video S4. Water droplets (20 µl) with difierent sliding speeds using an anisotropic superhydrophobic surface.

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Video S5. Control of water droplet motion using an anisotropic superhydrophobic surface.

Fig. 12. Wettability results of 3D printed samples with user-defined grid patterns after dip coating: (a) contact angles, (b) sliding angles, (c) contact angles after
5 months, and (d) sliding angles after 5 months.

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Fig. 13. Optical images of a water droplet on a user-defined line patterned surface with 1.0 mm step size (a) before dip coating and (b) after dip coating and on a user-
defined grid patterned surface with a 1.0 mm step size (c) before dip coating and (d) after dip coating.

Fig. 14. Demonstration of isotropic superhydrophobic surfaces: (a) half sphere bowl with porous holes and (b) a maze with water droplets.

In addition, the efiect of dip coating on the mechanical properties of and the samples were dried for 1 day. The dried 3D printed sample was
3D printed samples was evaluated by tensile test according to ASTM D- immersed in ethanol, washed with an ultrasonic cleaner for 10 min and
638. Tensile test samples were printed on the XY plane as Type I of dried at room temperature for 7 days to remove all ethanol from the
ASTM D-638 and five samples were prepared for each condition, such sample. The mechanical properties by tensile test are summarized in
as a 3D printed sample, a sample after dipping in pure MEK and a Table 1. The tensile strength after dipping in pure MEK and dip coating
sample after dip coating. Dipping in pure MEK and dip coating were decreased by 8.4% and the tensile modulus decreased by 2.2%, but the
done for 1 min as the previous superhydrophobic sample preparation difierence between the samples after dipping in pure MEK and dip

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Fig. 15. Demonstration of anisotropic superhydrophobic surfaces: (a) line patterned surfaces for sliding speed control of water droplets and (b) a round track for
motion control of water droplets.

Table 1
Tensile test results for 3D printed samples, samples after dipping in pure MEK, and samples after dip coating.
Mechanical properties Tensile strength (MPa) Tensile modulus (GPa) Elongation (%)

Average Standard deviation Average Standard deviation Average Standard deviation

3D Printing 45.4 0.736 2.29 0.125 5.12 0.53

3D Printing + MEK dipping 41.6 0.313 2.24 0.258 4.57 2.15
3D Printing + dip coating 41.6 0.614 2.24 0.296 5.94 1.54

coating was negligible. The reason for the reduction of the mechanical superhydrophobic behavior, and the grid patterned surfaces showed
properties was mainly the efiect of MEK on PLA, and silica coating may isotropic superhydrophobic behavior. In addition, several complex
not afiect the mechanical properties. Therefore, a small reduction in three-dimensional products with isotropic and anisotropic super-
mechanical properties should be considered before using this process. hydrophobic surfaces were demonstrated. This process can be easily
applied with commercial FDM 3D printers and can be used for many
5. Conclusion applications such as liquid position control and low water adhesion.
However, a small reduction in mechanical properties should be con-
In this study, superhydrophobic surfaces were fabricated on three- sidered before using this process.
dimensional products using a commercially available FDM 3D printer
with a widely used PLA filament material and dip coating with hy- Acknowledgements
drophobic silica nanoparticles. Nano or microscale structures and low
surface energy are generally necessary to make superhydrophobic sur- This research was supported by the 2017 Research Fund of
faces. The microscale structures on the surface were created by 3D University of Ulsan.
printing, and nanoscale structures with low surface energy were created
by dip coating with hydrophobic silica nanoparticles. The wettability References
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