Supercooled Water Drops Do Not Freeze During Impact on Hybrid

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Supercooled Water Drops Do Not Freeze During Impact on Hybrid Janus Particle-Based Surfaces Madeleine Schwarzer, Thomas Otto, Markus Schremb, Claudia Marschelke, Hisaschi T. Tee, Frederik R. Wurm, Ilia V. Roisman, Cameron Tropea, and Alla Synytska Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03183 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

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Chemistry of Materials

Supercooled Water Drops Do Not Freeze During Impact on Hybrid Janus Particle-Based Surfaces Madeleine Schwarzer1,2, Thomas Otto1,2, Markus Schremb3, Claudia Marschelke1,2, Hisaschi T. Tee4, Frederik R.Wurm4, Ilia V. Roisman3, Cameron Tropea3 and Alla Synytska1,2* 1Leibniz-Institut

für Polymerforschung Dresden e.V., Hohe Str. 6, 01069 Dresden, Germany Universität Dresden, Fakultät Mathematik und Naturwissenschaften, 01062 Dresden, Germany 3Technische Universität Darmstadt, Alarich-Weiss-Str. 10, 64287 Darmstadt, Germany 4Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany 2Technische

ABSTRACT: Icing of various surfaces is often a result of the collision of supercooled water drops with the substrates. Ice formation from supercooled water drops is initiated by nucleation when the size of an ice embryo reaches a critical value. The lack of controlling the inception of heterogeneous nucleation and the rate of solidification, which depend on the properties of the substrate, temperature, and the impact parameters of the liquid drop, poses a very serious challenge to the design of effective icepreventing materials. In this exploratory experimental study, we show how a significant nucleation delay during impact of supercooled water drops can be achieved by tuning of the properties of the substrate and, specifically, by introducing a chemical and topographical heterogeneity on the surfaces formed either by a mixture of polymer coated hydrophilic and hydrophobic particles or Janus particles. We have discovered that the nucleation rate during drop impact is significantly reduced on heterogeneous surfaces formed by a mixture of hydrophilic and hydrophobic particles. Exceptionally, freezing is completely prevented on surfaces made of amphiphilic Janus particles. Even after a repetition of 100 drop impact experiments, no single drop froze at all. After impact of the supercooled water drops a rebound occurs and afterwards smaller secondary drops are formed, which can be easily removed. Moreover, the designed surfaces demonstrate a good scratch resistance and robustness. The presented findings open a promising pathway for the rational design of effective passive ice-preventing coatings using Janus particles as building blocks.

Introduction Icing of wind turbines, power transmission lines, antennas, airframes of helicopters or aircrafts, is a severe problem, since it may lead to failures in energy supply or to dangerous transport accidents. Ice formation from supercooled water is initiated by nucleation when the size of an ice embryo reaches a critical value.1-3 The initial nuclei then trigger a fast, unavoidable expansion of a cloud of dendritic ice crystals.1-3 Two typical forms of drops contacting solid surfaces are sessile drops appearing due to water condensation from the vapor phase under common atmospheric conditions4-6 and impacting liquid drops.7-9 Apart from frost formation processes, the impact of supercooled water drops gives a further understanding of the freezing process. Icing due to the impacting drops is relevant to various applications like airframe, wind turbine icing, or helicopter blades. Several groups systematically examined nucleation during the impact of supercooled water drops on different inorganic and organic surfaces.8, 10-12 For instance, Schremb et al. showed that the freezing delay of impinging drops on pure Al and sandblasted glass surfaces varies over several orders of magnitude between few milliseconds up to several seconds.7 Moreover, they found that gas bubbles entrapped below the liquid during drop impact generally promote nucleation. As a consequence, high gas content in the impinging liquid results in an increased nucleation rate due to a decelerated bubble dissolving after impact. A statistical model, serving as an analysis tool for

experiments aiming on the rate of heterogeneous nucleation during drop impact, has been developed.7, 13 The model allows quantification of the formation of active nucleation sites during drop impact and by this it enables the examination of the statistical freezing delay. Other groups studied the impact of supercooled water drops on organic substrates.8, 10-12, 14 For instance, Aizenberg et al. have succeeded to design nanostructured superhydrophobic surfaces, coated by polymeric materials, that remain entirely ice-free.8 Poulikakos et al. investigated the influence of viscosity on micro-textured superhydrophobic surfaces during the impact of supercooled water-gylcerol drops.10 They showed that an increased viscosity leads to a reduced drop spreading and an increased contact time.10 The dynamic process of impacting supercooled drops was investigated on hydrophobic and super-hydrophobic surfaces and the typical impact behavior of supercooled large drops has been examined.7, 15 It was demonstrated that on superhydrophobic surfaces, the speed of ice formation is higher for the dynamic process of an impact than for sessile drops, because of the increase of the density of nucleation sites.15 Many existing studies are devoted also to the investigation of ice nucleation based on frozen sessile drops appeared due to water condensation or desublimation from the vapor phase (frost formation) or, then resulted de-icing of frozen surfaces using ice adhesion strength measurements. For this, various

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well-known strategies for the design of ice preventing materials i.e., surfaces were reported. 16-19 One way to inhibit ice formation and growth is based on the use of hydrophilic polymers and utilization of colligative properties of solutions.20, 21 For example, the icing rate can be reduced by using polyelectrolytes, which lower the freezing point of water.21, 22 An alternative biomimetic concept to depress the freezing point and to delay ice nucleation is based on anti-freeze proteins used by Antarctic fish.23, 24 In this case, the temperature of ice crystal formation is kinetically decreased. However, once nucleation sites become active, the growth rate of ice on such surfaces is very fast. Another strategy for the reduction of the hazard of icing implies lowering the surface energy of materials and designing hydrophobic, superhydrophobic25, 26 or lubricant-infused surfaces possessing low ice adhesion strength.14, 21, 22, 26-32 For instance, polydimethylsiloxane (PDMS), combines a low surface energy with an outstanding elasticity and thus reduces the ice adhesion.17, 33 In general, superhydrophobic coatings demonstrate ice preventing properties, but once an ice layer (frost) is formed, it can hardly be removed when water drops penetrate between the features on the structured surfaces.34 Also, very often, the removal of the ice layer leads to a loss of the superhydrophobic properties.34 In the case of lubricants, the formation of ice is prohibited due to the lubricant softness.21, 27, 30, 31 On the other side, the better robustness aspects should be addressed in the strategy development. Combination of both hydrophilic and hydrophobic properties in one material can lead to the synergism of properties and effects. This idea can be realized through surfaces with chemical heterogeneity.35-38 For instance, Van Dyke et al. showed that chemical heterogeneity, created on surfaces by photolithography in combination with modification of a hydrophobic substrate with hydrophilic octadecayltrichlorosilane, allowed for a suppression of the freezing temperature of water, freezing time delay and control of coalescence dynamics.37, 38 Murase et al. showed a reduction of ice adhesion and prevention of snow accretion on nanoscale surfaces based on organopolysiloxane modified with lithium.39 The effect of combining hydrophilic and hydrophobic regions of a native oxide on Si wafer on the icing process was investigated by Varanasi et al.36 The hydrophobic regions in these experiments have been modified with fluorosilane using micro contact printing. The freezing experiments have shown that the water drops favored to nucleate on the hydrophilic regions.36 Also, Aizenberg et al. showed that a combination of both topographical and chemical surface heterogeneity, introduced for instance through deposition of a hydrophilic polymer (polyvinyl alcohol) and particles on the tips of superhydrophobic surfaces, enabled control of condensation and freezing of water drops. 35 Recently, we reported on the design of chemically and topographically heterogeneous surfaces with effective anti-icing and de-icing capability using amphiphilic 1 µm large hybrid Janus particles (JP).40 These heterogeneous Janus-based surfaces exhibited special surface “edge” morphologies and showed formation of large unstable dendrites at the edges of heterogeneities, as a result of ice nucleation due to water condensation, as well as areas free of ice. Eventually, the synergism of both effects led to the extremely low ice adhesion.40 However, ice nucleation after

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the impact of supercooled water drops on such surface heterogeneities induced by chemical composition and topography has not been investigated before. In the present study the nucleation delays, the nucleation rate and the outcome of an impact of a supercooled water drop onto various heterogeneous surfaces is investigated using a high-speed video system. The substrates, used in the experiments, are formed either by a mixture of polymer coated hydrophilic and hydrophobic particles or Janus particles. The influence of the chemical heterogeneity and structure of the surface coating on the impact outcome and solidification delay is studied in order to determine the governing mechanisms of icing during drop impact. Results and discussion Synthesis of Hybrid Janus Particles and Design of Surfaces: Spherical hybrid 200 nm large core-shell Janus particles (Figure 1 b, d) and fully covered (FC, Figure 1 a, c) particles with hydrophilic poly(ethylene glycol) methacrylate (PEGMA) and hydrophobic poly(dimethylsiloxane) monomethylacrylate (PDMSMA) shells were fabricated and used for the preparation of coatings. The core composed of SiO2 was obtained by the Stöber method41 and the polymer shell was chemically immobilized on the core using the ARGET-ATRP approach. Briefly, for the JP a combination of ‘grafting from’ and ‘grafting to’ approaches was used.42 Before the ATRP, the functionalization of the particle surface was confirmed by zeta-potential measurements (Figure S1). Compared to the functionalization with (3-aminopropyl)triethoxysilane (APTES) (isoelectrical point: IEP = 8.2) the particles with ATRP initiator (-bromoisobutyrylbromide, BrIn) show a shift to a lower pH value (IEP = 3.2) and the subsequent polymerization lead again to a change of the IEPs (Figure S1). The resulting Janus ratio is 1:1, which was evaluated using scanning electron microscopy (SEM) and cryogenic transmission electron microscopy (cryo-TEM). The polymer layer thickness on the particle surfaces was calculated based on the results of a thermogravimetric analysis (TGA) (Table S1, Figure S2). The shell thickness for the FC particles was 7-8 nm. The grafting density of 0.24 nm-2 was evaluated for Janus-P(PEGMA)PDMS by gel permeation chromatography (Table S1) and TGA. Furthermore, the isoelectric point shifts to a pH value of 3.9 after grafting of P(PDMSMA), below a pH value of 2.5 after grafting of P(PEGMA), and to a pH value of 2.9 after grafting of P(PEGMA) and PDMS for the Janus particles as compared to the APTES-modified particles (Figure S1, black circles, IEP =8.2) and initiator-modified particles (Figure S1, red circles, IEP = 3.2) surface. Next, the particle-based layers were prepared using a drop casting method: different particle dispersions were dried on top of aluminum substrates pre-modified either with a poly(glycidyl methacrylate) (PGMA) or poly(methyl ethylene phosphate) maleimide (PMEPMI) as an adhesive promoter (see Experimental Section). Additionally, flat polymer brushes were grafted on aluminum substrates serving as a reference. The details on the synthesis are described in the experimental part. An overview of the prepared samples is given in Table 1. The various particle layers have different topographies. P(PEGMA)- and P(PDMSMA)-modified particles form almost monolayers (Figure 1 i, j). Coatings from a 50:50 mixture of P(PEGMA)- and P(PDMSMA)- modified particles form slightly structured layers without large aggregates


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(Figure 1 k) and with a low roughness (Table S2). Such coatings are rather important for this study since they can be considered as a step towards heterogeneity. The considerably different morphology was formed by Janus particles – these particles form relatively large structures, in the order of micrometers, obtained as a result of their self-assembly (Figure 1 l). As a result, the roughness for these surfaces increased (Table S2). In our previous work, it has been demonstrated, on the example of larger 1µm JPs, that hydrophilic-hydrophobic Janus particles are able to controllably agglomerate, which allows fabrication of superhydrophobic surfaces.43 In order to obtain statistical insight into the orientation of Janus particles in the prepared layers, their distribution and orientation were investigated by probing the adhesion properties of the top layer of the JPs using atomic force microscopy (AFM) direct force measurements. The evaluation of the adhesion properties via direct force measurements were performed under normal air pressure and a relative humidity of the ambient increased to 80%. It was found that the reference layers made of fully covered P(PEGMA)-particles are less adhesive than those made of the P(PDMSMA)-modified particles at these conditions (Figure 1e). The layers formed by bi-component Janus particles (JPs) exhibited different adhesion behavior to the colloidal probe. In the case of the Janus-based particle layers the adhesion strength depends on the particle orientation. There are different types of particle orientations: JPs turned with their hydrophobic side up, JPs with their hydrophilic side facing up, and JPs partially exposing both sides to different extents. Similarly to fully covered particle layers, the P(PEGMA)modified side of the JPs is less adhesive than the PDMSmodified one (Figure 1g). The adhesion strength for Janus particles exposing both P(PEGMA)-and PDMS-sides to the environment is in-between of those where oriented upward either with their hydrophilic or their hydrophobic parts(Figure 1 g; black line).

Figure 1. Representative SEM (top, a-b) and cryo-TEM (top, c-d) images of a, c) fully covered (P(PEGMA), P(PDMSMA)) and b,d) Janus particles; atomic force microscopy measurements on representative layers with Mixed-P(PEGMA)-P(PDMSMA) and Janus-P(PEGMA)-PDMS performed at 80% relative humidity: e)

direct force measurement on the Mixed-P(PEGMA)P(PDMSMA) (orange lines: P(PEGMA); green lines: P(PDMSMA); dashed line: approaching curve; solid line: retracting curve); f) false color mapping of the MixedP(PEGMA)-P(PDMSMA) (orange: P(PEGMA); green: P(PDMSMA)); g) direct force measurement on the JanusP(PEGMA)-PDMS (orange lines: P(PEGMA); green lines: PDMS; black lines: Janus particles with half P(PEGMA) and half PDMS; dashed line: approaching curve; solid line, retracting curve); h) false color mapping of the Janus-P(PEGMA)-PDMS (orange: P(PEGMA); green: PDMS; half orange half green: Janus particles with half P(PEGMA) and half PDMS and SEM images of surfaces prepared from FC particles i) Particle-P(PEGMA); j) Particle-P(PDMSMA); from mixture k) Mixed-P(PEGMA)P(PDMSMA); and from Janus particles l) Janus-P(PEGMA)PDMS.

Next, the statistical analysis of the particles orientation in the prepared layers in at least five 20 × 20 μm topographical AFM images was performed. For surfaces formed of mixedP(PEGMA) and P(PDMSMA) particles, a 47%:53% ratio between P(PEGMA):P(PDMSMA) was found. In contrary, on the surfaces based on Janus particles, it was found that 33 % of them were oriented in the fashion exposing both hydrophilic (P(PEGMA)) and hydrophobic (PDMS) sides to different extents. Moreover, 39% of JPs are turned with their hydrophilic P(PEGMA) (Figure 1 g orange line) and ca. 28% with the hydrophobic PDMS side up (Figure 1 g green line). Therefore, the overall distribution of the JPs is about one third for each side of the particle (P(PEGMA) : PDMS : P(PEGMA)/PDMS 39:28:33). And the overall contribution of hydrophilic and hydrophobic amount in JP layers is ca. 58:42, respectively. Wetting Properties of the Surfaces: Wettability measurements on unmodified aluminum substrates were performed by measuring the dynamic advancing and receding contact angles of water, and were found to be adv = 55° and rec = 16°, respectively (Figure 2). The aluminum substrates pre-coated with a PGMA layer are relatively hydrophilic (adv = 68°; rec = 29°). The immobilization of polymers and particles changes the wetting properties. While the immobilization of P(PEGMA) brushes onto flat aluminum substrates (Flat-P(PEGMA)) almost do not affect the wetting properties, immobilization of P(PEGMA)-modified particles leads to a considerable hydrophilization. The reduction of the contact angle is presumably a result of the water imbibition in the pores between the functionalized particles, i.e. a transition to the Wenzel wetting regime. All other coatings exhibit a hydrophobic behavior, whereby the hydrophobicity increases from the flat P(PDMSMA) polymer-coated aluminum substrates (Flat-P(PDMSMA)) to rough particle-based surface from FC 200 nm large P(PDMSMA)-modified ones. This effect is due to the implementation of roughness into the system using particles. The roughness on particle-based coatings reinforces the hydrophobicity, and thus the contact angles increase, which agrees with previous observations.43-45 Mixing of hydrophilic and hydrophobic particles in a 50:50 ratio leads to an intermediate wetting state between the monofunctionalized Particle-P(PEGMA) and Particle-P(PDMSMA) surfaces (Figure 2). An advancing contact angle of 96° and receding contact angle under 10° was detected on the heterogeneous surface formed by mixed hydrophilic (ParticleP(PEGMA)) and hydrophobic (Particle-P(PDMSMA)) particles in a 50:50 mixing ratio (Mixed-P(PEGMA)-


Chemistry of Materials P(PDMSMA)) (Figure 2). Water is imbibed into the layer upon liquid receding. Interestingly, highly hydrophobic (but not superhydrophobic) behavior was observed on JP coatings for the advancing as well as the receding water contact angle (adv = 130°; ec = 119°), resulting in a low contact angle hysteresis ( = 11 ± 2°). Surprisingly, the value of receding contact angle measured on layers of JPs is higher than that measured on layers of P(PDMSMA)-modified particles. The highly hydrophobic properties of layers made of 200 nm Janus particles can be explained by the rough morphology of these layers due to the self-assembly of individual JPs, as was also shown in our previous work 43. In fact, it is known that rough surfaces of intrinsically hydrophobic materials demonstrate such kind of increasingly hydrophobic behavior, which is due to entrapment of air in the pockets between particles, i.e. the water drop sits on top of the JP-aggregates. The Janus particles with both hydrophobic and hydrophilic sides form a randomly distributed layer. The neighboring particles form hydrophobic and hydrophilic clusters. This structure allows a local dewetting on the hydrophobic clusters, leading to liquid contact with the surface only at the hydrophilic clusters. This state is similar to the fakir CassieBaxter state46 which is typical to superhydrophobic, selfcleaning surfaces. Thus, unique geometry of layers of Janus particles results in a highly hydrophobic behavior of layers formed by these particles. 160

Water contact angle [°]

adv rec

120 80 40

MixedP(PEGMA)-P(PDMSMA) JanusP(PEGMA)-PDMS

Particle-P(PDMSMA)

Particle-P(PEGMA)

Flat-P(PDMSMA)

Flat-P(PEGMA)

Al-PGMA

0

Al-cleaned

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Figure 2. Water contact angle measurements (θadv: advancing contact angle; θrec: receding contact angle) on cleaned and PGMA-modified aluminum substrates and on freshly prepared coatings made of fully covered particles with P(PEGMA) and P(PDMSMA) shell and Janus particles with P(PEGMA) and PDMS.

In addition to these dynamic contact angle measurements, static water contact angle measurements at different temperatures (23°C, 0°C, -5°C and -10°C) were performed (Figure S3) to obtain an understanding of the behavior at lower temperatures.

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In general, the contact angle changes only slightly for varying temperature; it decreases with decreasing temperature. This tendency is very pronounced for the hydrophobic structured surfaces made of particles. This temperaturedependent wettability was also found by Heydari et al.47 and Wang et al.48 It was suggested by Wang et al. that the reduced saturation of the water vapor in combination with a liquefied water vapor within the particle structures leads to a decreasing contact angle.48 Wetting behavior during Condensation, Freezing and Thawing: The freezing and thawing behavior of condensed water on the various coated surfaces was observed using optical microscopy (Figure 3, S4, S5, S6). In particular, we observed the whole process starting from condensation of water on the surfaces, through freezing of small droplets, right up to thawing of the ice. The formation of many small frozen droplets with the size of approximately 10 µm on hydrophilic flat and particle P(PEGMA) surfaces and on mixed layers was observed (Figure 3 a, c, e). Relatively large frozen droplets (ca. 50 µm) were observed on hydrophobic surfaces: flat P(PDMSMA), P(PDMSMA) particle layer and layer of Janus particles (Figure 3 b, d, f). Thawing of ice also strongly depends on the substrate properties. For example, large individual drops (200-300 µm) are formed on the flat P(PEGMA) surface and the mixed particle layer. A continuous water layer is formed on the particle P(PEGMA) layer. Small water droplets (approx. 100 µm) are formed on the hydrophobic flat and particle P(PDMSMA) surfaces. Water rapidly condenses on hydrophilic surfaces which results in a large number of droplets eventually freezing, whereas on hydrophobic surfaces condensation results in fewer but larger drops. This is in direct connection with their wetting behavior. When the water reaches a certain size or a sufficient degree of supercooling, nucleation starts triggering freezing of the condensate drops. To understand these phenomena in more detail, drop impact experiments were performed and are described in the following. Ice Nucleation during Drop Impact: The hydrodynamics and ice nucleation of supercooled water drops during their impact onto the differently coated surfaces has been examined at a temperature of -16.0 °C using the experimental setup described below and shown in Figure S7. As shown in Figure 4 e, spreading on all surfaces is very similar since it is mainly governed by inertia and the forces associated with the substrate wettability are negligibly small. The maximum spreading is only slightly affected by the surface coating; it decreases for an increasing hydrophobicity of the surface. However, the behavior after maximum spreading significantly depends on the substrate chemistry and topography (Figure S8). In comparison to the hydrophilic surfaces, the receding rate is much higher on the hydrophobic surfaces. The dewetting behavior on heterogeneous Mixed-P(PEGMA)P(PDMSMA) is somewhere between the behavior on the respective homogeneous surfaces. As already shown in previous studies7, 13, the freezing process in an impinging drop appears as a dark expanding circular region (see Figure 4 a, g). In the experiments, both the substrate as well as the drop are well below the equilibrium freezing temperature of water and thus the ice grows dendritically.4 This first partial freezing of the supercooled


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liquid is followed by a slower quasi-isothermal stage characterized by solidification of the remaining liquid at the equilibrium freezing temperature (not shown in Figure 4).4

Figure 3. Representative optical microscope images from the formation of frozen and dewed drops during water condensation onto flat surfaces: a, g) Flat-P(PEGMA) and b, h) Flat-P(PDMSMA) and particle-based surfaces: c, i) Particle-P(PEGMA); d, i) ParticleP(PDMSMA); e, j) Mixed-P(PEGMA)-P(PDMSMA) and f, k) Janus-P(PEGMA)-PDMS. The upper row shows different surfaces after the freezing process at -2°C with different frozen drop structures and the bottom row shows the partially wetted surfaces after thawing at 5°C.

Due to a significantly decreased receding rate of the impinging liquid, water drops freeze in their flattened shape on hydrophilic Flat-P(PEGMA), Particle-P(PEGMA) and MixedP(PEGMA)-P(PDMSMA) layers (Figure 4 a, c, f). In contrast, water drops may further recede after impact on the hydrophobic Flat-P(PDMSMA) and Particle-P(PDMSMA) layers, resulting in a reduced area iced after nucleation (Figure 4 b, d). This effect is further intensified on the substrate coated with Janus particles: water drops contract very rapidly (Figure 4 e, h). After receding they adopt an almost spherical shape, which results in a minimized contact area between the drop and the surface (Figure S8 l), thereby also minimizing the area potentially iced after drop impact. By this, the most promising system for anti-icing applications is Janus-P(PEGMA)-PDMS since the rate and extent of dewetting is significantly enhanced in comparison to the remaining coatings, as shown in Figure 4 e. The impacting drop recedes followed by a partial break-up (Figure 4 h) and a rebound of the impinging liquid after retraction Figure 4 i). Within 10 ms after drop impact, the liquid receding, break-up and rebound are already finished and the final spherical shape of the resting primary and secondary drops is reached (Figure S8 l). Figure 4 j gives a quantitative comparison of the percentage of drops in which nucleation has been observed during the observation time on the differently coated surfaces. The total number of frozen drops was determined and the percentage of frozen drops out of 100 experiments was calculated. All investigated flat and rough P(PDMSMA)- and P(PEGMA)based systems show a low quantity of frozen drops being below 30 %. On the particle based coatings even less water drops froze after impact. Therefore, the surface morphology has a strong

influence on the nucleation process. Only approximately 15 % of the drops are frozen on Mixed-P(PEGMA)-P(PDMSMA) and even only less than 10 % of the drops on ParticleP(PDMSMA). Due to the significantly reduced contact time and contact area between the drop and the surface, all drops impacting on Janus-P(PEGMA)-PDMS do not freeze at all within the observation time of 8 s. In comparison to the remaining systems, this behavior is unique and the most remarkable effect of the Janus particle-based system. The characteristic drop survival curves illustrating the time dependence of nucleation on the different surfaces are shown in Figure 4 k. The relative amount of liquid drops Nliq/N0 is plotted over the freezing delay after impact, tdelay, where N0 denotes the total number of drop impact experiments under constant conditions, and Nliq is the number of drops being liquid until tdelay; each point in the graph represents nucleation in an individual drop. The curves for Flat-P(PEGMA) and ParticleP(PEGMA) are qualitatively very similar. In both cases, the nucleation rate which is proportional to the slope of the curves in the diagram is the highest in a short phase after impact and decreases over time. However, the relative number of liquid drops is higher on Particle-P(PEGMA) for all times after impact. The same vertical shift of the survival curves can be observed for the homogeneous P(PDMSMA) systems (FlatP(PDMSMA) and Particle-P(PDMSMA)), clearly indicating a significant influence of the surface morphology: nucleation is preferred on flat coated substrates. Astonishingly, heterogeneous mixing of hydrophobic and hydrophilic particles in the case of Mixed-P(PEGMA)-P(PDMSMA) does not result in a nucleation behavior which may be seen as a mixture of the nucleation behavior on the homogeneous systems. In fact, the nucleation rate on this coating is smaller



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than for both individual homogeneous systems, indicating a synergetic effect of the heterogeneous wettability distribution on the surface. This beneficial effect is even more pronounced in the case of the Janus-P(PEGMA)-PDMS surface. As already mentioned, no drop at all froze on that surface and therefore no drop survival curve is shown in Figure 4k for this coating. Concluding, a heterogeneous mixture of both properties leads to a significantly reduced nucleation rate, which is generally in accordance to our previous work.40

Figure 4. Kinematics and freezing of a drop impacting onto flat surfaces: a) Flat-P(PEGMA) and b) Flat-P(PDMSMA), and onto particlecoated surfaces: c) Particle-P(PEGMA); d) Particle-P(PDMSMA); f, g) Mixed-P(PEGMA)-P(PDMSMA) and h, i) Janus-P(PEGMA)PDMS. For each homogeneous system a picture is shown for the beginning of the nucleation (a, b, c, d). Moreover, a picture is shown for the heterogeneous systems for maximum spreading (f), propagation of the dendritic freezing front (g) for Mixed-P(PEGMA)-P(PDMSMA) and separation into smaller droplets (h), rebound of the drop (i) for Janus-P(PEGMA)-PDMS. Results from drop impact experiment: e) Curves obtained from the total wetted area calculated by the maximal expansion during and after drop impact; j) Relative number of frozen drops out of all drops for this experiment; k) Comparison of survival curves for drop impact experiment on flat and particle-based surfaces


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(grey: Flat-P(PEGMA), black: Particle-P(PEGMA), light blue: Flat-P(PDMSMA), dark blue: Particle-P(PDMSMA), orange: MixedP(PEGMA)-P(PDMSMA), red: Janus-P(PEGMA)-PDMS; dashed lines are extrapolated).

Coating Robustness: The robustness of the surface coatings is an important factor for the applicability of the investigated systems for their intended use. In particular surface stresses accompanying the volume expansion during drop freezing may cause damaging of the coatings. Therefore, it is important to have an overview of the before and after appearance of the coatings. Here, we studied the topographical and morphological surface properties before and after the impact of ten supercooled water drops with SEM and OM (Figure S9, S10). The flat coatings (Figure S11) show a homogeneous and smooth structure prior to drop impact. Damages caused by drop impact are found on Flat-P(PEGMA) at the edge of a spread drop. The homogeneous particle-coated surfaces are rough (Figure 1 i-l) and show a rather low robustness. ParticleP(PEGMA) and Particle-P(PDMSMA) are completely damaged after the impact. In case of Particle-P(PEGMA), large pieces, up to a size of 100 µm length, are cracked as shown in Figure 5a. For Particle-P(PDMSMA) the surface morphology changed completely (Figure 5 b). In contrast, the heterogeneous systems are more robust. Uniform surface topography before (Figure 1 k) as well as after the drop impact (Figure 5 c) is visible for MixedP(PEGMA)-P(PDMSMA). After drop impact a thin dark band at the edge of the drop appears. However, there is no severe damage, as observed for all other systems; it is a contamination that occurs as a result of evaporation of a drop, which is strongest at this border. The Janus particles form structured coatings, since they form self-assembled structures and aggregates (Figure 5 d). No distinctive changes of the surface morphology after the drop impact experiments were found for Janus-P(PEGMA)-PDMS layers. No remarkable changes for the water advancing contact angle before and after the drop impact experiments are observed (Figure S12). For the P(PDMSMA) coatings the contact line of the drop during the contact angle measurement is pinned at the largest diameter of the drop and only the volume varies.

Figure 5. Representative SEM images of the surfaces prepared from FC particles: a) Particle-P(PEGMA); b) ParticleP(PDMSMA); c) from mixture Mixed-P(PEGMA)-P(PDMSMA); and d) from Janus particles Janus-P(PEGMA)-PDMS after the drop impact experiment at the edge of the wetted area after drop impact.

Heterogeneous systems based on mixed fully covered and Janus particles have the great advantage over the homogenously structured surfaces, because they are very robust and do not change their surface morphology through the drop impact or the freezing of the drop. Even after ten drop impacts no changes are visible. Finally, the influence of the adhesive layer on the coating robustness was investigated. For all systems we used PGMA as a model system, where the particles are physically attached. In addition, we tried PMEPMI as an alternative to PGMA,

with the use of phosphate groups (Figure S13). The coating robustness was investigated applying a universal surface tester (UST) with a cutting diamond (Experimental Section). The scratch test was performed with a linearly increasing loading between 1 to 100 mN. We found that the surface modified with PGMA failed with a load of 34 mN, while the PMEPMI adhesive layer showed a higher load resistance of 45 mN. The PMEPMI-based coating sustains a higher load to be destroyed and therefore possesses a larger robustness. Accordingly, PMEPMI as an adhesive layer is highly promising and will be further investigated in more detail in future work. Conclusions Heterogeneous ice nucleation on a wetted solid substrate is the first, defining step of ice formation. Icing prevention is possible by a significant reduction of the adhesion forces or by delaying the nucleation event beyond the wetting time. In this exploratory experimental study, the influence of heterogeneity either induced by surface topography or chemical composition on the nucleation in supercooled water drops during drop impact and condensation is extensively investigated using a high-speed video system. Additionally, the synthesized coatings were tested regarding their scratch resistance and their robustness against stresses arising during drop impact and freezing. The chemically homogeneous and heterogeneous surfaces were prepared from hydrophilic and hydrophobic homogeneous, or amphiphilic hybrid Janus core-shell particles. Synthesized hybrid hairy Janus particles with hydrophilic and hydrophobic polymeric shells at their opposite sides form unique structured surfaces due to special selfassembly of the particles, which leads to a heterogeneous structuring of the coating, proved by AFM force distance measurements. On the other hand, the designed Janus particlebased layers demonstrated a highly hydrophobic (but not superhydrophobic) behavior. These particle-based surfaces can be easily prepared on a large scale using spraying or solvent casting techniques. Chemically homogeneous systems showed a high ice nucleation rate, which is strongly influenced by their wetting behavior and the structure of the surface. In contrast, chemically heterogeneous coatings led to a decreasing of the ice nucleation rate. We have discovered that the nucleation rate is significantly reduced on heterogeneous surfaces formed by a mixture of hydrophilic and hydrophobic particles. Exceptionally, freezing is completely prevented on the surfaces made of amphiphilic Janus particle-based coatings with modularity in chemical composition and final surface topography. Even after a repetition of 100 measurements, no single drop froze at all. After impact of the supercooled water drops a rebound occurs and afterwards smaller secondary drops are formed, which can be easily removed. Moreover, the scratch test measurements demonstrated that the homogeneous surfaces become damaged easily through the large forces during impact and freezing of a drop. Chemical heterogeneity solves this problem; the surfaces have an improved scratch resistance and robustness preventing severe damages. Due to the smaller hydrophilic and hydrophobic areas, no large domains can be removed through the forces during impact or freezing.



The presented work opens a promising pathway for the use of Janus particles for rational design of effective icepreventing coatings. Experimental Section Materials and Methods. Materials: (3-Aminopropyl)triethoxysilane (APTES, ABCR, 98%), ammonium hydroxide (NH4OH, ACROS, 28-30% solution in water), anisole (Fluka, 99%), L-ascorbic acid (Aldrich, 99+%), α-bromoisobutyryl bromide (BrIn, Aldrich, 98%), chloroform (Fisher Chemical, 99.8%), 2 chloro-2-oxo1,3,2-dioxaphospholane (COP, Sigma Aldrich, 95%), copper(II) bromide (CuBr2, Aldrich, 99.999%), dichloromethane (DCM, Acros Organics, dry 99.9%), diethyl ether (Sigma Aldrich, p.a.), N,N-dimethylformamide (DMF, 99.8%, Sigma Aldrich) , ethanol absolute (EtOH, VWR, 99.9%), ethylacetate (VWR Chemicals, p.a.), ethyl αbromoisobutyrate (EBiB, Aldrich, 98%), hydrogen peroxide (H2O2, VWR, 30%), 4 (maleinimido)phenyl isocyanate (Sigma Aldrich, ≥97.0%), methanol (Sigma Aldrich, anhydrous 99.8%), N,N,N’,N’’,N’’’-pentamethyldiethylene-triamine (PMDTA, Aldrich, 99%), tetraethyl orthosilicate (TEOS, Aldrich, 99.999%), tetrahydrofuran (THF, Acros Organics, dry 99.85%), tin(II)-2-ethylhexanoate (Aldrich, 95%), toluene (Sigma Aldrich, 99.8%), and triethylamine (Sigma, 99.5%) were used as received. Diazabicycloundecene (Sigma Aldrich, 98%), 2-(benzyloxy)ethanol (Sigma Aldrich, 98%) and trimethylamine (Roth, dried with CaH2) were distilled and stored over molecular sieves (4 Å). Monomethacryloxypropyl terminated polydimethylsiloxane (PDMSMA, asymmetric, 6-9 cSt., ABCR) and poly(ethylene glycol) methyl ether methacrylate (PEGMA, Mn: 475 g mol-1, Aldrich) were filtered through acidic, basic, and natural aluminum oxides (Sigma Aldrich) before polymerization. Poly(gylcidyl methacrylate) (PGMA, Mn: 42000 g mol-1, Polymer Source) was used without further purification. The water used in this work was always Millipore water (Millipore Milli-Q Advantage A10, conductivity: 0.055 µS/cm). 2-Methoxy-2-oxo-1,3,2-dioxaphospholane (MEP). A flamedried 1000 mL three-neck flask, equipped with a dropping funnel, was charged with 2 chloro-2-oxo-1,3,2dioxaphospholane (50 g, 0.35 mol) dissolved in dry THF (300 mL). A solution of dry methanol (11.24 g, 0.35 mol) and dry trimethylamine (35.42 g, 0.35 mol) in dry THF (45 mL) was added dropwise to the stirring solution of COP at -20 °C under argon atmosphere. During reaction, hydrogen chloride was formed and precipitated as triethylamine hydrochloride. The reaction was stirred at 4 °C overnight. The salt was removed by filtration and the filtrate concentrated in vacuo. The residue was purified by distillation under reduced pressure to give a fraction at 89-97 °C/0.001 mbar, obtaining the clear, colorless, liquid product MEP (37.21 g, 0.27 mol, yield: 77%). 1H-NMR (500 MHz, DMSO-d ): δ 4.43 (m, 4H), 3.71 (d, 3H). 6 13C{H} NMR (125 MHz, DMSO-d ): δ 66.57, 54.72. 31P{H} 6 NMR (202 MHz, DMSO-d6): δ 17.89. Polymerization of PMEPMI. N-cyclohexyl-N'-(3,5bis(trifluoromethyl)phenyl)thiourea (TU) was synthesized according to literature procedure49 and freeze dried with benzene prior use. MEP (1 g, 7.2·10-3 mol), TU (268 mg, 7.2·10-4 mol) and 0.56 mL dry dichloromethane were introduced into a flame dried Schlenk tube. A stock solution of

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the initiator 2-(benzyloxy)ethanol in dry dichloromethane was prepared. All solutions were cooled down to 0 °C. 1.25 mL of the initiator stock solution (18.85 mg/mL dichloromethane) was added to the stirring solution of MEP and TU to give a total reaction concentration of 4 mol/L MEP in dichloromethane. The polymerization was initiated by rapid addition of 0.1 mL DBU (110 mg, 7.2·10-4 mol) to the stirring monomer, TU and initiator with a syringe. The polymerization was quenched after 60 min by addition of an excess of 4-(maleinimido)phenyl isocyanate (111 mg, 5.2·10-4 mol) in dichloromethane. The polymer was purified by two times precipitation from dichlormethane into icecold ethylacetate and one time precipitation into icecold diethylether. Followed by dialysis in deionized water over night (MWCO: 3500). The polymer was obtained after freeze drying in quantitive yield (Figure S14). 1H NMR (300 MHz, DMSO-d6) δ 8.16 – 6.71 (m, 9H), 4.44 (s, 2H), 4.29 – 3.93 (m, 132H), 3.78 – 3.53 (m, 104H). 13C{H} NMR (75 MHz, Chloroform-d) δ 66.51, 54.81, 54.73. 31P{H} NMR (121 MHz, DMSO-d6) δ 1.10, -0.12, 1.28 (Figure S15). Atomic Force Microscopy (AFM): All measurements were carried out using a Bruker Dimension Icon (Bruker, USA) in contact mode. Triangular sharp Si3N4 cantilevers (DNP-D, Bruker, USA) with a spring constant of 0.06 N m-1 were used. Several topographical images with the respective force curves were recorded for each sample in air at 80 % relative humidity. The “Point & Shoot” option in the AFM software was utilized to accurately trigger force spectroscopic measurements on individual particles within the AFM images acquired beforehand. Average force−distance curves corresponding to each particle layer were calculated using five representative curves. Contact Angle Measurements (CA): The water contact angles were measured using a conventional drop shape analysis technique for sessile drops (OCA 35xl, DataPhysics Instruments GmbH, Germany). For the measurement of the advancing angle a drop of 20 µL fresh Millipore water was generated on the sample surface using a rate of 0.25 µL s-1. Afterwards the water was removed at the same rate to measure the receding contact angle. For the sessile drop measurement a drop (7 µL) was generated in a cooling chamber with a Peltier cooling stage with an automatic temperature control (23°C, 0°C, -5°C, -10°C). The contact angles were determined using the tangential method. The measurements were repeated four times at different positions of the surface and the average values were calculated. The dynamic measurements were performed at 23°C and 50 % relative humidity. Cryogenic transmission electron microscopy (cryo-TEM): Cryogenic transmission electron microscopy (TEM) images were taken with a Libra 120 cryo-TEM from Carl-Zeiss NTS GmbH equipped with a LaB6 source. The acceleration voltage was 120 kV and the energy filter with an energy window of 15 eV was used. The particles were dispersed in water (0.5 mg ml-1) by ultrasonication for 20 minutes. Prior to the analysis 3.5 ml of the sample were taken, blotted and vitrified in liquid ethane at -178°C. Ultimately, an approximately 200 nm thick ice film was examined in the TEM. Dynamic Light Scattering (DLS): The hydrodynamic radius was measured with a Zetasizer Nano ZS (Malvern Instruments Ltd, UK.). For the measurement 1 mg core-shell particles were dispersed in 2 mL solvent (P(PEGMA) in water and P(PDMSMA) in toluene) and filled in a cuvette ((P(PEGMA)


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in a polystyrene cuvette and P(PDMSMA) in a quartz cuvette). The measurement was recorded three times and the average was calculated. Electrokinetic Measurements: The measurements of the pHdependent electrokinetic potential were carried out using a Zetasizer Nano ZS with MPT-2 autotitrator (Malvern Instruments Ltd, UK.). The particles were dispersed in 10-3 M KCl in water. The pH control was ensured by the addition of 0.1 M KOH or HCl aqueous solutions. The measurement was repeated three times for each sample at each pH value. Gel Permeation Chromatography (GPC): The GPC (Gradient HPLC HP Series 1100, Agilent Technologies Inc., U.S.A.) was used to evaluate the molecular weight of bulk polymers after precipitation. The polymers were dissolved in the eluent (DMAc, 2 vol% water, 3 g/L LiCl), filtered (0.2µm) and injected in a column (Zobrax PSM Trimodal-S). The flow rate was 0.5 mL/min and as a standard Poly(methyl methacrylate) (PMMA) was used. For the calculation of the grafting density of the attached polymer chains the equation of ref.42 was used. Optical Microscopy (OM): The images from the optical microscopy were taken using an Olympus BX51 microscope with a digital camera (Olympus UC30) and Streaming Essentials software (Olympus Soft Imaging Solutions GmbH). Scanning Electron Microscopy (SEM): All scanning electron microscopy images were acquired on a scanning electron microscope (NEON 40 EsB CrossBeam, Carl Zeiss NTS GmbH), operating at 3 kV in the secondary electron (SE) mode. In order to enhance electron density contrast, samples were coated with platinum (3.5 nm) using a sputter coater (Leica EM SCD500). Thermogravimetric Analysis (TGA): The polymer layer thickness on the SiO2-particles was determined using a TGA Q 5000IR (TA Instruments Co., U.S.A.) by the equation described elsewhere.42 The measurements were realized under air atmosphere and the temperature was heated up to 900 °C using a temperature ramp of 10 K/min. Drop Impact Experiments: The impact experiments have been performed using an experimental facility (see Figure S7) which was already used in several previous studies.7, 13, 50 It comprises a drop generation system, a cooling system and an observation system. Water drops (Milli-Q Type 1, electrical conductivity σ = 5.5 · 10-6 S m-1 at 25 °C) are generated above the impact surface at the tip of a PTFE tube using a micro pump. Due to a small volume flow rate of the micro pump, the water drops detach from the tube with a nearly constant size. Using an external liquid chiller and a heated bypass flow, the temperature of a cooling plate and the atmosphere around a growing drop can be controlled independently. Polymer coated aluminum specimen with a size of 28 x 28 x 4 mm3 are placed in a sample holder which is placed on top of the cooling plate. The PTFE tube for drop generation ends in a stainless steel tube which is wrapped in a copper coil. Cooling fluid from the heated bypass which flows through the copper coil provides a cold atmosphere in the stainless steel tube. In this atmosphere, water drops may be supercooled during their growth at the PTFE tube down to T = -18 °C. The drop temperature is continuously measured with a thermocouple immersed into the drop, as illustrated in the detail of Figure S7. The steel tube and the cooling plate are encapsulated in styrofoam chambers which are filled with gaseous nitrogen to prevent the buildup

of frost and condensate on any parts of the setup. The relative humidity in the cooling tube and in the impact chamber is at any time RH < 10% and RH < 5%, respectively. The impact process is captured in a top-view using a highspeed video camera (Phantom V12). A LED outside of the impact chamber and a diffusor screen above the impact surface in the chamber are used to indirectly illuminate the impact process. It is recorded with 2500 frames per second and an optical resolution of approximately 21 µm/pixel to allow an accurate determination of the nucleation event during the first 8 s after drop impact. The experiments have been performed with a constant drop diameter dd and impact velocity vd, which were determined in separate calibration experiments in a side view to dd = 3.2mm and vd = 2.2m/s. Both the drop and surface temperature are kept constant at -16 ± 0.3 °C for all experiments. To obtain data of statistical significance, the impact experiments are repeated under the same conditions at least 100 times for all coatings. The sample holder takes five specimens which are coated with the same polymer. Two subsequent impacts can be performed side by side onto each of the specimens and the sample holder is horizontally shifted to allow in sum ten subsequent impact experiments. After a set of ten impact experiments, the sample holder is warmed up outside the impact chamber and the impact surfaces are rinsed with isopropanol. Frost Layer Formation Experiment: During the freezing/thawing experiments, the coated aluminum substrates (28 x 28 x 4 mm3) were placed on a Peltier element and observed during freezing and thawing of condensate drops under the microscope using a digital camera. The initial plate temperature was 10 °C; it was gradually decreased to −20 °C at a ramp rate of 30 K min−1, heated up to -2 °C with the same ramp of 30 K min-1 and afterwards heated up to 5 °C with 2 K min-1. The air temperature was 20 ± 0.5 °C at 40% RH. Scratch resistance test: For the scratch test a Universal Surface Tester® (UST) from INNOWEP GmbH was used. As test probe a diamond with 5° was used. The load on the surface was stepwise raised from 1 to 100 mN on a 10 mm long distance. The produced scratches were investigated with SEM. Synthesis of Core-Shell Particles Synthesis and modification of monodisperse SiO2 particles: 200 nm-sized silica particles were synthesized using a multistep hydrolysis-condensation procedure of TEOS in ammonia hydroxide–ethanol solution based on the Stöber approach41. TEOS was added sequentially into a mixture of ethanol and ammonia solution. The particles produced within one step were used as seeds for the next step. Each reaction was carried out by stirring the mixture at 500 rpm overnight at room temperature (starting from the last addition of TEOS). Subsequently, the particles of the desired size were separated from the solvent by centrifugation yielding monodisperse silica particles. The purified particles were dried in a vacuum oven under reduced pressure at 60 °C. Afterwards, the particles were stirred for 24 hours in a 5% APTES solution in ethanol to introduce amino groups onto the surface. The APTES-modified particles were purified by repeated washing and centrifugation cycles in ethanol, and dried at 60 °C. The dried APTES-modified particles were suspended in dry dichloromethane (100 ml). α-bromoisobutyryl bromide and



trimethylamine were added to the suspension. The reaction mixture was stirred at room temperature for two hours. The BrIn-functionalized particles were collected by centrifugation, washed in dichloromethane and ethanol, and dried under reduced pressure at 25 °C. Synthesis of Janus particles: The 200 nm silica particles were prepared as described before for FC particles. After the modification with APTES the colloidosomes were prepared via the wax-water Pickering emulsion approach described elsewhere.42, 43 α-bromoisobutyric acid (ATRP-initiator) was immobilized onto the exposed particle surface and the wax was dissolved in hexane.51 Afterwards, the initiator-covered particles were used for polymerization. Grafting of P(PEGMA) and P(PDMSMA) using surfaceinitiated ATRP (“Grafting from” approach): Poly(poly(ethylene glycol) methyl ether methacrylate) (P(PEGMA)) was grafted from the initiator modified particle surface as follows: ethanol (3 mL), PEGMA (3 mL), PMDTA (60 μL, 0.5 M solution in DMF), CuBr2 (60 μL, 0.1 M solution in DMF), and EBiB (0.15 μL) were added to the particles. The mixture was sonicated and purged with Ar, followed by the injection of ascorbic acid (200 μL, 1 M solution in DMF). The polymerization was performed under continuous stirring at 70 °C in a water bath for 60 min. Further, particles with the grafted polymer were washed eight times by centrifugation in ethanol and dried under vacuum at 25 °C. P(PDMSMA) was grafted similar: anisole (3 mL), PDMSMA (3 mL), PMDTA (64 μL, 0.5 M solution in DMF), CuBr2 (64 μL, 0.1 M solution in DMF), and EBiB (0.1 μL) were added to the particles. The mixture was sonicated and purged with Ar, followed by the injection of tin(II)-2-ethylhexanoate (200 μL). The polymerization was performed under continuous stirring at 70 °C in a water bath for 15 min. Further, particles with the grafted polymer were washed eight times by centrifugation in anisole, chloroform, toluene and ethanol and dried under vacuum at 25 °C. 800 nm and 200 nm particles were grafted in a similar way. Grafting of carboxy terminated PDMS (“Grafting to” approach): The grafting of the second polymer onto the P(PEGMA) modified Janus particles was done using the “grafting to” approach. The JPs were dispersed in a 1 wt % carboxy terminated PDMS solution (20 mL) and stirred for 2 h. Next, the solvent was evaporated, and the particles were annealed at 150 °C overnight. The ungrafted polymer was removed by repeatedly dispersing the particles in chloroform or toluene and subsequent centrifugation. As a result, bicomponent P(PEGMA)/PDMS-Janus particles were obtained. Preparation of Coatings with Core-Shell Particles Silicon wafers (23 x 23 mm, polished single-crystal silicon wafers of ⟨100⟩ orientation, Si-Mat Silicon Materials, Landsberg, Germany) were cleaned using a mixture of H2O2/NH4OH/H2O (1:1:1) to get a uniform SiO2 layer with silanol groups. The thickness of this layer was 1.5 ± 0.2 nm and determined by null-ellipsometry. Aluminum substrates (28 x 28 mm, polished) were cleaned in absolute ethanol in a ultrasonic bath and in O2-plasma cleaner. The wafers/substrates were coated with a PGMA solution (2 wt% in THF) or PMEPMI (2 wt% in H2O) and annealed for 20 min at 150 °C in a vacuum oven in order to chemically graft PGMA/PMEPMI. The layer thickness was 80 ± 3 nm. The core shell particles were dispersed in a mixture of chloroform/toluene and deposited dropwise on the pre-coated

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wafers/substrates using a solvent casting method. The wafers/substrates were again annealed for 2 h at 150°C in a vacuum oven. Superfluous multilayers were removed by sonication in toluene. The quality of the prepared coatings was checked by optical microscopy and SEM. Overview of the prepared coatings: Table 1. List of the prepared samples for the icing experiments. Sample ID

Description

Flat-P(PDMSMA)

P(PDMSMA) polymer coated wafer

Flat-P(PEGMA)

P(PEGMA) polymer coated wafer

ParticleP(PDMSMA)

Surface from FC 200 nm large SiO2 particles with P(PDMSMA) shell

Particle-P(PEGMA)

Surface from FC 200 nm large SiO2 particles with P(PEGMA) shell

Mixed-P(PEGMA)P(PDMSMA)

Surface from FC 200 nm large SiO2 particles with P(PEGMA) and P(PDMSMA) shell in a 50:50 mixing ratio

Janus-P(PEGMA)PDMS

Surface from 200 nm large SiO2 Janus particles with two different polymer (P(PEGMA) and P(PDMSMA)) shells

ASSOCIATED CONTENT Supporting Information. Chemical characterization of the particles: Zeta-potential, GPC, TGA, DLS data); characterization of the particle-based surfaces: wetting behavior at different temperatures, optical microscope pictures during condensation and freezing process, experimental setup for supercooled water drop experiment, drop impact images from supercooled water drop experiment, OM image, SEM images of flat surfaces, wetting behavior; chemical structures of adhesives, SEC elugram, 1H and 31P-NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (A.S.) Tel.: +49 (0)351 4658 475. Fax: +49 (0)351 4658 474.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources Deutsche Forschungsgemeinschaft DFG (Grants SY 125/4-1, IO 68/1-3 and WU 750/6-1), Collaborative Research Center SFBTRR 75 (TP-C3) and Heraeus Medical.

ACKNOWLEDGMENT M.S., T.O., C.M. and A.S. gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (Grants SY 125/4-1 and IO 68/1-3) and the Leibniz Institute of Polymer Research Dresden (IPF) for generous financial support. M.S., I.R. and C.T. gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft within the Collaborative Research Center SFB-TRR 75 (TP-C3 and TP-C4).


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H. T. and F. W. gratefully acknowledge financial support from Heraeus Medical and DFG WU 750/6-1. The authors thank coworkers from IPF Dresden for valuable contributions: Mrs. K. Arnhold and Mrs. L. Häußler for TGA investigations, Mrs. A. Caspari and Mrs. M. Priebs for electrokinetic measurements, Mrs. P. Treppe for SEC measurements and Dr. I. Raguzin for Cryo-TEM measurements.

ABBREVIATIONS AFM atomic force microscopy; APTES3aminopropytriethoxysilane; ARGET activators regenerated by electron transfer, ATRP atom transfer radical polymerization; BrIn α-bromoisobutyrylbromide; COP 2 chloro-2-oxo-1,3,2dioxaphospholane; DBU 1,8-Diazabicyclo(5.4.0)undec-7-ene; FC fully covered; IEP isoelectric point; JP Janus particles; MEP 2Methoxy-2-oxo-1,3,2-dioxaphospholane; OM optical microscopy; PEGMA poly(ethylene glycol) methacrylate; PDMS polydimethylsiloxane; PDMSMA poly(dimethylsiloxane) monomethylacrylate; PGMA poly(glycidyl methacrylate); PMEPMI poly(methyl ethylene phosphate) maleimide, P(PEGMA) Poly (poly(ethylene glycol) methacrylate); P(PDMSMA) Poly (poly(dimethylsiloxane) monomethylacrylate);PTFE polytetrafluoroethylene; SEM scanning electron microscope; TEM transmissions electron microscopy; TGA thermogravimetric analysis; TU N-cyclohexylN'-(3,5-bis(trifluoromethyl)phenyl)thiourea. REFERENCES 1. Fletcher, N. H., Liquid water and freezing. In The Chemical Physics of Ice, Cambridge University Press: Cambridge, 1970; pp 73-103. 2. Fletcher, N. H., Crystal growth. In The Chemical Physics of Ice, Cambridge University Press: Cambridge, 1970; pp 104-129. 3. Na, B.; Webb, R. L. A fundamental understanding of factors affecting frost nucleation. Int. J. Heat Mass Transfer 2003, 46, (20), 37973808. 4. Schremb, M.; Tropea, C. Solidification of supercooled water in the vicinity of a solid wall. Phys. Rev. E 2016, 94, (5), 052804. 5. Boinovich, L.; Emelyanenko, A. M.; Korolev, V. V.; Pashinin, A. S. Effect of wettability on sessile drop freezing: when superhydrophobicity stimulates an extreme freezing delay. Langmuir 2014, 30, (6), 1659-68. 6. Alizadeh, A.; Yamada, M.; Li, R.; Shang, W.; Otta, S.; Zhong, S.; Ge, L.; Dhinojwala, A.; Conway, K. R.; Bahadur, V.; Vinciquerra, A. J.; Stephens, B.; Blohm, M. L. Dynamics of Ice Nucleation on Water Repellent Surfaces. Langmuir 2012, 28, (6), 3180-3186. 7. Schremb, M.; Roisman, I. V.; Tropea, C. Transient effects in ice nucleation of a water drop impacting onto a cold substrate. Phys. Rev. E 2017, 95, (2), 022805. 8. Mishchenko, L.; Hatton, B.; Bahadur, V.; Taylor, J. A.; Krupenkin, T.; Aizenberg, J. Design of Ice-free Nanostructured Surfaces Based on Repulsion of Impacting Water Droplets. ACS Nano 2010, 4, (12), 7699-7707. 9. Jung, S.; Dorrestijn, M.; Raps, D.; Das, A.; Megaridis, C. M.; Poulikakos, D. Are superhydrophobic surfaces best for icephobicity? Langmuir 2011, 27, (6), 3059-66. 10. Maitra, T.; Antonini, C.; Tiwari, M. K.; Mularczyk, A.; Imeri, Z.; Schoch, P.; Poulikakos, D. Supercooled water drops impacting superhydrophobic textures. Langmuir 2014, 30, (36), 10855-61. 11. Khedir, K. R.; Kannarpady, G. K.; Ishihara, H.; Woo, J.; Asar, M. P.; Ryerson, C.; Biris, A. S. Temperature-dependent bouncing of super-cooled water on teflon-coated superhydrophobic tungsten nanorods. Appl. Surf. Sci. 2013, 279, 76-84. 12. Zheng, L.; Li, Z.; Bourdo, S.; Khedir, K. R.; Asar, M. P.; Ryerson, C. C.; Biris, A. S. Exceptional superhydrophobicity and low velocity impact icephobicity of acetone-functionalized carbon nanotube films. Langmuir 2011, 27, (16), 9936-43. 13. Schremb, M. Hydrodynamics and Thermodynamics of Ice Accretion through Impact of Supercooled Large Droplets: Experiments, Simulations and Theory. PhD thesis, Technische Universität Darmstadt, Darmstadt, 2018.

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Supercooled Water Drops Do Not Freeze During Impact on Hybrid

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