Formation of liquid marbles using pH-responsive particles: Rolling vs

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Formation of liquid marbles using pH-responsive particles: Rolling vs electrostatic methods Kohei Kido, Peter Matthew Ireland, Takafumi Sekido, Erica J. Wanless, Grant Bruce Webber, Yoshinobu Nakamura, and Syuji Fujii Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04204 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Formation of liquid marbles using pH-responsive particles: Rolling vs electrostatic methods Kohei Kido1, Peter M. Ireland2,3, Takafumi Sekido1, Erica J. Wanless2,4 Grant B. Webber2,3, Yoshinobu Nakamura5,6, Syuji Fujii5,6* 1

Division of Applied Chemistry, Graduate School of Engineering Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku Osaka, 535-8585, Japan.

2

Priority Research Centre for Advanced Particle Processing and Transport University of Newcastle, Australia 3

Discipline of Chemical Engineering, University of Newcastle, Australia 4

Discipline of Chemistry, University of Newcastle, Australia

5

Department of Applied Chemistry, Faculty of Engineering Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku Osaka 535-8585, Japan. 6

Nanomaterials Microdevices Research Center

Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku Osaka 535-8585, Japan.

* Author to whom correspondence should be addressed ([email protected])

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Abstract Aqueous dispersions of micrometer-sized, monodisperse polystyrene (PS) particles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA) colloidal stabilizer on their surfaces were dried under ambient conditions at pH 3.0 and 10.0. The resulting dried cake-like particulate materials were ground into powders and used as a stabilizer to fabricate liquid marbles (LMs) by rolling and electrostatic methods. The powder obtained from pH 3.0 aqueous dispersion consisted of polydisperse irregular-shaped colloidal crystal grains of densely packed colloids which had hydrophilic character. On the other hand, the powder obtained from pH 10.0 aqueous dispersion consisted of amorphous and disordered colloidal aggregate grains with random sizes and shapes, which had hydrophobic character. Reflecting the hydrophilic-hydrophobic balance of the dried PDEA-PS particle powders, stable LMs were fabricated with distilled water droplets by rolling on the powders prepared from pH 10.0, but the water droplets were adsorbed into the powders prepared from pH 3.0. In the electrostatic method, where an electric field assists transport of powders to droplet surface, the PDEA-PS powders prepared from pH 3.0 jumped to an earthed pendant distilled water droplet to form a droplet of aqueous dispersion. Conversely the larger powder aggregates prepared from pH 10.0 did not jump due to cohesion between the hydrophobic PDEA chains on the PS particles, resulting in no LM formation.

Keywords: liquid marble · pH-responsive particle · electrostatics · air-water interface · adsorption

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Introduction Liquid marbles (LMs) are liquid droplets stabilized by solid particles adsorbed at the gas-liquid interface1-6, which behave as non-wetting soft objects. No LMs can be stabilized by molecular surfactants, making them different from emulsions and foams, which can be stabilized by both solid particles and molecular surfactants. LMs are generally fabricated by gentle rolling of a liquid droplet on a hydrophobic powder bed, which allows encapsulation of the liquid by the particles7-11.

Liquid droplets can be

deposited onto the dried powder bed using a micropipette, and the volume of the LMs can be controlled by tuning the droplet volume. It has been demonstrated that simply drying the droplets on the hydrophobic powder bed leads to droplet self-coating by the powder and the formation of LMs12. Droplet impact onto a powder bed can also lead to LM creation13. Recently, it was shown that electrostatic fields can induce particle coating of pendant droplets14,15. The dried particles deposited on an electrically biased substrate can jump to a grounded water drop when they were brought close to each other. Introduction of stimuli-responsive characters into the LMs is of significant interest in various research fields, including material chemistry, physical chemistry, colloid science and nanotechnology4. Chemical and physical changes above micrometer scales in the liquid and gas phases can act on the LM stabilizers and lead to the modification of the liquid-gas interfacial properties; consequently, these modifications can change the LM structure and stability. Development of LMs whose structure and stability can be well controlled by external stimuli also enables the insertion of new functional materials into the LMs and delivery of these materials on demand. Wettability of the particles at the gas-liquid interface and their adsorption energy play


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a crucial role in the fabrication of LMs and control their structure and stability. Relatively hydrophobic particles that adsorb at the gas-liquid interface are generally utilized to stabilize the LMs, whereas relatively hydrophilic particles are utilized to stabilize bubbles/foams. LMs stabilized with polystyrene (PS) particles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA) colloidal stabilizer (PDEA-PS particles) were the first to be reported as stimuli-responsive16,17. PDEA is a weak polybase with a pKa of 7.3 that is soluble in aqueous media below pH ~7 because of protonation of its tertiary amine groups. At pH 8 or above, PDEA exhibits either very low or zero charge density, and hydrophobic character. The PDEA-PS particle-stabilized LMs prepared using distilled water droplets exhibit long-term stability (over 90 min) when transferred onto the planar liquid water surface, provided that the solution pH of the subphase is above pH 8. In contrast, the use of acidic solutions leads to immediate disintegration of these LMs. The LMs placed on a water surface at pH 8 can be damaged by the addition of acid aqueous solutions because protonation of the PDEA stabilizer on the PS particle surfaces leads to spontaneous desorption of the particles from the water droplets. Hence, the LMs immediately disintegrate with concomitant dispersal of the particles into the acidic solution. Just as acids can induce disruption of LMs in the systems described above, it is possible to fabricate LMs exhibiting complementary behavior that are disrupted by the addition of bases. To obtain a base-induced disruption mechanism, particles carrying polyacid stabilizer, such as poly[6-(acrylamido)hexanoic

acid]18

or

succinic

anhydride-esterified

poly(2-hydroxypropyl methacrylate)19, on their surfaces have been used. The LMs stabilized by these particles are stable when placed on the surface of acidic solutions because the particles have hydrophobic surfaces with protonated polyacids and remain


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adsorbed to the LM surface. In contrast, the LMs rapidly disintegrate upon the addition of a base because the particles contain anionic water-soluble deprotonated polyacids with hydrophilic character. Ma and Ngai have recently indicated that drying conditions of pH-responsive latex particles

should

be

carefully

taken

into

account

in

formulating

LMs20:

poly(styrene-co-methacrylic acid) particles freeze-dried in their protonated state can work as efficient LM stabilizers, but it is difficult to stabilize LMs by using particles dried in their deprotonated state. These results indicates that the initial state of the particles is also important, even if the same particles are used, as well as external stimuli applied after formation of LMs. In this study, dried powders consisting of PDEA-PS particles were used as LM stabilizers, and effects of the pH of the continuous phase aqueous medium before drying on the formation of LMs were investigated in detail. The effects of the fabrication methods of LMs, namely the rolling method and electrostatic method, on LM formation were also compared.


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Experimental Materials Styrene, α,α’-azobisisobutyronitrile (AIBN), isopropanol (IPA, 99%), hydrochloric acid (0.5 M aqueous solution), ammonia (28% aqueous solution) and aluminum oxide (activated, basic, Brockmann 1, standard grade, ~150 mesh, pore diameter 58 Å) were purchased from Sigma-Aldrich. 2-(Diethylamino)ethyl methacrylate (DEA, >98.5%) was purchased from Tokyo Chemical Industry Co. Ltd. Styrene and DEA were treated with the basic alumina to remove the inhibitor and then stored at -18 ˚C prior to use. Distilled water for solution preparation was first ion exchanged to a resistance of 18.2 MΩ·cm and then distilled (Advantec MFS RFD240NA: GA25A-0715).

Preparation of PDEA homopolymer by solution polymerization The initiator AIBN (0.4 g, 2.44 mmol) and IPA (400 mL) were stirred in a round-bottomed 1 L flask equipped with a magnetic stirrer bar until dissolved completely and bubbled for 30 min with nitrogen gas to purge oxygen at room temperature. Under a stream of nitrogen and with constant stirring at 250 rpm, the monomer DEA (40 g, 216 mmol), which was purified by passage through an aluminium oxide column, was injected into the flask to start the polymerization at 70 ˚C using a temperature controlled magnetic stirrer. After the polymerization for 24 h, the reaction solution was cooled to room temperature. The number-average molecular weight and its distribution (Mw/Mn) of the PDEA stabilizer used in this study were determined to be 29900 g·mol-1 and 2.7 by gel permeation chromatography.

Drying method of PDEA homopolymer


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The PDEA homopolymer prepared in IPA medium was purified using a dialysis membrane (Spectra/Por® 7, MWCO: 3.5 kD, Spectrum Laboratories, Inc.) for at least 10 days (IPA in the dialysis container was replaced twice per one day to remove impurities). The IPA solution of PDEA homopolymer (2.6 g, 38.5 wt%) was poured into HCl aqueous solution with a pH of 3.0 (40 mL) and pH was adjusted to 3.0 using HCl aqueous solution before freeze drying at 15.3 Pa for 48 hour (FDU-1200, Tokyo Rikakikai Co. Ltd.). The same approach was used for the preparation of PDEA homopolymer dried from pH 10.0 using NH3 aqueous solution for pH adjustment.

Characterization of PDEA homopolymer Chemical composition Chemical compositions of the PDEA homopolymers obtained from pH 3.0 aqueous solution and pH 10.0 aqueous dispersion were determined by CHN elemental microanalysis (Yanaco CHN-Corder MT-5). Cl elemental microanalysis was conducted by combustion ion chromatography method by A Rabbit Science Japan Co., Ltd. (Kanagawa, Japan).

Thermogravimetric analysis Thermogravimetric analysis (TGA) was conducted using a TGA instrument (SII, TG/DTA 6300).

Dried samples were heated up to 1000 °C under nitrogen at a heating

rate of 10 °C·min-1.

1

H pulse NMR analysis

Pulse nuclear magnetic resonance (pulse NMR) measurements were conducted using a


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pulse NMR instrument (JNM-MU25, JEOL. Ltd., Tokyo, Japan) by solid echo method at temperatures from -60 to 120 °C with a sampling time from 400 µs to 4 ms to study the molecular mobility of PDEA molecules dried from pH 3.0 and 10.0. Free induction decay (FID) curves were analyzed according to the method proposed by Urahama et al21. Relaxation time and the relaxation spectrum of protons in the sample were graphed as the X and Y axis, respectively. The number of points obtained in one measurement was limited to 2000 points. For this reason, five measurements at different measuring ranges of 1-400, 1-1000, 1-4000, 1-10000, and 1-40000 µs were performed and the five results were combined.

Probe tack test The probe tack test was conducted on films prepared from IPA solution of PDEA homopolymer after exposure to HCl or NH3 vapor for 2 weeks. In the present study, probe tack studies were conducted at a rate of 10 mm·s-1, which is close to the actual peeling speed, based on ASTM-D2979-71 and JIS Z0237-01. Tack of the adhesive films was measured using a probe tack tester (TE-6002, Tester Sangyo, Saitama, Japan) with a stainless-steel (SUS 304) probe (5 mm diameter) with a flat end covered by PDEA film at 23 ± 1 °C22. The PDEA films placed on glass substrates were attached to the film support (9.8 g), and the film support was set on the supporting board. The supporting board was reclined at 10 mm·s-1 and the probe brought into contact with the sample adhesive film. After a constant contact time (30 s), debonding occurred when the supporting board was elevated at 10 mm·s-1. The stress-displacement (S-D) curve of the debonding process was recorded, and the tack was defined as the maximum stress of the


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curve. The pressure of the film support was 5.1 × 103 Pa (5.0 × 102 kg·m-2).

Preparation of PDEA-PS particles by dispersion polymerization The PDEA-PS particles were synthesized in the same way reported previously23. Briefly, the IPA solution of PDEA homopolymer prepared by solution polymerization (293.8 g, 10.21 wt%) and IPA (2660 mL) were mixed in a 5 L three-necked round-bottomed separable flask fitted with a thermometer and a reflux condenser and equipped with a magnetic stirrer bar, and bubbled for 30 min with nitrogen gas to purge oxygen at room temperature. Under a stream of nitrogen and with constant stirring at 250 rpm, a mixture of the monomer styrene (300 g, 2.88 mol), which was purified by passage through an aluminium oxide column, and the initiator AIBN (3.0 g, 1.83 mmol) were injected to the flask to start the polymerization at 70 ˚C using magnetic stirrer and thermostatic bath. After the polymerization for 24 h, PDEA-PS latex was cooled to room temperature. The latex was purified by centrifugation/redispersion cycles with IPA (4 cycles; 3000 rpm, 1207 g, 15 min) and then deionized water (4 cycles; 4500 rpm, 2717 g, 15 min) using a centrifuge (Hitachi, CF16RXII type centrifuge with a Hitachi T15A 36 rotor).

Drying method of PDEA-PS particles Aqueous dispersions of PDEA-PS particles with pH values of 3.0 and 10.0 (40 g, 10 wt%) were placed in 50 mL glass vials (Nichidenrika-Glass Co., Ltd., Japan, SV-50A: caliber, 40 mm; bottom inner diameter, 37 mm; height, 75 mm). pH values were adjusted using HCl and NH3 aqueous solutions before the drying process. Drying of these dispersions were conducted at 21 °C, 0.1 MPa and 42.8-87.4 RH% in air. The


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PDEA homopolymer was also dried in the same manner at pH 3.0 and 10.0. The obtained dried cake-like white chunks were ground into powders using a pestle and mortar. Digital camera (Stylus SH-2, Olympus, Japan) was used to obtain optical images of the samples.

Characterization of PDEA-PS powders Optical microscopy (OM) The PDEA-PS powders were placed on a microscope slide glass and observed using an optical microscope (Shimadzu Motic BA200; Shimadzu Corp., Kyoto, Japan) fitted with an objective lens (Shimadzu, EF-N Plan achromat, × 10) and a digital system (Shimadzu Moticam 2000).

Scanning electron microscopy (SEM) The PDEA-PS powders were placed on an aluminium stub and sputter-coated with gold (a few tens nm thickness) using an Au coater (SC-701 Quick Coater, Elionix, Japan) in order to minimize sample-charging problems. SEM studies were carried out using a Keyence VE-8800 SEM operated at 5 kV.

Contact angle measurement Contact angles for water droplets (10 µL) placed on pressed pellets prepared from PDEA-PS powders dried from pH 3.0 and 10.0 (pelletized at 300 kg·cm-2 for 10 min using a Shimadzu SSP-IOA hand press) were determined using a DropMaster apparatus (DMs-401, Kyowa Interface Science, Japan) and a software (FAMAS, Kyowa Interface Science, Japan) at 25 ˚C.


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Preparation of LMs Rolling method Water droplets were deposited onto the dried PDEA-PS particle powder bed using a micropipette (Nichipet EX, Nichiryo: 2 to 20 µL). By gently rolling the aqueous droplet (15 µL) on the powder bed, the liquid was entirely encapsulated by the PDEA-PS powder, resulting in a LM.

Electrostatic method A schematic of the experimental apparatus is shown in Figure S1. A bed of dried PDEA-PS particle powder was spread on a glass slide, which rested on a stainless steel plate connected to a high voltage power supply. The metal plate was held at a constant negative potential relative to earth, and was gradually raised at a rate of 50 µm·s-1 toward a pendant drop of Milli-Q water on the end of an earthed metal capillary syringe of 1.2 mm outer diameter. The nominal drop volume was 5 µL, as dispensed by a syringe pump (Harvard Apparatus 11Plus), before the application of the electric field and loading with particles. When the separation between the particle bed and drop became sufficiently small, the powder was transferred across the gap to the drop, encapsulating it to form a LM. The process was recorded with either a high-speed camera (Fastec IL-5) or a normal high-resolution video camera, as appropriate.

Results and Discussion PDEA-PS particles Latex particles have been used in a dried state or after dispersal of the dried particles


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in a liquid medium. There are many studies on investigation of the relationship between drying conditions and the structures of the resulting dried colloidal dispersions24-29. Although it has been empirically known that the physical properties of the dried latex particles such as wetting and dispersability in liquid media depends on drying conditions, there are few systematic studies on the relationship among these parameters. The PDEA-PS particles synthesized by dispersion polymerization were nearly monodisperse and had a number-average diameter (Dn) of 2.20 µm and a coefficient of variation of 2 %, as indicated by SEM studies (Figure S2). PDEA loading% was determined to be 2.66 wt% by elemental microanalysis. XPS studies on the PDEA-PS particles dried from IPA dispersion determined the surface coverage by PDEA to be approximately 47% and that the PDEA is mainly located at the surface of the PS particles. At pH 3.0, where the PDEA is protonated and cationic, the PDEA-PS particles were well dispersed in aqueous medium, and at pH 10.0, where the PDEA is unprotonated and neutral, the PDEA-PS particles formed aggregates in an aqueous medium. After 2 months, the aqueous medium was completely evaporated from the aqueous dispersions of the PDEA-PS particles and cake-like white particulate materials were obtained at the bottom of glass vials in both pH 3.0 and 10.0 systems. The dried particulate material obtained at pH 10.0 was more fragile and easily broken when ground using mortar and pestle compared with that obtained at pH 3.0. These observed relative mechanical strengths of the dried particulate materials accord well with those obtained by indenter experiments23. Digital camera and optical microscopy images confirmed that the resulting powders consisted of polydisperse and irregular-shaped particle aggregate grains and their Heywood diameters were determined to be 153 ± 74


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Figure 1

(a,d) Optical and (b,e) SEM images and (c,f) schematic diagrams of

PDEA-PS particles dried from aqueous dispersions with (a-c) pH 3.0 and (d-f) pH

10.0 µm (n = 700) and 40 ± 31 µm (n = 1200) for pH 3.0 and 10.0 systems (Figure 1a, d and Figure S3). The PDEA-PS powders prepared from pH 3.0 and pH 10.0 aqueous dispersions were observed by SEM (Figure 1b, e). In the case of pH 3.0, colloidal arrays were observed on the surface of the particle aggregate grains, which had edges and planes on their surfaces. In the case of pH 10.0, random packing of the particles was observed on the surface of the aggregate grains. The degree of particle ordering in the dried particulate materials can be correlated with the fracture strength. Wettability of water to the PDEA-PS particles was studied by measuring static contact angles for deionized water drops on pressed pellets prepared from the dried PDEA-PS powders (Figure 2). Contact angles measured through water on the pellets of PDEA-PS powder prepared from pH 10.0 aqueous dispersion were >90˚ for 10 min, which indicates hydrophobic surface character due to the non-protonated PDEA stabilizer on the PS particles. The contact angle slowly decreased from 100˚ to approximately 90˚ during 10 min as seen in Figure 2(b). The initial advancing angle was measured to be approximately 100˚. Attempts to measure receding contact angles by


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Figure 2 Relationship between contact angle and time measured for a water droplet (10 µL) on pressed pellets prepared from PDEA-PS particles dried from aqueous dispersions with (a) pH 3.0 and (b) pH 10.0. (The pressed pellets were fabricated at 3.8×104 kPa.) withdrawing the water via a syringe were prevented by pinning of the three phase contact line until all the water was removed. On the other hand, contact angles of around 20˚ were determined on the PDEA-PS powder prepared from pH 3.0 aqueous dispersion. The contact angle rapidly decreased to 0˚ within 7 s as shown in Figure 2(a). This indicates that the surface of dried PDEA-PS particles is hydrophilic. The rapid absorption of water into the pellet precluded the measurement of any advancing or receding contact angles. These contact angle results are well correlated with the LM formation ability of the PDEA-PS particles (see below). The PDEA-PS particle powder prepared from pH 3.0 rapidly started to disperse into distilled water once added to a planar air-water interface prepared in a beaker (Figure S4). On the other hand, the PDEA-PS particle powder prepared from pH 10.0 remained floating on the air-water interface (Figure S4).


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PDEA homopolymer The wettability difference of the dried PDEA-PS particles to water should be due to the difference in chemical status of PDEA colloidal stabilizer existing on the particle surfaces dried from pH 3.0 and 10.0. Therefore, the chemical status of the PDEA homopolymers dried from pH 3.0 and 10.0 was investigated in detail. Figure 3a, b shows optical photographs of PDEA homopolymers dried from pH 3.0 and 10.0. The PDEA homopolymer obtained from pH 3.0 was solid-like and pale-yellow colored, and that obtained from pH 10.0 was highly viscous and dark-yellow colored. Optical microscopy studies confirmed that the PDEA obtained from pH 3.0 had a sponge-like structure, whose pores/hollows are believed to be formed during evaporation of the aqueous medium, resulting in a material that could scatter visible light (Figure S5a). On the other hand, the PDEA obtained from pH 10.0 was transparent with a wrinkled surface (Figure S5b).

Figure 3

(a,b) Optical photographs and (c,d) thermogravimetric and

differential thermal analysis data obtained for PDEA homopolymer freeze dried from aqueous solutions with (a,c) pH 3.0 and (b,d) pH 10.0. 15 ACS Paragon Plus Environment

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Figure 3c, d shows TGA results obtained for the PDEA homopolymers. The weight loss of the PDEA homopolymer obtained from pH 10.0 started at approximately 250 °C and ended at approximately 460 °C via one shoulder at approximately 380 °C; there were two peaks in the differential TG curve at 355 and 429 ˚C. On the other hand, weight loss of the PDEA homopolymer obtained from pH 3.0 started at approximately 200 °C, ~50 degrees lower than that observed in the pH 10.0 system, and ended at approximately 460 °C via two shoulders at 300 °C and 380 °C; there were three peaks in a differential TG curve at 239, 318 and 428 ˚C. The first peak observed in the differential TG curve could be observed only in pH 3.0 system and should be due to evaporation of volatile compounds in the PDEA sample. The PDEA homopolymer obtained at pH 3.0 exhibited an overall weight loss of 97%, and black-colored residue was observed after the TGA experiment. It is noteworthy that 4% weight loss was detected below 100 ˚C in the pH 3.0 system which should be due to evaporation of adsorbed water and was not clearly observed in the pH 10.0 system. This result indicates that the PDEA obtained from pH 3.0 is more hygroscopic compared to that obtained from pH 10.0.

Table 1. Chemical compositions of PDEA homopolymers dried from pH 3.0 and 10.0 aqueous solutions, determined by elemental microanalyses C/%

H/%

N/%

Cl / %

pH10 (exp.)

63.4

9.9

7.4

0.0

pH10 (theor.)a)

64.9

10.3

7.6

0.0

pH3 (exp.)

60.2

9.8

7.1

7.2

pH3 (theor.)b)

54.2

9.0

6.3

16.0

a) b)

Calculated assuming 100 % non-protonation of DEA units Calculated assuming 100 % protonation of DEA units with HCl


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In order to characterize the chemical compositions of the PDEA homopolymers, elemental microanalysis studies were conducted (Table 1). Carbon, hydrogen and nitrogen contents of the PDEA homopolymer dried from pH 10.0 accorded well with those theoretically calculated. On the other hand, carbon, hydrogen and nitrogen contents were estimated to be smaller for the PDEA homopolymer dried from pH 3.0 compared to that dried from pH 10.0, which is due to the presence of chloride. By comparing the chloride content of the PDEA homopolymer dried from pH 3.0 determined from elemental microanalysis to theoretical value calculated assuming all the DEA units are protonated and carry chloride ions, it was estimated that 45% of DEA units were protonated. It is surprising that DEA units remained protonated even after storage in vacuum (15.3 Pa) for 2 days, considering the boiling point of HCl is -85 ˚C30. Interestingly, poly(allylamine hydrochloride) was also shown to be protonated by HCl even under high vacuum condition by XPS studies31. The mobility of the PDEA molecules dried from pH 3.0 and 10.0 aqueous solutions can be evaluated by using pulse NMR. The FID curves obtained at 25 °C for the PDEA homopolymers prepared from pH 3.0 and 10.0 aqueous solutions were analyzed according to the method proposed by Urahama et al21. The measured and calculated FID curves were normalized and differentiated by the logarithm of relaxation time (Figure 4a). The curve obtained was referred as "the relaxation spectrum". For the PDEA homopolymer dried from pH 3.0, peaks were observed at shorter relaxation time compared with that dried from pH 10.0, which indicates that the mobility of the PDEA molecules was restricted. This decrease in mobility of the PDEA homopolymer prepared from pH 3.0 could be due to ionic dipolar interactions32,33. Richard and Maquet32 reported that carboxylated styrene-butadiene copolymer latex films prepared


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Figure 4

(a) Pulse NMR spectra obtained for PDEA homopolymer freeze dried from

aqueous solutions with pH 3.0 and pH 10.0. (b) Maximum stress in tack measurements obtained from PDEA homopolymer films. The PDEA homopolymer films were exposed to HCl or NH3 vapor for 2 weeks, followed by drying. from aqueous dispersion with basic pH values showed higher elastic moduli compared to those prepared from aqueous dispersion with acidic pH values. They proposed that the counterions that neutralized the carboxylate groups at high pH values resulted in dipoles, which can attribute the higher moduli to ionic dipolar interactions; monovalent (not only di- or trivalent) ions can participate in the dipolar interactions, which leads to a type of crosslinking. In the PDEA case, diethyl amino groups protonated with HCl could be dipoles and the PDEA mobility decreased due to their ionic dipolar interactions. The mobility of the PDEA should be related to tackiness of the PDEA. Considering the existence of PDEA on the surface of the PDEA-PS particles, the tackiness should affect the cohesion between the particles and therefore their floc size. The probe tack test is a powerful method to investigate the adhesion properties of soft polymers22, 34-37. Figure S6 shows representative stress-displacement curves measured for the PDEA homopolymers after exposure to HCl and NH3 vapors using the probe tack tester. In 18 ACS Paragon Plus Environment

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both cases, the stress reached a maximum point immediately after test initiation and then decreased rapidly. The maximum stresses were determined to be 0.0422 ± 0.0183 MPa and 0.0892 ± 0.0459 MPa for acid and base systems, respectively (Figure 4b), which indicates that the dried PDEA homopolymer prepared from basic condition is more tacky comparing to that prepared from acidic condition. The lower adhesion properties observed in the acid system should be due to smaller contact area between PDEA substrates compared to the base system, because of the lower mobility of PDEA.

Formation of LMs Rolling method First, the rolling method was used to investigate the LM formation ability of the PDEA-PS particles. In the case of the PDEA-PS powder dried at pH 10.0, individual LMs could be immediately prepared by rolling a de-ionized water droplet over these particles. The PDEA-PS powder coating on the water droplet rendered it both hydrophobic and non-wetting, and resulting LMs were stable even after transfer from the powder bed onto solid substrates, such as glass slide and poly(methyl methacrylate) (see Figure 5b). These LMs clearly have significant surface roughness, which indicates that particle aggregate grains, rather than a monolayer, coated the droplet. The diameter of the LMs was 4.4 ± 0.2 mm (n = 5), which was larger than the calculated value (3.0 mm), because of the PDEA particle aggregate grain shell layer on the water droplet. The weight ratio of water and the PDEA-PS particles was estimated gravimetrically to be 89/11. The LM shape deviated a little from a spherical shape due to gravitational force. (Note the capillary length of water is 2.7 mm; the capillary length is ⁄, where γ is the surface tension of water, g = 9.81 m·s-2 and ρ is the density of water.) Slow


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evaporation of the water from the LM at 25 °C led to the buckling on the LM surface (Figure 5d). The aggregate grains of PDEA-PS particles are irreversibly adsorbed at the

Figure 5 Optical images of liquid marbles prepared by (a-d) conventional rolling method and (e,f) electrostatic method (2.5 kV substrate potential) by using PDEA-PS

particles dried from aqueous dispersions with (a,c,e) pH 3.0 and (b,d,f) pH 10.0. Figures 5c,d are taken after evaporation of water from the liquid marbles shown in Figure 5a,b, respectively. Figures 5e,f show an aqueous dispersion pendant droplet of PDEA-PS particles and a water pendant droplet hanging on the end of earthed metal capillary syringes. 20 ACS Paragon Plus Environment

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air-water interface due to high adsorption energy, and the total surface area of the LM remains constant. Thus the LM should adjust its surface-to-volume ratio by undergoing deformation. The LMs can also be transferred intact onto the surface of planar liquid water surface, where they can move easily thanks to low friction38-42. The adsorbed PDEA-PS aggregate grains on the LM surface should have multiple contact points with the planar bulk water surface and separate the LM interior and exterior bulk liquids. The LMs broke up and inner water was released within a few seconds on lowering the pH of the bulk aqueous solution by addition of HCl aqueous solution (Supporting video 1), whereas the LMs remained stable for at least 4 h after addition of distilled water (n = 5); some LMs shrunk due to evaporation of inner water after 4 h, rather than disruption. This is because the PDEA-PS particles, which originally had unprotonated and hydrophobic PDEA colloidal stabilizer, acquired cationic and hydrophilic surface character on protonation of the PDEA colloidal stabilizer.

Hence the resulting

PDEA-PS aggregate grains spontaneously desorb from surface of the LMs and disperse into the bulk water solution, leading to rapid break-up of the LMs. SEM studies on the dried LM confirmed that the PDEA-PS particle aggregate grains existed on the LM surface and the PDEA-PS particles formed disordered structure (Figure 6g-i). Cross sectional SEM observation indicated that the LM had a flattened shape and a disordered structure of the PDEA-PS particles was again found inside (Figure 6j-l). In the case of the dried PDEA-PS powder prepared from pH 3.0 aqueous dispersion, a de-ionized water droplet was absorbed into the powder once placed on it. If the water droplet was rolled on the PDEA-PS powder quickly after the droplet was set on it, a white-colored ball was formed (Figure 5a). The weight ratio of water and PDEA-PS particles for the ball was gravimetrically estimated to be 26/74, which indicates that the


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Figure 6

SEM images of dried liquid marbles prepared by using PDEA-PS

particles dried from aqueous dispersions with (a-f) pH 3.0 and (g-l) pH 10.0. (d-f,j-l) Cross-section images of the dried liquid marbles. (b and c, e and f, h and i, k and l) Magnified images of the areas shown in (a,d,g,j), respectively. PDEA-PS aggregate grains were wetted and were dispersed into water phase. Slow evaporation of the water component from the LM at 25 °C led to the size reduction of the LM with the shape remained (Figure 5c), rather than buckling or formation of wrinkles. SEM studies confirmed that there were both PDEA-PS particle aggregate


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regions and PDEA-PS colloidal crystalline regions on the LM surfaces (Figure 6a-c). Only the PDEA-PS particle colloidal crystalline structures were observed inside of the dried sample in the cross sectional SEM images (Figure 6d-f). These colloidal crystalline structures should be formed during slow evaporation of the aqueous dispersion of PDEA-PS particles in the ball.

Electrostatic method

The electrostatic method used in the present study for forming liquid-particle agglomerates was first demonstrated with hydrophilic particles in 201343. Its use was then extended to the formation of genuine LMs with PS particles15, and ‘complex liquid marbles’ with a core consisting of a suspension of hydrophilic particles, enclosed in a layer of hydrophobic particles44. Since this electrostatic method requires no direct contact between the drop and the particle bed, the encapsulation process for LMs is physically quite different to rolling contact, potentially leading to differences in the structure and stability of the resulting marble. As already described, the particle bed was held at a constant potential while being gradually lifted toward the pendant drop. This resulted in particles jumping toward the drop, and either being internalised by or coating the drop, depending on their wettability. The separation distance at which particle transfer began was determined by the electric field, which became more concentrated beneath the drop as the separation decreased14, by the weight of the particles, and by the cohesive forces in the particle bed43. Perhaps surprisingly, no PDEA-PS particle powder prepared from pH 10.0 jumped to the pendant drop (pH approximately


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6.5) under any experimental conditions used in this study (1.5 - 3.0 kV on the substrate) (Figure 5f), even when the drop-bed separation was reduced to zero. This is a strikingly different result from that of the rolling method, where the LMs were easily prepared. The reason for this difference is not clear, but it is likely that the PDEA-PS particle aggregate grains formed larger aggregates due to their sticky surfaces (see Figure S6 for the probe tack results on the PDEA), and that these aggregates were too large and heavy to jump to the pendant droplet. Figure S7 shows the interaction of PDEA-PS particle powder with a pendant droplet (1 µL) after direct contact. The PDEA-PS particle aggregate grains appear to weakly adhere to the pendant droplet, and hang loosely at the base of the water droplet. The size of the aggregate hanging on the droplet is approximately 900 µm which is larger than that of the independent PDEA-PS particle aggregate grain observed in OM and SEM images. This observation relates to a wider class of cohesive and frictional mechanisms within the bed that are thought to impede particle transfer, some related to the presence of the electric field12,14. It is also possible the aggregate grains in the bed were impeded from jumping because of steric hindrance due to their undefined shapes.


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Figure 7 Snapshots of formation of liquid marbles by electrostatic method (2.5 kV substrate potential) by using PDEA-PS particles dried from the aqueous dispersion with pH 3.0. Water droplet is distilled water. In contrast to these cases, the dried PDEA-PS powder prepared from pH 3.0 aqueous dispersion successfully jumped to the droplet (Figure 5e, Figure 7 and Supporting video 2), although it did so in the form of sizeable aggregate grains (

Formation of liquid marbles using pH-responsive particles: Rolling vs

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