1

Supplementary Information: Lubricated nanostructured fabrics displaying robust

omniphobicity

Cicely Shillingford1, Noah MacCallum1, Tak Sing Wong1, Philseok Kim1, Joanna Aizenberg1*

School of Engineering and Applied Sciences, Wyss Institute for Biologically Inspired

Engineering and Kavli Institute for Bionano Science and Technology, Harvard University,

Cambridge, Massachusetts 02138, USA

2

SupplementaryMaterialsandMethods

Materials

As specified, lubricants used to create SLIPS fabric were the perfluorinated liquids

Krytox 100 and 102 (Dupont Inc., Wilmington DE). Their densities are similar at 1.87 and 1.91

g/mL, respectively. Their kinematic viscosities of 12.4 and 38 cSt, respectively, at 20°C are

different, but this only influences the speed at which droplets roll off the surface and does not

affect the results for our purposes. The organic test liquids, including n-heptane (biotech. grade,

≥99.0%), octane (puriss, ≥99.0%), decane (ReagentPlus®, ≥99.0%), dodecane (ReagentPlus®,

≥99.0%), tetradecane (ReagentPlus®, ≥99.0%), and hexadecane ((ReagentPlus®, 99%), were

obtained from Sigma (Sigma Aldrich Inc., St. Louis, MO). Kaydol® heavy mineral oil was

obtained from Ruger Chemical Company, Linden, NJ. Pure, ACS Grade Isopropyl Alcohol was

from BDH. Drierite calcium sulfate Desiccant was purchased from W A Hammond Drierite Co.

Ltd., Xenia, OH. Teflon membranes in the breathability experiments of average pore size 200

nm, 1 µm or 20 µm, as specified, were purchased from Sterlitech Corporation, WA, USA.

TEOSOptimization

Tetraethyl Orthosilicate (TEOS) is used to functionalize the surface of amine cleaved

polyester materials for compatibility with silica-microbead deposition and silane fluorination [1].

Namely, the largest achievable concentration of terminal hydroxyl groups is required to maxmize

particle polymerization and silane condensation. To optimize the surface actuation, flat,

commercial grade polyethylene terephthalate (polyester) was cleaved with APTES, submerged in

TEOS for varying reaction times, fluorinated with perfluorododecyl-1H,1H,2H,2H-

triethoxysilane, and observed for hydrophobicity. The contact angle increases as a function of

TEOS reaction time, which is proportional to the amount of terminal hydroxyl groups available

for reaction and therefore the degree of fluorination. Polyesters must be reacted with TEOS for a

sufficient amount of time to maximize their water repellency.

3

AqueousLiquidRepellencyTestResults(AATCC193)

Testing was performed as per the industrial standard protocol AATCC 193 [2]. STable 3

shows the standard liquids used with the corresponding score. Water:IPA mixtures of

progressively lower surface tensions were placed on the surface of the fabric sample, and the first

fluid that wets the fabric determines the reported score. The maximum score of 8 corresponds to

a 60% IPA solution.

HydrocarbonResistanceTest(AATCC118)

Testing was performed as per the industrial standard protocol AATCC 118 [2]. Organic

liquids (ranging from Kaydol heavy mineral oil to n-heptane) of progressively lower surface

tensions were placed on the surface of the fabric sample; the first fluid that wets the fabric

determines the score (STable 4). A maximum score of 8 corresponds to n-heptane.

•DropImpactTest

Drop Impact Test was 10 µL droplet is dropped from a measured height and impacts the

surface of the fabric with a known velocity and thus a known dynamic pressure. Immediately

after impact we measured the sliding angle to assess whether the droplet is pinned and/or the

surface is compromised in any way. This exposes a known shortcoming in lotus-effect

superhydrophobic surfaces: even low pressures (typically ~400 Pa [3]) can cause an irreversible

Cassie-Wenzel state transition whereby the droplet is forced into the air pockets of the roughened

surface and strongly held in place. While past experiments show that SLIPS is tolerant to

dynamic pressures exceeding 6000 Pa, this was not feasible to test on out fabric samples because

when the droplet impacts the fabrics with such a great speed the macroscale imperfections cause

it to explode into many tiny droplets, thus precluding sliding angle analysis. Hence, we used the

highest dynamic pressure possible that left the droplet intact after impact: 1520 Pa for

tetradecane and 1990 Pa for Water, and these values do not indicate an upper limit to the

pressure tolerance of the SLIPS fabrics.

4

FTIRAnalysisofUntreatedFabrics

We used a Bruker Vertex70 fourier-transform infrared spectrometer (FTIR) with the

ZnSe crystal ATR attachment to confirm the chemical composition of the polyester fabrics. Dry

samples were simply placed on the detector and the spectra were measured for a range of

wavenumbers from approximately 4000 – 600 cm-1.

QualitativeOmniphobicityTest

The presence of SLIPS functionality on a fabric can be easily seen by applying various

low surface tension fluids to the fabric and observing wetting behavior. Once the SgB and SiM

methods were established, the fabric was lubricated with Krytox 102 (Dupont Inc., Wilmington

DE) and sprayed with water, cooking oil, and hexadecane, the latter two of which would wet

untreated and superhydrophobic fabrics. The results of this test are shown in Supplementary

Movie 2; all test liquids slid off of the SLIPS fabrics, and none were wetted after the test. This

information was used to progress beyond the optimization stage and onto Phase I

characterization of the fabrics.

QualitativeStainResistanceTest

The Muslin cotton sample displayed large sliding angle values, indicating pinning. This

test shows that the droplet is still repelled by the surface (i.e., the droplets are not wetting the

surface) while being pinned by loose fibers of the fabric. As shown in Supplementary Movie 3, a

Muslin cotton sample is fixed to a tilting stage tilted to ~30°, a red 20 µL de-ionized water

droplet is applied to the surface of the fabric, pins to the surface, and is fully removed when

contacted with the Kimwipe.

Washingmachinetesting

The washing machine test is a preliminary investigation into the tolerance of SLIPS

fabric to a single washing machine cycle in a standard top-loading washing machine (STable 2).

The sliding angle of a 10 µL water droplet on an un-lubricated and lubricated SiM fabric sample

is measured to set baseline values. The sample was placed inside a clean sock (so the sample

isn’t lost in the machine), the sock was tied shut, and the sample was washed at the local

5

laundromat (normal cycle, cold temperature) with standard CVS brand detergent (CVS

Pharmacy Inc., Woonsocket, RI). The sample was allowed to air dry for 24 hours and the sliding

angle was measured. To test whether the lubricant was removed and could be effectively

replenished, the sample was re lubricated, and sliding angle was measured again.

6

SupplementaryDiscussion

OptimizationofSiMTreatment

Polyester samples were treated with (3-aminopropyl)triethoxysilane (APTES) and

tetraethyl orthosilicate (TEOS) to activate the surface for further surface modification, using a

procedure outlined by Fadeev et al. [1]. After amine cleavage the samples showed drastically

different wetting characteristics, and it is notable that the samples were difficult to submerge in

water for hydrolysis.

The next consideration is the use of TEOS to amplify the newly created hydroxyl groups

(from the previous APTES reaction) to ultimately maximize reactivity with the silica microbeads

and subsequent fluorosilane. A fluorosilane will react proportionally with the amount of

available hydroxyl groups available for reaction; therefore, the contact angle has a direct

correlation with the concentration of surface hydroxyl groups available for reaction (SFigure 1).

The effectiveness of the TEOS reaction was optimized by applying the reaction to flat poly-

ethylene terephthalate (i.e., polyester) surfaces obtained from commercial PET products. To

optimize the TEOS reaction, we performed the reaction for durations ranging from 0 to 8 hours,

fluorinated with a fluorosilane, and measured the static contact angle. At least 10 independent

measurements were taken for each sample and the initially exponential correlation between

reaction time and contact angle can be seen in SFigure 1. After 7 hours the static contact angle

began to plateau and approached the 120° theoretical maximum for flat surfaces. Thus, to

maximize the presence of hydroxyl groups and thereby the superhydrophobicity, 8-hour reaction

times were used for all polyester fabrics.

We measured the static contact angle and contact angle hysteresis for APTES-activated

and unmodified polyester fabrics after silane fluorination, and compared the results with

untreated polyester (STable 1). The contact angle hysteresis of lubricated samples is lowered

from 23.40o ± 10.15 with no treatment to 15.5 ± 4.7° for direct fluorosilanization and 12.67o ±

5.13 for fluorination of APTES-activated polyester. Though the differences are generally close,

7

we chose to continue with APTES activation before silane reaction because of the lower contact

angle hysteresis values and the fact that the silane is covalently linked to the surface.

The in situ silica bead treatment followed the TEOS reaction step to create covalently

bound surface roughness surrounding the threads of polyester fabrics. Note that cotton fabrics

can undergo silica treatment directly. SEM images of the crepe and Nike polyester after silica

bead treatment can be seen in Figure 4. The silica forms a tightly bound shell around each fiber

and polymerizes into submicron-sized spheres with diameters ranging from 100 – 500 nm.

Lessening the TEOS concentration gave a much lower density of particle formation and resulted

in pinning even after fluorination.

All SiM samples were made superhydrophobic by liquid-phase fluorination with a

fluorosilane solution. We used Jersey cotton for simplicity, since it is already abundantly

hydroxylated. Higher silane concentrations and longer reaction times than those used, namely,

4.8% and 60 minutes, respectively, appear to deliver no decrease in the wettability of the surface.

Thus our reactions proceeded for the minimum amount of time required for completion.

Increasing the time, for which the fabric is dipped into the silane solution from 2 to 4 minutes,

only increased the average contact angle by approximately 2.4%. Acetic acid concentrations that

fall outside the range of 0.67-0.83 M cause complete wetting of the fabric by water.

BreathabilityofPTFEMembranes

We tested the breathability of PTFE membranes to observe the breathability of a SLIPS

system with controlled, known pore sizes (Table 4 and SFigure 7). We investigated the

breathability of lubricated and un-lubricated 200 nm, 1 µm and 20 µm PTFE membranes to

observe whether breathability was present in SLIPS made from any of these pore sizes. No

significant breathability was observed in any of these lubricated membranes. Next, to measure

still larger pore sizes, we punctured 200 nm PTFE membranes with small needles that produced

elliptical holes approximately 390 x 80 µm. It is important to note that this membrane, when

lubricated, still effectively repelled hexadecane (SMovie 8). There is a particularly large variance

in the breathability data for this membrane because these holes are below the capillary length of

8

Krytox 102 (which is ~1 mm), and as such a punctured hole can be refilled after the initial

lubricant removal. The data indicate that there is a nonzero breathability increase, but it is not a

statistically significant result (STable 5). We can therefore conclude that we require pores larger

than 80µm in size for the holes to be large enough to prevent refilling of the holes with Krytox;

determining a specific number would require a less flexible material and the creation of holes

closer to the capillary length of Krytox.

With hierarchical pore sizes, controlled application of the lubricant and careful removal

from the holes (e.g., by spinning, forcing air through the membrane, dabbing with an absorbent

membrane, etc.) it should be possible to engineer breathability in a SLIPS fabric. However, the

presence of large pores in the fabric weave lowers the omnirepellency, as evidenced by the

Muslin Cotton achieving the lowest score of the Phase III fabrics in the Aqueous Liquid

Repellency and Hydrocarbon Resistance tests. Hence, Muslin Cotton (or any breathable SLIPS

fabric) fills a niche between the extremely omniphobic, non-breathable SLIPS of the Dense

Polyester and traditional superhydrophobic and breathable fabrics.

9

SupplementaryTables

AATCC 193 Aqueous Liquid Repellency Score

Composition of Test Liquid (% IPA in water,

v/v) Surface Tension (mN/m)

0 None (Fails 2% IPA)

1 2 59.0

2 5 50.0

3 10 42.0

4 20 33.0

5 30 27.5

6 40 25.4

7 50 24.5

8 60 24.0

STable 1. AATCC 193 Aqueous Liquid Repellency Test Liquids [2]. Surface tension values

(Korosl & Kovats, 1981)are for T=25°C

10

AATCC 118 Hydrocarbon

Resistance Score Test Liquid

Surface Tension (mN/m)

0 None (Fails Kaydol)

1 Kaydol Heavy Mineral Oil Not Measured

2 65:35 Kaydol:n-hexadecane

(v/v) Not Measured

3 n-hexadecane 27.2 (Wong, 2012)

4 n-tetradecane 26.55 (Korosl, 1981)

5 n-dodecane 25.3 (Wong, 2012)

6 n-decane 23.6 (Wong, 2012)

7 n-octane 21.4 (Wong, 2012)

8 n-heptane 19.9 (Wong, 2012)

STable 2. AATCC 118 Hydrocarbon Resistance Test Liquids [2]. Surface tension values

(Korosl & Kovats, 1981) [5] are for T=20-24°C.

11

Fabric: Dense Polyester Static Contact Angle

(±SD) Contact Angle

Hysteresis (±SD) Fluorination Method Lubricant

None - 149.6 ± 6.4° 23.4 ± 10.2°

None K100 104.9 ± 3.9° 22.8 ± 8.8°

Direct Fluorosilanization - 152.7 ± 5.8° 24.9 ± 5.5°

Direct Fluorosilanization K100 113.5 ± 2.8° 15.5 ± 4.7°

APTES + Fluorosilanization - 154.7 ± 6.6° 19.2 ± 6.8°

APTES + Fluorosilanization K100 111.9 ± 1.4° 12.7 ± 5.1°

STable 3. Static Contact Angles and Contact Angle Hysteresis Measurements for

different Fluorination Methods. For effective SLIPS, the fluorocarbon polymer lubricant

(Krytox) should be chemically similar to the roughened substrate. The two fluorinated fabrics are

prepared differently: one receives no pre-treatment before reaction with the fluorosilane, while

the other is reacted with APTES followed by hydrolysis and polymerization in TEOS to increase

the reactivity with fluorinated alkoxysilanes. Low contact angle hysteresis indicates that the

lubricant is tightly bound to the fiber, resulting in reduced pinning and a good quality SLIPS

coating. The APTES Silanization was used for future studies because it resulted in lower contact

angle hysteresis and increased stable and covalent association with the fabric.

12

Step Condition Sliding Angle

1 * ** Un-lubricated, Pre-Wash 20.0 ± 5.1°

2 * *** Lubricated with K102 13.8 ± 1.0°

3 *** Washing Machine Cycle 18.1 ± 3.6°

4 ** Re-lubricated with K102 15.8 ± 4.0°

STable 4. Effect of a washing machine cycle on SLIPS fabric performance. SiM-SLIPS

Nike PE fabric was used as a representative fabric to investigate the tolerance of SLIPS fabric to

washing. The fabric sample was lubricated (K102), washed at a laundromat, and re-lubricated;

the sliding angle of a 10 µL water droplet was measured at each stage. The sliding angle

increases after the wash, indicating that some of the lubricant is washed away. However, when

re-lubricated the sliding angle decreases once again, indicating that SLIPS performance is

restored. Pairs of asterisk groups indicate statistically significant difference determined by

student’s two-tailed t-test, for p<0.05. Data are mean ± SD, n≥7.

13

# Fabric WVTR (g/24h/m^2)

Comparing P-Value Comparing P-

Value Trial 1 Trial 2 Trial 3

1 No Membrane 920.9 929.8 991.9 2 to 1 0.00003

2 Parafilm 16.3 44.4 50.8

3 µfiber 428.3 518.9 545.2 4 to 3 0.00280 4 to Parafilm 0.564

4 µfiber 38.1 44.1 51.7

5 Nike PE 426.5 536.9 558.9 6 to 5 0.00290 6 to Parafilm 0.640

6 Nike PE 29.1 43.3 60.3

7 Dense PE 233.9 270.3 307.2 8 to 7 0.00092 8 to Parafilm 0.704

8 Dense PE 18.8 51.4 61.8

9 Cotton 489.5 478.4 511.9 10 to 9 0.00001 10 to Parafilm 0.002

10 Cotton 125.8 159.7 132.8

11 200 nm PTFE 450.1 515.1 456.5 12 to 10 0.00075 12 to Parafilm 0.706

12 200 nm PTFE 42.4 33.8 49.8

13 1 µm PTFE 451.7 518.1 442.8 14 to 12 0.00034 14 to Parafilm 0.732

14 1 µm PTFE 49.3 12.0 33.6

15 20 µm PTFE 446.1 538.9 465.2 16 to 14 0.00068 16 to Parafilm 0.856

16 20 µm PTFE 53.1 14.0 35.4

17 P. 200 nm PTFE 446.8 524.1 470.7 18 to 17 0.00289 18 to Parafilm 0.159

18 P. 200 nm PTFE 218.8 83.1 91.5

STable 5. Raw data and Statistical tests of Water Vapor Transmission Rate (WVTR) of

tested fabrics and membranes. Italicized fabric names denote fabrics/membranes lubricated

with K102. “P. 200nm PTFE” means “punctured PTFE”. Shown P-values are for a one-tailed t-

test comparing the stated groups. All comparisons between lubricated/unlubricated groups

14

reached the p<0.05 significance level, while no lubricated membranes except for Muslin Cotton,

showed significant difference from the Parafilm membrane.

15

SupplementaryFigures

SFigure 1. Optimization of tetraethyl orthosilicate (TEOS) reaction time. TEOS was used

to functionalize polyester with terminal hydroxyl groups, which facilitates covalent linkage of a

fluorinated silane. The static contact angle of a water droplet increases proportionally to the

degree of fluorination, which in turn is a function of the terminal hydroxyl functional groups

available for reaction. As shown in the figure, TEOS should be reacted for ≥ 7 hours to achieve

maximal concentration of terminal hydroxyl groups on a polyester surface.

80

85

90

95

100

105

110

115

0 1 2 3 4 5 6 7 8

Sta

tic

Co

nta

ct

An

gle

(°)

Elapsed Time (h)

16

SFigure 2. FTIR Analysis of untreated fabrics. Each polyester fabric was purchased from

commercial sources; these spectra confirm their composition to ensure that our chemical

approach will work properly. Note that the microfiber fabric is 80% polyester and 20% nylon,

hence the presence of unique spectral peaks.

0

0.1

0.2

0.3

0.4

0.5

0.6

600110016002100260031003600

Co

un

ts

Wavenumber (1/cm)

Crepe Dense PE Gav. µfiber Nike PE

17

SFigure 3. Aqueous Liquid Repellency Test images. Representative series of images from

the AATCC 193 Aqueous Liquid Repellency test. Un-lubricated Dense PE conditionally passes

liquid 6 (40% IPA in water, ϒ=25.4 mN/m) and fails liquid 7 (50% IPA in water, ϒ=24.5 mN/m)

to achieve a score of 5.5.

18

SFigure 4. Hydrocarbon Resistance Test Images. Representative series of images from the

AATCC 118 Hydrocarbon Resistance test. SiM-SLIPS treated Dense PE achieved a maximum

score of 8 in the AATCC 118 hydrocarbon resistance test, and is not wetted by any hydrocarbon

with surface tension greater than or equal to heptane (ϒ=19.9 mN/m).

19

SFigure 5. Images of breathability testing capsule. The breathability of fabrics and PTFE

membranes was measured with this capsule. Desiccant is sealed within a capsule with the

membrane to be tested, and the mass change of the whole unit (A) is measured to determine the

amount of water vapor transmitted from the controlled external environment.

20

SFigure 6. Breathability of SLIPS-Fabric samples. We measured the mass change of a

desiccant-containing capsule sealed with SiM-treated phase III fabrics, including µfiber (A),

Nike PE (B), Dense PE (C), and M. Cotton (D). Chambers sealed with Parafilm and open

chambers act as controls to display the functional range of this system. All lubricated fabrics do

not exhibit statistically significant breathability (significance level: p<0.05) when compared with

the Parafilm control, except for SLIPS-M. Cotton, which shows 4-fold better breathability than

the control.

A B

C D

21

SFigure 7. Breathability of PTFE Membranes. We measured the mass change of a

desiccant-containing capsule sealed with PTFE membranes of 200 nm (A), 1 µm (B), and 20 µm

(C) pore size. Chambers sealed with Parafilm and open chambers act as controls to display the

functional range of this system. All PTFE membranes exhibited good breathability when dry, but

did not show a statistically significant difference (significance level: p<0.05) from the parafilm

control when lubricated with Krytox 102 (K102).

A

B C

22

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1

Supplementary Movies: Lubricated nanostructured fabrics displaying robust

omniphobicity

Cicely Shillingford1, Noah MacCallum1, Tak Sing Wong1, Philseok Kim1, Joanna Aizenberg1*

School of Engineering and Applied Sciences, Wyss Institute for Biologically Inspired

Engineering and Kavli Institute for Bionano Science and Technology, Harvard University,

Cambridge, Massachusetts 02138, USA

2

SMovie 1. Bouncing Droplet Demonstration. Nike Polyester fabric is rendered superhydrophobic when treated with silica microparticle deposition and subsequent fluorina8on. A stream of water is sprayed from approximately 10 cm above the fabric and bounces off the repellent fabric.

 

SMovie 2. Omniphobic liquid repellency demonstration of SLIPS-Fabrics. The liquid repellency of various SLIPS fabrics. In each frame, untreated fabric is on the left, sol-gel boehmite (SgB)-SLIPS is in the middle, and silica microparticle (SiM)-SLIPS is on the right. A variety of liquids (water, cooking oil and hexadecane) are applied to the fabrics and repelled by the two SLIPS fabrics while the untreated fabric becomes wet and stained.

3

 

SMovie 3. Stain resistance of SLIPS-Fabrics. SLIPS-fabrics exhibit stain resistance even when droplet pinning occurs. Muslin Cotton has a loose weave pattern with many loose fibers that cause this fabric to have a high sliding angle despite being omnirepellent. To demonstrate, a droplet is pinned to a tilted fabric and easily and totally removed by wiping with a Kimwipe.

SMovie 5. Rub testing of SiM-Treated SLIPS-Fabrics. Abrasion resistance is an important consideration. This demonstration shows preliminary indica8ons that the repellency of silica micorparticle (SiM) modified Nike Polyester (un-lubricated) remains after rubbing with a Kimwipe. SEM characterization confirms that the microstructures withstand the abrasion (Figure 6).

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SMovie 6. Rub testing of SgB-Treated SLIPS-Fabrics. Abrasion resistance is an important consideration. This demonstra8on shows preliminary indications that the repellency of sol-gel boehmite (SgB) modified Nike Polyester (un-lubricated) remains after rubbing with a Kimwipe. SEM characterization reveals that there some cracking of the boehmite structure results from this experiment, indicating that this is not a robust hydrophobic coating for fabrics.

 

SMovie 7. Droplet Impact Testing Demonstration. We performed droplet impact tes8ng on phase 3 fabrics to observe impact resistance – Nike PE is shown as a representa8ve example. A water droplet is dropped from a height of 20.3 cm. The water repellency of unlubricated fabrics (i.e., conven8onal superhydrophobic surfaces) relies on the maintenance of microscale air pockets on the surface. Under pressure, water penetrates the air pockets and the droplet becomes stuck. SLIPS-fabrics are tolerant to pressure and maintain low sliding angles after impact from a water droplet.

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SMovie 8. Slippery performance of a punctured SLIPS membrane. To see whether breathable SLIPS might be possible, we punctured a PTFE membrane with 26 1⁄2 gauge needles, lubricated with Krytox 102 and assessed the breathability and SLIPS performance. This demonstrates a droplet of hexadecane sliding over the punctured holes, experiencing only minor pinning from the edges of the holes.