Thermoresponsive Mobile Interfaces with Switchable Wettability

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Thermo-Responsive Mobile Interfaces with Switchable Wettability, Optical Properties, and Penetrability Yijun Zheng, Xiao Liu, Jiajia Xu, Huaixia Zhao, Xinhong Xiong, Xu Hou, and Jiaxi Cui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12354 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Applied Materials & Interfaces

Thermo-Responsive Mobile Interfaces with Switchable Wettability, Optical Properties, and Penetrability Yijun Zheng†#, Xiao Liu‡§#, Jiajia Xu†, Huaixia Zhao†∥, Xinhong Xiong†, Xu

Hou⊥&^*, Jiaxi Cui†*.

†INM - Leibniz Institute for New Materials, Campus D2 2, Saarbrücken, 66123 Germany ‡Key Laboratory for Biomechanics and Mechanobiology of the Ministry of Education, School of Biological Science and Medical Engineering, Beihang University, Beijing 100191, China §Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston 02139, MA, USA ∥Institute

for Fundamental and Frontier Science, University of Electronic Science and Technology of China, Chengdu 610054, China ⊥College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

&College of Physical Science and Technology, Xiamen University, Xiamen 361005, China ^Collaborative Innovation Center of Chemistry for Energy Materials, Xiamen 361005, China

KEYWORDS: LCST, slippery surface, wettability, optical properties, penetrability.

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ABSTRACT:

Liquid-based mobile interfaces, in which liquids are being utilized as structural long-term components, have shown their multi-functionality in material science, such as the hydration layer of polyelectrolyte brushes used for artificial implants, stabilized lubricants for anti-biofouling, anti-icing, self-cleaning, optical control etc. However, these systems currently available do not usually show a response to environment stimuli. Here, we describe a strategy for preparing thermo-responsive mobile interfaces made from novel silicone-based lubricants that display lower critical solution temperature (LCST) and demonstrate their capabilities on controlling in-situ water wetting and dewetting, thermo-gating penetration, and optical properties. These properties allow the mobile films to form a kind of erasable recording platforms. We foresee diverse application in liquid transport, wetting and adhesion control, and transport switching.


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Introduction

Liquids are rarely considered as structural materials used in artificial systems because of their dynamics.1 In contrast, living organism utilizes liquids in a fundamentally same way with other solid materials to meet some of the life’s toughest challenges. For example, human body creates a thin liquid film on pounding knees to keep bones gliding smoothly over each other;2 carnivorous plants catch water to form a slippery surface to prey on insects;3 lacrimal glands secrete aqueous on our eyes to provide a perfectly smooth refractive surface and also keeps away bacteria;4 tree frogs play liquid films on their toes in an adaptable mode to switch adhesion and then climb over dripping leaves or flooded rocks,5 and so on.6-7 These incredible functions are attributed to the intrinsic characteristics of liquids, i.e., mobility and incompressibility. These features allow them automatically form defect-free surface, infinitely change shape, bear pressure without compromising the slippery feature, etc.8-9. Unlike the way we use liquids for lithographic printing, coating, device fabrication, heat transfer, or lubrication in which the liquids ultimately dry, cure, evaporate, or remain enclosed, liquids here are structural long-term interface components. To this end, nature develops fine structured templates to entrap, assemble, and shape liquid components.1

Inspired by nature, synthetic approaches are being developed to create liquid structural materials for the multifunctional interface. Polyelectrolyte brushes attached on surfaces are able to entrap water to form hydration layer which resists mutual



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molecular interpenetration — as structural materials do.10,11 Because of the fluidity of the hydration layer, these liquid-structural interfaces exhibit extremely low friction coefficients even at high pressures. It is therefore utilized to design lubricated surfaces in artificial implants.12 Polymer networks can load various liquids, such as silicone oil,13-15 paraffin,16,17 fluorocarbon,18 ionic liquid19 or even water,20-21 etc., to form mobile surfaces for anti-biofouling,22-23 anti-icing,24,25 self-clean,13, 26 optical control,16, 18, 27 and so on.28-33 A general approach closed to nature path for robust control over the assembly of liquids was developed recently by Aizenberg et al.34 Inspired by the carnivorous plant, they used rigid porous substrates to stabilize perfluorinated oils to create mobile surfaces that show almost perfect slipperiness toward various complex liquids, organisms, and solid materials. This strategy can be applied to different materials such as polymers,35-37 inorganic particles,38-39 and metal40 on various substrates and therefore shows potential in wide applications including biomedical fluid handling, fuel transport, optical imaging, harsh environments and so on.41-51 More than surface properties, liquids entrapped in porous membranes also supplies a novel gating mechanism for selective penetration of fluids.52 Consequently, the use of liquids as structural materials is emerging as a robust approach to design multifunctional interfaces. Current attention in this field, however, is mainly attracted to the development of new substrates for stabilizing liquids or the exploit of novel applications and therefore, research on the system containing novel liquids which response to external stimuli without compromising their intrinsic mobility is still rare.


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In this report, we described a strategy for creating thermo-responsive mobile interfaces with switchable surface wettability and optical properties as well as tunable penetrability to fluids (Figure 1). The idea based on a class of novel silicone-based lubricants that displayed lower critical solution temperature (LCST53) in water (named as oilLCST), which can infuse into a hydrophobic porous membrane to form a stable slippery surface. Polymers with LCST have been widely applied to create smart materials for various applications such as drug delivery systems, soft actuators, energy-saving coatings etc.54-56 Herein we applied this concept to fabricate smart slippery surface. In a case that the oils have a refractive index (RI) close to the substrate, the oil-infused film could be optically homogeneous (thus transparent). At the temperature higher than the LCST of the oil infused, the membrane is expected to exhibit a slippery and water-repellent surface as slippery substrates currently available.4 When the temperature was lower than the LCST of the oil infused, water is miscible with the oil and therefore would wet the surface through diffusing into the oil phase. The mixing changes the refractive index of the liquid phase and then altered the optical properties of the membrane. Thermo-induced phase separation between water and the oil is reversible. Thus increasing temperature is believed to induce in-situ water dewetting accompanied with a recovery of membrane’s transmittance. In addition, the penetration of water through this oil-infused membrane should also be thermo-tunable: water is blocked at the temperature higher than the LCST of the oil but allowed to pass through at the temperature lower than the LCST of the oil.

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Synthesis of D4-TEO. To the mixture of 1,3,5,7-tetramethylcyclotetrasiloxane (6.0 g, 25 mmol) and allyloxy(triethylene oxide) methyl ether (20.8 g, 100 mmol) Pt Pt catalyst (20 µL) was added at room temperature under stir. The temperature of the solution sharply increased and the solution became yellow. The mixture was allowed to react under stir at room temperature for overnight and resulted light yellow oil was passed through a short silica gel column to give a colorless oil (yield 99%). 1H NMR (250 MHz, CDCl3, δppm): 3.65-3.62 (m, 40H, OCH2), 3.57-3.52 (m, 16H, OCH2), 3.36 (s, 12H, OCH3), 1.61-1.55 (m, 8H, CH2CH2CH2), 0.50-0.43 (m, 6.8H, SiCH2), 0.05 (s, 12H, SiCH3). MALDI-MS m/z: 1079.4 [M+Na] (Calculated for C44H96O20Si4: 1079.5). Elemental Analysis calculated for C44H96O20Si4: C, 49.97; H, 9.15; Si, 10.62. Found: C, 49.3; H, 9.0; Si, 10.29. Density: 1.06 g/mL. Refractive index: 1.4549 (20 o

C). Surface tension: 32 mN/m in air (20 oC) and 9 mN/m in water (40 oC). Noted that

the product is a mixture of D4-TEO and its isomers. All other oils were prepared by the same method. In the case that trimethylsiloxy terminated poly(methylhydrosiloxane)s were used as starting materials instead of 1,3,5,7-tetramethylcyclotetrasiloxane, the reactions were carried out at 70 oC for better grafting yield. The grafting density was 96% estimated by 1H NMR spectroscopy.

Measurement of the LCST. The LCST of the oils in water were estimated by both turbidity and DSC measurements. For turbidity measurement, a solution containing 2 vol% of oils was used. The transmittance of the solution was in-situ monitored at 500 nm with a heating/cooling rate of 1 oC/min in the temperature range of 20-30 and 6 ACS Paragon Plus Environment

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35-40 oC but 0.1 oC/min in the range of 30-35 oC. The data in Figure 2 in main text were measured by this method. On the other hand, the LCST of the mixture solutions of water and oil with various ratios were obtained by DSC measurements. The heating/cooling rate is 5 oC/min from 20 to 70 oC. All the samples were measured for two times. The original data are shown in Figure S2.

Measurement of polymer-solvent interaction parameters (χij). A swelling method was used to measure the measurement of χij based on the Flory-Rehner theory of

welling

equilibrium.57

Slightly

cross-linked

poly[(triethylene

glycol)methylhydrosiloxane]s were prepared by using poly(ethylene glycol) diacrylate (avarage Mn 575) as a crosslinker (2 mol% of allyloxy(triethylene oxide) methyl ether). Briefly, to the mixture of the poly(methylhydrosiloxane)s, allyloxy(triethylene oxide), and the crosslinker (vinyl/hydrosiloxane group) was added the catalyst (0.5 vol%). After short shaking for mixing (5 sec), the mixture (gelling in five minutes) was stored for one day at room temperature before use. The specimens were prepared under same conditions and were cut into well-defined shape for measurements. These specimens were immersed in deionized water at 25, 30，35 and 40 oC, respectively, for 24 hours. After the samples were moved out and wiped to give away the residual water sticking on the sample surface, the weight changes of the samples were recorded carefully. On one hand, the specimens were compressed for calculating the effective crosslinking density by using equation (1): G = RT∗ , ,

/ /

(1)



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in which G is the shear modulus, R is the gas constant, T is absolute temperature, νp,r is the polymer volume fraction in the relaxed state and νp,s is the volume fraction of the polymer in the swollen state. The shear modulus was equal to the slope of the curve of stress vs. (λ-λ-2) (λ: deformation ratio, defined as L/L0) at small strains (lower than 50%). A ∗ of 1.8×10-4 mol/cm3 was obtained for the materials. Polymer-solvent interaction parameters have been determined by the following equation (2) applied to these results obtained from swelling a polymer sample in deionized water: χij= -

ln1 - , + , + ∗ , !" , 2

% #, & % #, $ ' ( )+ ( #, #,

. -

(2)

,

in which /0 is the molar volume of water (18 cm3/mol). By using the swelling ratios

of the materials in water, ∗ value obtained from compression measurements, and the volume fraction of polymer under different state, the χij was calculated. The data

in Table S1 were collected by this method.

Dissipative particle dynamics (DPD) simulation. To understand the phenomena of thermoresponsive phase transition, we used the dissipative particle dynamics (DPD) simulations to investigate the oil/water system under temperatures close to LCST.58-60 The DPD system contains two kinds of particles, representing water and oil,

respectively.

The

oil

is

represented

by

18

beads,

in

which

the

tetramethylcyclotetrasiloxane was expressed by two beads (Bead A, SiO) and each


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tail has four beads with each representing one ethylene oxide (Bead B, EO). These beads are bonded together by springs. The water molecule is represented by one bead (Bead C). The time evolution of each bead is controlled by Newton’s equations of motion:

dri dt

= vi

，

ri ,

where

mi

vi ,

dv i dt

= fi

and

fi are the position vector, velocity vector and force vector of

the ith bead, whose mass

mi

is simplified as 1. Actually, the density of oil (1.06

g/cm3) is very close to that of water. fi contains three parts of conservative, random, and dissipative forces: fi

= ∑ (FijC +FijR + FijD )

(rij < rc )

i≠ j

fi is the total force on the ith bead by all other beads within a certain cutoff

where radius

rc .

The conservative force is a soft repulsion taking the form

FijC

α ij (1 − rij / rc )rîj (rij < rc ) (rij > rc ) 0

=

where

α ij

is a maximum repulsion between beads i and j,

between them (

rij = ri − r j

interaction parameter

χij

α ij

, and

rij = ri − r j

) and

rîj

rij

is the distance rîj =

is the unit vector (

rij rij

). The

between beads is related to the Flory-Huggins parameter (

) as

α ij = α ii + χ ij / 0.306



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where

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αii is the interaction parameter between the same type bead and calculated

by

αii = 75kBT / ρ ρ is the number density and set as 3.

The random force (

FijR

) and dissipative force (

FijR

= σ wR (rij )θij rîj

FijD

= −γ wD (rij )(rîj ⋅ vij )rîj

Where

v ij = v i − v j

coefficient;

σ

FijD

) are given by

R D , w and w are weight functions;

is the noise amplitude;

θ ij

γ is the friction

is a fluctuating variable. The detailed

information about these parameters is given by Groot and Warren2. The size of the simulated box is set as 20*20*20, which contains a total of 3000 beads. The length (

rc ), mass ( mi ) and time ( kBT ) scales are set as units for

simulation convenience without rescaling. Equilibrium can be reached before 10,000-time steps. Therefore 20,000 DPD steps per simulation are used and sufficiently accurate. In addition, the time step of simulation is chosen as 0.05. Water contact angle and surface tension. Water sliding angles, water contact angles, and surface tensions were recorded on a contact angle goniometer (KSV CAM101) equipped with a thermal control table. Surface tension was calculated by a pendant drop method.

Water contact angle and surface tension. Water sliding angles, water contact angles, and surface tensions were recorded on a contact angle goniometer (KSV


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CAM101) equipped with a thermal control table. Surface tension was calculated by a pendant drop method.

Estimation of total interface energies. The total interface energies of the iPP with D4-TEO with or without water were estimated by equation (1) and (2). At 40 oC, water and D4-TEO have contact angles of 97.8 ± 0.7 o (θw) and 49.6 ± 0.2

o

(θo) on

flat PP film while D4-TEO. The roughness factor R of pPP film was obtained by comparing the water contact angle (WCA) at room temperature (R = cosθr / cosθf, θr and θf are the WCA of pPP and flat PP film, respectively). A θr of 151±0.1 o and a θf of 103 ± 0.3 o were found for pPP and flat PP film, respectively and thus, the R is 4.2. The surface tension was calculated by pendent droplet method. At 40 oC, γo, γw, and γwo are 32, 72, and 9.1 mN/M, respectively. Therefore, ∆E1 and ∆E2 are 119.0 and 168.1 mN/M, respectively, at 40 oC. At 20 oC, the parameters of θo, θw, γo, γw, and γwo are 51.6± 0.8 o, 103 ± 0.3 o, 33 mN/M, 76 mN/M, and 0 mN/M, respectively. As a result, ∆E1 and ∆E2 are 157.9 and 200.9 mN/M, respectively, at 20 oC.

Results and discussion Synthesis and characterization of oilLCST The oilLCST (Figure 2a) were synthesized by a platinum-catalyzed hydrosilylation reaction between methylhydrosiloxanes and allyloxyl(triethylene oxide) methyl ether. The reaction is extremely “green”: solvent-free, conducted at room temperature, quantitative yield, and especially, products for directly using. Both cyclic and linear oilLCST were prepared. The cyclic oil, D4-TEO has a well-defined structure and low



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viscosity (48 cSt) while the linear ones have high molecular weights (7800 -12400 Da) and therefore are non-volatile even at a high temperature (90 oC), implying a high stability. All these oilLCST can mix with deionized water in any ratio at room temperature but undergo phase separation when the temperature is higher than their LCST. In this soluble state, the oilLCST, self-assembled into micelle in water due to their surfactant-like structure (checked by dynamic light scattering, Figure S1). Both cyclic and linear oils display a LCST of 36.5 oC at a concentration of 2 vol% (Figure 2b, unless specifically mentioned, D4-TEO was used for all the demonstrations), a value very close to human body temperature. The LCST increases with the oil's fraction (Figure S2). The liquid-liquid phase transition is extremely sharp and there is nearly no hysteresis between heating and cooling curves. It is attributed to the liquid nature of the oils and the absence of intramolecular hydrogen-bonding in bulk state.53

To theoretically understand the phenomenon of thermoresponsive phase transition, we measured the interaction parameter (χij) of water/oil mixture under temperatures close to LCST to describe their interaction (swelling method was used to obtain approximate parameter, see detail in method section and Table S1). The parameter increases with the temperature and a sharp change occurs in the temperature region of 30-40 oC. It was consistent with the observation in the LCST. On the other hand, we also used the molecular simulation based on the atomistic structures to estimate the χij parameters (see detail in method section and Supporting Information, Table S2 and S3).58-60 The simulation results show that the χij parameter between A-C (SiO-water) is much higher than that between B-C (EO and water), indicating that D4-TEO has a 12 ACS Paragon Plus Environment

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structure of hydrophobic ring backbone and a hydrophilic side chain. In addition, the interaction parameter among beads A-B (SiO-EO) and A-C (SiO-water) is decreased when the temperature changes from 25 to 40 oC. In contrast, the χij between EO and water is increasing with the increasing of temperature. It is known that EO forms hydrogen bonds with water at low temperature but disorders at high temperature leading to increased χij.61,62 Therefore, at the temperature higher than the LCST, the increasing χij between EO and water may make the TEO side chain also hydrophobic, implying a tendency to phase separation. The implication is verified in the DPD simulation results. The snapshots of the DPD simulation system with the oil/water ratio as 5:5 (Figure 2c) demonstrate good mixings at 25 oC but a phase separation at 40 oC. The averaged density profiles at 25 oC are quite even in the total unites but they are quite rough at 40 oC, which further indicates some phase separation. Thermo-responsive wetting and dewetting The thermo-responsive mobile surface was prepared by infusing the oilLCST into a porous polypropylene (pPP) substrate (Figure 3a, loading ratio (Woil/Wsubstrate) of 6.1). Resulting infused porous polypropylene (iPP) membranes can suffer various mechanical deformations and acute swing without losing any oil (measured by the weight). The bare pPP film has a hydrophobic surface (water contact angle: 151 oC, Figure S3) while the surface of oilLCST-iPP films is hydrophilic. At the temperature higher than the LCST of the oils (40 oC), a water contact angle (WCA) of 27 o was observed (Figure 3bi). The surface is water-repellent: the water droplets slide from the surface without leaving any trace (Figure 3bii and movie 1). The sliding angle for a 13 ACS Paragon Plus Environment


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water droplet of 5 µl is smaller than 15 o, which is significantly lower than the pPP surface (sliding angle: 87 o). Note that the oil has strong affinity to water, which made the water droplet slide slowly at a low angle in the region of 5-10°. When the temperature is lower than the LCST of the oils, water wets the surface, showing a contact angle of < 5 o on the surface (Figure 3biii). After passes through the surface (Figure 3biv and movie 2), dyed water droplets leave a color trace. The trace cannot be removed by a wipe since water molecules have diffused into the substrate. To further confirm that the transition can occur in the hydrophobic substrate, we tested the loading capability of pPP to the mixture of oilLCST and water and found that the maximum water fraction is up to 70 vol%.

The liquid-liquid transition is reversible, which results in an in-situ switching in wettability (Figure 3c and movie 3). During the temperature change from 40 to 20 oC, water droplets wet the surface. Unlike the simple wetting on the hydrophilic surface on which water only stay on the top of the surface, in this case, water molecules diffuse into the iPP film by mixing with oilLCST. When the temperature is switched back to 40 oC, the spreading water layer retreats to the condensed state again in a two-step way: the water on the surface recedes quickly at first, leaving a trace which also recedes late but at a slower speed. The trace was attributed to the water entrapped inside the iPP film. These water molecules underwent a phase separation and then were pushed out from the hydrophobic PP film at the high temperature. Although water molecules can go into the film, the iPP is stable and water cannot remove the oil stabilized by the substrate even at a temperature lower than the LCSToil (the LCST of 14 ACS Paragon Plus Environment

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the oil). The stability of the iPP was evaluated through the interfacial energy (E), by the equations34

∆E1 = R(γocosθo – γwcosθw) –γwo > 0

∆E2 = R(γocosθo – γwcosθw) + γw – γo > 0

(1)

(2)

Where E1 and E2 are the total interfacial energies with or without water on the top, respectively, γw, γo, and γwo are the surface tensions of water, oilLCST, and water-oilLCST interface, θo and θw are the equilibrium contact angles of oilLCST and water on a flat solid surface (i.e. PP film), and R is the roughness factor of pPP (R = 4.2). Compared to the equation applied in previous researches13,63, the parameter R was used in the current system because the surface energy of the oil and membrane depended on their contact surface area. D4-TEO has a γo of 32 mNm-1, a θo of 49.6 o, and a γwo of 9.1 mNm-1 at 40 oC, indicating a ∆E1 of 119.0 mNm-1 and a ∆E2 of 168.1 mNm-1. At 20 oC, D4-TEO is miscible with water (Figure S4) and therefore, γwo could be considered as zero. In this case, ∆E2 did not change very much while ∆E1 became even bigger. Therefore, pPP has a very strong affinity to D4-TEO regardless of the temperature. In other words, the PP film enhanced the water—D4-TEO separation at the high temperatures. Thermo-responsive optical properties Bare pPP is opaque because of the light scattering induced at the air/solid interface (RI: 1.0 for air and 1.49 for PP). After infused with the oilLCST (RI: 1.46), the membrane becomes transparent. Since water has a lower RI (1.33) than the oil, the 15 ACS Paragon Plus Environment


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mixture of water and the oilLCST displays a decrease in RI with increasing the fraction of water (Figure 4a). Therefore, the wetting of water droplets on the iPP membrane, in which water molecules diffuse into the oil phase and decreased the total RI of the liquid phase, leads to a watermark. Taking advantages of the reversible wettability and the water-dependent transparency, the oilLCST-infused substrate can be used as an erasable recording platform with water as ink (Figure 4b and c). Although water molecules can diffuse into the membranes to induce opaqueness at room temperature, they are not able to replace the oil (both ∆E1 and ∆E2 >0). Upon heating, water ink comes out from the membrane and thus can be removed easily by tilting as a consequence of the slippery surface. The resulted platform is clean and can be rewritten again. Thermo-responsive penetration of water Using oil-infused pore to gate the penetration of fluids is a novel concept.52 OilLCST is proposed to further extend this concept to the field of external stimuli-responsiveness (Figure 5). The bare pPP membrane allows air to pass through but blocks water, due to the capillary effect (Figure 5a and b). The infusion of oilLCST alters this penetrability. In the iPP membrane, air is blocked but water can pass through at room temperature (Figure 5c). The penetration of water in the iPP is thermo-responsive. Water passes through the membrane at the temperature lower than the LCSToil but is blocked at the temperature higher than the LCSToil (Figure 5d). When the water passed through the membrane, the oil was continual losing, leading to a varying rate of penetration. In addition, the water that passed through the membrane 16 ACS Paragon Plus Environment

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also induced a temperature change in the microenvironment of the membrane. These factors induced difficulty to obtain reliable penetration rate. This oilLCST–infused PP membrane is anti-fouling at high temperature and easy-to-clean at low temperature. The color aqueous solution passed through the iPP without inducing any trace at 40 o

C (Figure 5e), suggesting anti-fouling properties as normal liquid-infused porous

systems.52 The film was contaminated by the color solution at 20 oC. However, the contaminated compounds could be removed easily by washing the film at high temperature (a control of pPP does not exhibit this anti-fouling/easy-to-clean behavior). When water passed through the membrane and took away partially the D4-TEO at 20 oC, there should be an interface liquid layer with high concentration of D4-TEO remained due to the strong affinity (∆E1). This lubricating liquid layer prevented direct contamination and therefore, the water-soluble "contaminant" can be removed easily by water-washing at high temperature. Because of this antifouling mechanism, the membranes can be filled by oil to elongate their longevity for water penetration (without refilling, the iPP could only be used for several times for this purpose). However, because of its selfThis is the first time to get controllable critical pressure property by responsive gating liquid when it could only be achieved previously by changing the specific pore size and chemical surface property of the porous membranes.52

Conclusion



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By structuring thermo-responsive liquids which can be easily prepared by a green method, we have described a strategy for preparing stimuli-responsive mobile interfaces through transferring the well-studied LCST-type liquid-liquid transition into liquid-solid transition at which the interface is sharp and easy to separate. Among vigorous development of various liquid-infused slippery surface, the thermo-responsive mobile interface allows controllable water wetting, in-situ dewetting, and thermo-triggered penetration of water in addition to the slippery and switchable optical properties. Although the longevity of current strategy was lower than typical SLIPS64 system because of the loss of the oil when water passed through, the iPP film maintained the anti-fouling properties. The novel surfaces could be potential in multiple extractions, water collection and transport, temperature sensing, control release etc, specifically, such as one-time injection, visible temperature alarming system, micro extracting systems for the reactions in microfluidics.

Supporting Information. Supporting figures and data including DSC curves, Flory-Huggins parameters, DPD images, water contact angles, instruction of movies etc.

AUTHOR INFORMATION Corresponding Author *Jiaxi Cui, email: [email protected] and Xu Hou, email: [email protected]

Author Contributions


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J.C. Y.Z. X.L. and X.H. conceived the concepts of the research. J.C. supervised the research. Y.Z. designed the experiments. Y.Z., J.X., H.Z., and X.X. performed the experiments. X.L. did theoretical analysis and simulation. J.C., Y.Z., X.L., and X. H. wrote the manuscript. #These authors contributed equally.

ACKNOWLEDGMENT J. C. acknowledges the support from BMBF under the project of the Leibniz Research Cluster "Organic/synthetic multifunctional micro-production units ─ New ways to the development of the active ingredient" with an award number 031A360D. X.H. acknowledges the support of Recruitment Program for Young Professionals, China, and National Natural Science Foundation (21673197), China and Research Institute for Biomimetics and Soft Matter, Fujian Provincial Key Laboratory for Soft Functional Materials Research, Xiamen University, supported by the 111 Project (B16029). X.L. acknowledges the support from National Natural Science Foundation (31570947)

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Figure 1. Schematic of the mobile interface made from the oil with LCST in water (oilLCST). LCSToil: the LCST of the oil infused. The oil is immiscible with water at a temperature higher than the LCSToil but becomes water-miscible at a temperature lower than the LCSToil.



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Figure 2. Silicone-based oilLCST. a, The synthesis of oilLCST. b, The plots of transmittance at 500 nm as a function of temperature measured for the solution of D4-TEO in dyed water (2 vol%). Insets show images of the mixture of water and D4-TEO (1:1) at 20 oC, a homogeneous state, and at 40 oC, a liquid-liquid phase separation state with oil on the bottom. c, Snapshots of the dissipative particle dynamics (DPD) simulations of D4-TEO oil (A: red bead of SiO; B: green bead of OE)/water (B: blue bead) system at 1:1 oil/water ratio under different temperatures.


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Figure 3. Thermo-switchable wetting on the surface of the oilLCST-infused substrate. a, Photographic and SEM images of porous polypropylene (pPP) and oilLCST-infused polypropylene (iPP) membranes. b, Water droplet (5 µL) on iPP at 40 o C (i) and (ii), or at 20 oC (iii) and (iv). c, Water droplets on iPP in a temperature cycle of 40-20-40 oC. Amplified areas (right) show that the dewetting involves a relative slower liquid-liquid phase separation in the substrate.



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Figure 4. OilLCST-infused substrate used as an erasable recording platform. a, the refractive index (RI) of the mixture of D4-TEO and water. b, Demonstration on iPP with water ink. c, Transmittance comparison in water-free and water-wetted regions. The experiments in a and c have been conducted at room temperature (20 oC).


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Figure 5. Thermo-tunable gating of fluid transport in the oilLCST-infused substrate. a, Setup used for demonstrating fluid transport. b, Air passes through the bare pPP but is blocked by iPP. c, Dyed water passes through iPP but is blocked by pPP at room temperature (20 oC). d, Thermo-gating penetration of water in iPP. e, iPP at different states.



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Table of contents


Thermoresponsive Mobile Interfaces with Switchable Wettability

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