Nanoscale Approach for Studying Scale Formation and

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A Micro/Nanoscale Approach for Studying Scale Formation and Developing of Scale-Resistant Surfaces Hossein Sojoudi, Srinivasa Kartik Nemani, Kaitlyn Mullin, Matthew G Wilson, Hamad Al-Adwani, Haitham M.S. Lababidi, and Karen K. Gleason ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18523 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

A Micro/Nanoscale Approach for Studying Scale Formation and Developing of Scale-Resistant Surfaces Hossein Sojoudi1‫٭‬, Srinivasa Kartik Nemani1, Kaitlyn M. Mullin2, Matthew G. Wilson2, Hamad Aladwani3, Haitham Lababidi3, and Karen K. Gleason2‫٭‬ 1.

Department of Mechanical, Industrial, and Manufacturing Engineering, University of Toledo, 2801 W. Bancroft Street, Toledo, OH 43606, USA

2.

Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

3.

Department of Chemical Engineering, College of Engineering & Petroleum, Kuwait University, P.O. Box 5969, 13060 Safat, Kuwait

KEYWORDS: scaling, iCVD polymers, QCM, surface energy, adhesion

ABSTRACT:

Blockage of pipelines due to accretion of salt particles is detrimental in

desalination and water harvesting industries as they compromise productivity, while increasing maintenance costs. We present a micro/nano-scale approach to study fundamentals of scale formation, deposition, and adhesion to engineered surfaces with wide range of surface energies fabricated using initial chemical vapor deposition (iCVD) method. Silicon wafers and steel substrates are coated with poly (1H, 1H, 2H, 2H-perfluorodecylacrylate) or pPFDA , poly 1 ACS Paragon Plus Environment

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(tetravinyl-tetramethylcyclotetrasilohexane) or pV4D4, poly(divinylbenzene) or pDVB, poly(1,3,5,7- tetravinyl-1,3,5,7-tetramethylcyclotetrasilohexane) or pV3D3, and crosslinked copolymers of poly(2-hydroxyethylmethacrylate) and polyethylene glycol diacrylate or p(PHEMA-co-EGDA). Particles of salt (CaSO4.2H2O) are formed and shaped with a focused ion-beam (FIB) and adhered to a tip-less cantilever beam using a micro-manipulator setup to study their adhesion strength with a molecular force probe (MFP). Adhesion forces were measured on the substrates in wet and dry conditions to evaluate the effects of interfacial fluid layer and capillary bridges on net adhesion strength. The adhesion between salt particles and the pPFDA coatings decreased by 5.1 ± 1.15 nN in wet states, indicating the influence of capillary bridging between the particle and the liquid layer. In addition, scale nucleation and growth on various surfaces is examined using a quartz crystal microbalance (QCM), where supersaturated solution of CaSO4.2H2O is passed over bare and polymer-coated quartz substrates while mass gain is measured in real-time. The salt accretion decreased by two-folds on pPFDA-coated substrates when compared to p(HEMA-co-EGDA) coatings. Both MFP and QCM studies are essential in studying the impact of surface energy and roughness on the extent of scale formation and adhesion strength. This study can pave the way for design of scaleresistant surfaces with potential applications in water treatment, energy harvesting, and purification industries.

INTRODUCTION

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Scales are hard, chalky, and grey-white salt deposits found in boilers1, heat-exchanger systems2, reverse-osmosis (RO) membranes3-4, de-salination pipe-line systems5, and household plumbing.6 Evaporation of water containing dissolved compounds of calcium, magnesium, and sometimes, other divalent and trivalent metallic elements primarily results in formation and accumulation of these deposits. Scale build-up is a major concern as it leads to choked liquid flow-rates and wear of metal parts causing deleterious impact on thermal conduction resulting in overall drop in thermal efficiencies and increase in operational costs, due to long and expensive processes of scale removal.7 Scaling is primarily observed in elevated temperature zones where the solubility of the dissolved compounds decreases resulting in rapid sedimentation of salts on the surface. Calcium sulfate di-hydrate or gypsum (CaSO4.2H2O) scaling occurs mostly in desalination and reverse-osmosis plants due to mixing of incompatible waters where the sea water containing high sulfate content is harbored at off-shore locations for maintaining pressure and undergoing further treatment. A change in water PH level, temperature, pressure, and stoichiometric ratio of the lattice ions further accelerates scale formation in various systems.8 Prevention of scale build-up is imperative to minimize rising maintenance costs and to delay the onset of further scaling. Currently available methods rely on mechanical means or chemical treatment to remove scale deposits from surfaces,9-10 or use of inhibitor mixtures in the liquid medium to delay the onset of crystallization on surfaces. However, most of these solutions have minimal or no impact to inhibit the growth of scales or prevent their nucleation 3 ACS Paragon Plus Environment


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at first place.11-12 In addition, solvent-based inhibitors come at an additional cost of deteriorating the chemistry of the liquid and the environment thereby restricting their use to a few select applications where liquid contamination is insignificant. In recent years, alternative methods such as thin-film coatings13-14 and polymeric mixtures15 have gained wide spread attention to inhibit the build-up of scales on surfaces and are of interest due to two reasons: first to attenuate nucleation and/or deposition of salt scales and second to reduce the adhesion strength scale, in case of any formation and deposition. Moreover, mechanisms to elevate energy barriers of scale initiation have been explored to inhibit scale accumulation. Masoudi et al. have reported magnetic slippery surfaces which provide liquid-liquid interactions by boosting the free energy barrier for salt nucleation, thus reducing the adhesion strength of scales.16 Subramanyam et al. studied the effect of liquid-impregnated surfaces for decreasing the density of nucleation sites and lowering the surface energy of the functional substrates.17

Similarly, iCVD coatings provide durable and robust surfaces with relatively

lesser surface energies and roughness which inhibit deposition and scale initiation. Moreover, the compatibility of the developed functionalized coatings on various host substrates with minimal impact on their robustness renders them ideal for large scale implementation in various industrial setups. In the present study, micro/nano-scale approaches were utilized to explore the fundamentals of scale formation, deposition, and adhesion strength on silicon as well as steel substrates coated with iCVD polymers using both molecular force probe (MFP) and quartz crystal microbalance (QCM) methods. A single salt (CaSO4.2H2O) crystal was 4 ACS Paragon Plus Environment

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formed and cut into a few microns in shape with a focused ion beam (FIB) which was mounted on tip-less cantilevers using a micro-manipulator setup to study their adhesion strength. The MFP measurements were conducted in dry as well as wet states, to study the effect of capillary bridging between the salt particle and the wet surfaces on the measured adhesion forces. In addition, scale nucleation and growth on silicon and steel surfaces is examined using QCM measurements,18-20 by allowing supersaturated solution of CaSO4.2H2O to flow over quartz substrates and monitoring the kinetics of mass growth at sustained supersaturation levels. While a steel substrate helps in simulating an environment similar to commercial water transport medium in desalination plants, a silicon substrate provides a relatively flat and smooth surface enabling us to eliminate the effect of roughness while studying scale formation and adhesion. Salt scale initiation and subsequent accretion is governed by two modes of nucleation. Heterogenous nucleation of salts on surfaces can be explained using the classic nucleation theory (CNT)21 where a seed crystal or an impurity triggers crystal growth in the medium which must reach a critical mass and size for sustaining and propagating scale formation. The impurity may be present either on the surface or in the liquid, while external factors such as PH level, temperature, and pressure play an important role in the growth rate and propagation of scales. Gypsum crystals preferentially nucleate by a surface controlled reaction.22 The Gibbs free energy (ΔG*) of the nucleus on planate mirror-like surfaces depends on the interfacial surface energies of the salt-solution, salt-substrate, and substrate-solution. This activation 5 ACS Paragon Plus Environment


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barrier is reported to be high on surfaces with lesser surface energies,23-24 with the precipitated salts essentially forming at the sites of the impurities and then subsequently growing over time. Figure 1 shows a schematic representation of scale nucleation and growth with subsequent accretion and sedimentation/adhesion on surfaces. While heterogenous nucleation requires some metastable sources for scale initiation; the mechanism of scale formation consequentially is much slower in homogenous nucleation mode, where majority of crystal growth occurs within the liquid phase which precipitates and sticks on the solid surfaces after considerable mass gain due to gravity. While previously formed scales can deposit and accrete on surfaces with pre-existing debris, there is direct accumulation of scales in some instances where the mechanism of scaling is primarily due to heterogeneous nucleation. Despite extensive studies on physics of nucleation and growth of salt scales on particulate and bulk systems,25-26 there is little data on their formation and adhesion strength to surfaces with controlled surface energies, their polar and apolar components, and roughness. There is still a need to investigate the effect of surface roughness and the impact of scale crystallization on its adhesion strength to various polymer coatings. In this work, we study these two scaling mechanisms using engineered coatings with controlled surface energy and roughness developed via the iCVD method.27-28 We have recently reported the design and deposition of iCVD polymer coatings of pPFDA, pV3D3, pV4D4, pDVB, and co-polymer of pHEMA-co-pEGDA on silicon and steel substrates.29-30 We have also investigated the durability of similar coatings developed via iCVD 6 ACS Paragon Plus Environment

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and in-situ grafting mechanism which enhanced the elastic modulus and hardness of the films through the bilayer structure while evaluating their performance against wear and variable environments such as extreme cold conditions and abrasive flows in our previous studies.31-32 It was observed that functionalized coatings of organosilanes such as pV4D4, and pV3D3 exhibited a 60% drop in the mass gain rate of scales when compared to bare silicon substrates. In addition, the salt adhesion force for pV4D4 coatings on silicon were measured to be 12.5 nN and 7.5 nN for dry and wet substrates establishing the effect of capillary bridging observed in wet samples. It was also observed that coatings with lower values of polar component of surface energies exhibited lower adhesion strengths to the pPFDA-coated substrates, exhibiting the lowest salt adhesion forces of 11.1 ± 1.6 nN and 6.0 ± 2.75 nN in dry and wet stat, respectively. This indicates the effect of polar component of surface energies in developing efficient scale-resistant engineered surfaces.

RESULTS AND DISCUSSION Estimation of surface energy FTIR spectroscopy was used to study the chemical composition of the films and verify the presence of conformal coatings on the substrate (See Supporting Information, Figure S1). The contact angles of polar and apolar solvents were determined to compute the surface energies of the thin film coatings. A summary of advancing and receding contact angles of DI water,



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diiodomethane (DIM), and ethylene glycol (EG) are reported in Table 1. The surface energies were estimated using Van Oss-Chaudhury-Good theory of contact angles:33-34

 l ,i 1  cos i   2



 s LW  l ,i LW   s  l ,i    s  l ,i 



(1)

where  l ,i is the known surface tension for reference probe liquids, i is the measured contact angle of the liquid i on the substrate,  s LW is the apolar component of surface energy, and

 s  ,  s  are the components associated with the Lewis acid and base zones, respectively. The values of  l ,i LW ,  l ,i  ,  l ,i  were obtained for known reference liquids from literature.34 The polar component of surface energy,  s AB is calculated from the Lewis acid and base parameters of the surface energy using the following correlation:

 s AB  2  s    s 

(2)

where the subscript AB refers to the acid-base polar interactions in the liquid. The net surface energies of the substrates were calculated using Equation (3). Table 2 shows the total (  total ), Van-der-Waal’s (  LW ), acid-base components (  AB )

of the surface energies for various

polymer surfaces along with their corresponding RMS roughness calculated using optical profilometer.

 total   LW   AB

(3)

The samples coated with pPFDA have the lowest surface energy, due to lesser inherent tendencies to acts as Lewis acids and bases since the fluorine atoms possess a full octet of electrons in their 2s22p6 orbitals which reduces their surface polarity. Polymers with cyclic 8 ACS Paragon Plus Environment

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monomers as in the case of pV4D4 and pV3D3 or aromatic rings (pDVB) exhibited larger surface energies due to greater polarizing affinity (high  LW values). Micromanipulator setup for studying salt adhesion An in-house micromanipulator setup (World Precision Instruments) with high resolution lens (Navitar) was utilized to adhere CaSO4.2H2O salt particles to tip-less cantilevers of MLCT-010 AFM probe, as shown schematically in Figure 2. Tip-less cantilevers with various stiffnesses available on the MLCT-010 probe were tried and tested for adhering the salt particle. Eventually, the cantilever “F” with stiffness of 0.6 N/m was used for adhering salt microparticles to allow measurement of adhesion forces up to 30 nN in magnitude (See Supporting Information, Figure S2 for a photograph of the set-up and SEM image of various tip-less cantilevers on the AFM probe). The salt particle was adhered to the cantilever with glue and the position was aligned simultaneously using fine adjustments. Figure 3a shows the SEM images of as grown salt crystal mounted on the cantilever. However, the mounted salt particle was observed to be uneven with high surface roughness which was detrimental for the adhesion measurements in this study. Hence, in order to establish a relatively smooth surface contact between the substrate and the salt crystal adhered to the tip-less cantilever, the salt particle was trimmed and smoothened with a focused ion beam (FIB) (See Supporting Information, Figure S3) to generate particles of equal sizes and to enable better Hertzian contact between salt particle and the surfaces as shown in Figure 3b.



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Salt adhesion force measurements Plasma treated35 silicon wafers followed by polymer coatings in dry and wet condition were brought in contact with the salt particles adhered to the tip-less cantilever, while the adhesion forces were recorded with a molecular force probe (MFP). While the same salt particle mounted on the cantilever is used in the MFP measurements, the impact of particle size on the effective adhesion strength is an interesting area of experimental research under dynamic environmental conditions where the effect of environmental factors such as PH, concentration, temperature, and the nature of flow should be taken into consideration. The adhesion between the salt particles and the surfaces (bare and polymer coated) is primarily governed by Van-der-Waals and Coulombic forces in microscopic particle interactions. Moreover in natural systems, the presence of moisture and high humidity can impact the growth rate of scales by capillary bridging consequentially due to attraction between the salt particle and interfacial fluid retained by the substrates.36 To simulate more environmentallyidealistic conditions where presence of moisture can impact the adhesion strength of the scales on surfaces, wet samples were prepared by passing a thin film of water on the substrate with a micro-pipette (7-10 µl). The impregnated liquid layer gets stabilized by the effect of capillary bridging due to forces generated from the microscopic texture of the thin films which leads to a better surface performance in harsher conditions.37 Figure 4a shows the adhesive forces between the salt microparticles and the surfaces (pPFDA, pV4D4, pV3D3, pDVB, and pHEMA-co-pEGDA polymeric films, and bare silicon) with and without intermediate liquid 10 ACS Paragon Plus Environment

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layer between them, as a function of total surface energies of each substrate. The readings were averaged at sixteen randomly selected spots on the samples to achieve greater accuracies (See Supporting Information, Figure S4). The salt adhesion force to the wet pPFDA films was measured to be 6.0 ± 2.75 nN, lower than its value to the dry pPFDA film which exhibited an adhesion force of 11.1 ± 1.6 nN. While bare silicon wafers exhibited net adhesion forces of 14.8 ± 2.4 nN and 16.3 ± 0.1 nN in wet and dry conditions respectively. Films coated with pV4D4, pDVB, and pV3D3 showed moderate reduction in adhesion forces when compared to bare silicon. The adhesion forces were relatively commensurate for pV4D4 and pDVB films, measured at 7.6 ± 3.6 and 7.5 ± 2.8 nN in wet conditions while the dry films exhibited adhesion forces of 12.8 ± 0.6 and 12.5 ± 0.4 nN. The substrates with coatings of cross-linked pHEMAco-pEGDA polymer film exhibited very high adhesion forces of 15.4 ± 0.3 nN and 17.5 ± 5.7 nN in dry and wet states respectively, which is due to their inherently high surface energies.38 The roughness of the polymer thin films was analyzed with AFM measurements (See Supporting Information, Figure S5). Overall, a reduction in salt adhesion force was observed with a decrease in total surface energies of the examined substrates. In addition, the salt adhesion force was lower when the substrates were wet, suggesting weaker Van-der-Waal’s forces due to consequent capillary bridging occurring between the liquid layer and the salt particle. A schematic diagram of proposed capillary bridging between the salt particle and the liquid interface is shown in Figure 4c. Adhesive forces were also plotted as a function of the

  AB ratio of polar and apolar components of the surface energies  LW 

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  . Figure 4b shows the  11


variation in adhesive forces against

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 AB . It was observed that the substrates with higher polar  LW

components exhibited greater adhesion strengths, with cross-linked polymer films of pHEMAco-pEGDA showing relatively higher adhesive tendencies in wet as well as dry substrates. The samples coated with pDVB, pV4D4, and pPFDA films exhibited a great reduction in adhesion forces with lesser values of polar component of surface energies which were measured to be 0.5, 1.9, and 0.1 mN/m, respectively. This suggests that the engineered surfaces with low polar components of surface energies reduce the tendencies of already-formed salt particles to adhere to the surface, which is crucial in developing scale-resistant coatings.39 Similar set of experiments were performed on stainless steel substrates to highlight the practical applicability of the polymer coatings, and to eliminate the effect of inherent micro-level roughness associated with the bare substrates as discussed earlier (See Supporting Information, Figure S6).

Scale formation and mass-gain measurement In addition to scale formation in the liquid medium leading to subsequent deposition and adhesion to surfaces, scales can also nucleate and grow directly on the surface. To examine the scale formation and the subsequent mass growth on the test surfaces, another set of experiments were performed by flowing supersaturated solution of calcium sulfate dihydrate over the substrates. The solution was prepared by dissolving excess amount of CaSO4.2H2O in water at 45°C while stirring. As bought, and iCVD polymer coated quartz crystals were exposed 12 ACS Paragon Plus Environment

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to the supersaturated salt solution for 72 hours which resulted in CaSO4 scale deposits on the quartz test substrates due to subsequent evaporation of aqueous phases. Mass gain rates were recorded for each sample over a period of 7 hours using a quartz crystal microbalance with dissipation monitoring (QCM-D) setup as shown schematically in Figure 5a. Figure 5b compares the mass gain per unit area (ng/cm2) over time on bare silicon, and quartz crystals coated with pPFDA, pDVB, pV3D3, pV4D4, and PHEMA-co-PEGDA polymeric films. It is observed that the substrates with pV4D4 coating exhibited the lowest mass gain rate of ~ 600 ng per unit sq. area after 4 hours of exposure to saturated CaSO4.2H2O solution, which is attributed to lesser surface energies associated with the polymer surface which inhibit the accumulation of scales as a result of higher energy barrier. A considerable decrease in mass gain rate was also observed for samples coated with pPFDA compared to pHEMA-co-pEGDA, which accrued 1600 ng/cm2 in 5.5 hours. The pPFDA samples exhibited an average mass gain of 812 ng/cm2 after 5.5 hours of exposure to saturated solution with the rate of salt accumulation accelerating, while the inherently greater surface roughness of the pPFDA coated samples resulted in higher noise in the readings. This may be due to enhanced heterogenous salt nucleation on the pPFDA surfaces as a consequence of their greater surface roughness which propagates scale initiation on the microtextured crevices of the surface. Surface profilometry studies on the samples reaffirmed the mechanism of nucleation with pPFDA exhibiting surface roughness as high as 28.3 ± 3.3 nm resulting greater mass gain due to abundance of nucleation sites on the surface. It may be concluded that the mechanism of 13 ACS Paragon Plus Environment


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growth of salt deposits on pPFDA samples was primarily due to surface crystallization40 resulting in higher mass accumulation on the surface of pPFDA films. The pV4D4 and pV3D3 samples exhibited lower mass gain rates over time due to a relatively smoother substrate suggesting lesser salt nucleation sites on these surfaces. Overall, the mass gain due to salt formation/accretion was lowered significantly by lowering the surface energy of the substrates. However, rough surfaces such as pPFDA films experienced lesser reduction in mass gain when compared to their relatively smoother counterparts. Figure 5c shows the mass gain rate on sample substrates as a fraction of scale formation per unit area on bare silicon and CVD coated silicon substrates. A 60% reduction in scale formation was observed for substrates coated with pV4D4 in comparison to bare silicon with an average scale formation ratio of 0.42 which may be attributed to smoother surface and relatively low surface energies associated with the polymer film.

The accumulation on samples was

qualitatively examined after 30 minutes post QCM-D experiments by passing supersaturated CaSO4.2H2O solution on the quartz crystal. The SEM images (See Supporting Information, Figure S7) exhibited a great reduction in salt accumulation and mass growth on samples coated with pPFDA, pV3D3, and pV4D4 when compared to bare silicon substrates primarily due to their lower surface energies. Coatings deposited on stainless steel also exhibit inhibited scale accumulation over extended time periods. Effect of surface energies and RMS roughness


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We observed that the adhesion forces between the salt particles and the substrates (bare and polymer coated) were primarily affected by the polar component of surface energies (  LW ), suggesting that the surfaces with low polar components might be a significant aspect to mitigate the onset of scaling. However, the surface roughness of the substrates was also a contributing factor for the overall mass accumulation on the surfaces such as pPFDA with low polar component of surface energy along with inherently high surface roughness resulting in reduced salt adhesion on it. However, its rough surface drives crystallization and heterogeneous nucleation of salt particles on the substrate leading to higher mass gain irrespective of a lower total surface energy (  total ). CONCLUSION In summary, we have evaluated the adhesion forces and mass accumulation rates on various iCVD polymer films by utilizing a novel micro/nanoscale approach where a salt particle adhered to a tip-less cantilever was employed to measure the salt adhesion forces. The net mass accumulation per unit surface area was also determined by QCM-D measurements real-time. These results demonstrated a 60% drop in net mass gain of scales on surfaces with coatings of organosilanes pV4D4 and pV3D3 and polar components surface energies of 1.9 mN/m and 3.2 mN/m, respectively. These results consequently emphasize the effect of polar component of surface energy in salt adhesion. It was also observed that the net mass accumulation on pPFDA coatings with total surface energy of 8.3 mN/m exhibited higher mass gains due to a dominant 15 ACS Paragon Plus Environment


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heterogenous nucleation and surface crystallization attributed to a greater surface roughness in the pPFDA. It is evident that the reduction in surface energies may not decrease the rate of scale growth with their polar and apolar components playing a vital role in governing the dynamics of adhesion forces on the surface. The results of this study can be helpful in designing surfaces with controlled surface energies in desalination plants. Selection of coatings based on this study can help in reducing the surface proneness to nucleation sites and inhibit subsequent accretion of scales on surface. Such selectivity and controllability can prevent recurring losses and reduce costs in preventive maintenance of systems, hereby enabling scope for developing more reliable, efficient, and durable scale-resistant surfaces.

MATERIALS AND METHODS iCVD coating on steel and silicon substrates 1H,1H,2H,2H-Perfluorodecylacrylate (97%), 2,4,6,8-Tetramethylcyclotetrasiloxane (98.5%), 1,3,5,7- tetravinyl-1,3,5,7-tetramethylcyclotetrasilohexane (98.5%), divinylbenzene (80%), ethylene glycol diacrylate (90%), 2-hydroxyethylmethacrylate (>99%), and the initiator tert-butyl peroxide (98%), were obtained from Aldrich and were used as received. Silicon wafers (Waferworld Inc.) and steel coupons (McMaster-Carr) were first cleaned and dried using solvents (acetone, iso-propyl alcohol from Aldrich), followed by rinsing in deionized (DI) water (>16 MΩ.cm). The substrates were then treated with oxygen plasma for 10 min prior to deposition of polymer films using iCVD.31,

41

iCVD polymerizations were carried out in a custom-built

cylindrical reactor (diameter 24.6 cm and height 3.8 cm), with a supported array of 14 chromo16 ACS Paragon Plus Environment

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alloy filaments (Good-Fellow) suspended parallelly at 2 cm above the stage. The thickness was monitored real time with a quartz top (2.5 cm thick) and a 633 nm He-Ne laser source reflecting off the substrate and recording the interference signal as a function of time. The initiator, TBPO was kept at room temperature (Tf = 25°C) and was delivered at a constant flow rate of 3 sccm during the polymerization of DVB, and 1 sccm for V3D3, V4D4, and PFDA. The monomers were vaporized in glass jars, followed by their introduction to the reactor through needle valves at constant flow rates. The filament temperature was maintained at 280 °C while the substrates were maintained at Ts=30 °C, using a recirculating chiller/heater (NESLAB RTE-7). All temperatures were measured with k-type thermocouples (Omega Engineering). The working pressures were maintained at 300 mTorr in DVB and PFDA polymerization and 350 mTorr for PHEMA-coPEGDA, V3D3, and V4D4 polymerization. The setup, deposition, and grafting method of these polymer films using the iCVD method are extensively discussed in our previous works.29-32, 42-44 Surface chemistry and characterization To determine the surface morphology of the substrates, the prepared samples were characterized using atomic force microcopy (AFM-Veeco Metrology Nanoscope IV). FTIR spectroscopy (Nicolet Nexus 870 ESP spectrometer) was performed in normal transmission mode equipped with a mercury-cadmium- tellurium MCT detector and K-Br beam splitter to confirm the presence of functional groups on the surface of the polymers. A non-contacting optical profilometer (Zeta-20, KLA-Tencor Co.) was used to determine the surface roughness of the polymer coatings, quantitatively. A scanning electron microscope (JSM-6610LV) was used at an accelerating voltage of 20 kV to qualitatively evaluate scale deposition on various surfaces and imaging salt microparticles before and after cutting with the FIB. 17 ACS Paragon Plus Environment


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Mass gain measurements Mass gain on developed polymer coatings was measured using a quartz crystal microbalance with dissipation monitoring (QCM-D) setup (Sycon Instruments) with a sensor (Maxtek PM 740) for detecting mass change. The coated quartz crystal was mounted on the deck with a constant inflow of supersaturated CaSO4 solution to allow mass accumulation. All experiments were performed at constant temperatures of 40 °C. Contact angle measurements Contact angles of the three probe liquids (diiodomethane, ethylene glycol, and DI water purchased from Aldrich) on various surfaces were measured using a Ramé-Hart M500 advanced goniometer equipped with an automatic dispenser. The surface energies of the bare and polymer-coated surfaces polymer were computed from the measured contact angle values. The advancing and receding contact angles were measured with a droplet size of 5µl, increasing the volume in 0.15 µL increments until advancement in the liquid meniscus was observed followed by decreasing the volume with the same rate until the initiation of the receding motion. Advancing contact angles were considered to be the maximum angles observed during the droplet growth, while receding contact angles were calculated from fitting the drop profile just before the interface started receding. Contact angles were taken to be an average of 15 measurements at laboratory level temperature and pressure. ASSOCIATED CONTENT Supporting Information Figure S1: Molecular models and FTIR spectra of polymer coatings. 18 ACS Paragon Plus Environment

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Figure S2: Micro-manipulator set-up and SEM images of tip-less cantilever. Figure S3: Schematic of FIB cut salt particles. Figure S4: Force vs deflection plot of MFP. Figure S5: AFM images of polymer coatings on silicon. Figure S6: Adhesion forces vs polar and apolar surface energies of polymers on steel substrates. Figure S7: SEM images of salt accumulation on bare silicon, and iCVD polymer coatings. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (K.K.G). *E-mail: [email protected] (H.S). FUNDING RESOURCES This work was supported by Kuwait-MIT Center for Natural Resources and the Environment (CNRE), which was funded by Kuwait Foundation for the Advancement of Sciences (KFAS). H.S. and S.K.N are grateful for the start-up fund from the University of Toledo. REFERENCES (1) Najibi, S. H.; Müller-Steinhagen, H.; Jamialahmadi, M. Calcium Sulphate Scale Formation During Subcooled Flow Boiling. Chem. Eng. Sc. 1997, 52 (8), 1265-1284. (2) Karabelas, A. J. Scale Formation in Tubular Heat Exchangers—Research Priorities. International J. of Therm. Scs. 2002, 41 (7), 682-692. (3) Hasson, D.; Drak, A.; Semiat, R. Inception of CaSO4 Scaling on RO Membranes at Various Water Recovery Levels. Desalination 2001, 139 (1), 73-81. (4) Shmulevsky, M.; Li, X.; Shemer, H.; Hasson, D.; Semiat, R. Analysis of the Onset of Calcium Sulfate Scaling on RO Membranes. J. of Membr. Sci. 2017, 524, 299-304.



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(5) Gude, V. G. Chapter 4 - Geothermal Source for Water Desalination—Challenges and Opportunities. In Renewable Energy Powered Desalination Handbook; ButterworthHeinemann: 2018; pp 141-176. (6) Richards, C. S.; Wang, F.; Becker, W. C.; Edwards, M. A. A 21st-Century Perspective on Calcium Carbonate Formation in Potable Water Systems. Environ. Eng. Sci. 2017, 35 (3), 143158. (7) Dash, S.; Rapoport, L.; Varanasi, K. K. Crystallization-Induced Fouling during Boiling: Formation Mechanisms to Mitigation Approaches. Langmuir 2018, 34 (3), 782-788. (8) Amjad, Z. Calcium Sulfate Dihydrate (gypsum) Scale Formation on Heat Exchanger Surfaces: The influence of Scale Inhibitors. J. of Coll. and Int. Sci. 1988, 123 (2), 523-536. (9) Elmabrouk, W. M. M. S. K. In Assessment of Scale Removal by Chemical and Underbalance Mechanical Treatments, International Conference on Industrial Engineering and Operations Management, Rabat, Morocco,, IEOM Society International: Rabat, Morocco, 2017. (10) Al-Sabagh, A. M.; El Basiony, N. M.; Sadeek, S. A.; Migahed, M. A. Scale and Corrosion Inhibition Performance of the Newly Synthesized Anionic Surfactant in Desalination Plants: Experimental, and Theoretical Investigations. Desalination 2018, 437, 45-58. (11) Al Helal, A.; Soames, A.; Gubner, R.; Iglauer, S.; Barifcani, A. Influence of Magnetic Fields on Calcium Carbonate Scaling in Aqueous Solutions at 150°C and 1bar. J. of Coll. and Int. Sci. 2018, 509, 472-484. (12) Younes, A. A.; El-Maghrabi, H. H.; Ali, H. R. Novel Polyacrylamide-Based Solid Scale Inhibitor. J. of Hazar. Mat. 2017, 334, 1-9. (13) Zhao, Q.; Liu, Y.; Wang, S. Surface Modification of Water Treatment Equipment for Reducing CaSO4 Scale Formation. Desalination 2005, 180 (1), 133-138. (14) Wang, C.; Wang, H.; Hu, Y.; Liu, Z.; Lv, C.; Zhu, Y.; Bao, N. Anti-Corrosive and Scale Inhibiting Polymer-Based Functional Coating with Internal and External Regulation of TiO2 Whiskers. Coatings 2018, 8 (1). (15) Zhang, Y.; Yin, H.; Zhang, Q.; Li, Y.; Yao, P.; Huo, H. A Novel Polyaspartic Acid Derivative with Multifunctional Groups for Scale Inhibition Application. Environ. Tech. 2018, 39 (7), 843-850. (16) Masoudi, A.; Irajizad, P.; Farokhnia, N.; Kashyap, V.; Ghasemi, H. Antiscaling Magnetic Slippery Surfaces. ACS Appl. Mater. Inter. 2017, 9 (24), 21025-21033. (17) Subramanyam, S. B.; Azimi, G.; Varanasi, K. K. Designing Lubricant-Impregnated Textured Surfaces to Resist Scale Formation. Adv. Mat. Inter. 2014, 1 (2), 1300068. (18) Richter, R. P.; Brisson, A. QCM-D on Mica for Parallel QCM-DAFM Studies. Langmuir 2004, 20 (11), 4609-4613. (19) Chen, Q.; Xu, S.; Liu, Q.; Masliyah, J.; Xu, Z. QCM-D Study of Nanoparticle Interactions. Adv. in Coll. and Inter. Sci. 2016, 233, 94-114.


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(20) Cao, B.; Stack, A. G.; Steefel, C. I.; DePaolo, D. J.; Lammers, L. N.; Hu, Y. Investigating Calcite Growth Rates using a Quartz Crystal Microbalance with Dissipation (QCM-D). Geochimica et Cosmochimica Acta 2018, 222, 269-283. (21) Fletcher, N. H. Size Effect in Heterogeneous Nucleation. The J. of Chem. Phy. 1958, 29 (3), 572-576. (22) Lebedev, A. L. Kinetics of Gypsum Dissolution in Water. Geochem. Int. 2015, 53 (9), 811824. (23) Russell, K. C. Nucleation in solids: The Induction and Steady State Effects. Adv. in Coll. and Inter. Sci. 1980, 13 (3), 205-318. (24) Diao, Y.; Myerson, A. S.; Hatton, T. A.; Trout, B. L. Surface Design for Controlled Crystallization: The Role of Surface Chemistry and Nanoscale Pores in Heterogeneous Nucleation. Langmuir 2011, 27 (9), 5324-5334. (25) Al-Anezi, K.; Johnson, D. J.; Hilal, N. An Atomic Force Microscope Study of Calcium Carbonate Adhesion to Desalination Process Equipment: Effect of Anti-Scale Agent. Desalination 2008, 220 (1), 359-370. (26) Vazirian, M.; Neville, A., An Investigation Into the Effect of Hydrodynamic Conditions and Surface Characterization on Adhesion/Deposition Processes of Carbonate/Sulphate Scale in the Oil and Gas Industry. In Oilfield Chemistry Symposium Norway, 2015. (27) Reeja‐Jayan, B.; Kovacik, P.; Yang, R.; Sojoudi, H.; Ugur, A.; Kim Do, H.; Petruczok Christy, D.; Wang, X.; Liu, A.; Gleason Karen, K. A Route Towards Sustainability Through Engineered Polymeric Interfaces. Adv. Mater. Inter. 2014, 1 (4), 1400117. (28) Wang, M.; Wang, X.; Moni, P.; Liu, A.; Kim, D. H.; Jo, W. J.; Sojoudi, H.; Gleason, K. K. CVD Polymers for Devices and Device Fabrication. Adv. Mater. 2016, 29 (11), 1604606. (29) Sojoudi, H.; Walsh, M. R.; Gleason, K. K.; McKinley, G. H. Investigation into the Formation and Adhesion of Cyclopentane Hydrates on Mechanically Robust Vapor-Deposited Polymeric Coatings. Langmuir 2015, 31 (22), 6186-6196. (30) Sojoudi, H.; Walsh, M. R.; Gleason, K. K.; McKinley, G. H. Designing Durable VaporDeposited Surfaces for Reduced Hydrate Adhesion. Adv. Mater. Inter. 2015, 2 (6), 1500003n/a. (31) Sojoudi, H.; McKinley, G. H.; Gleason, K. K. Linker-Free Grafting of Fluorinated Polymeric Cross-Linked Network Bilayers for Durable Reduction of Ice Adhesion. Mat. Horizons 2015, 2 (1), 91-99. (32) Sojoudi, H.; Arabnejad, H.; Raiyan, A.; Shirazi, S. A.; McKinley, G. H.; Gleason, K. K. Scalable and Durable Polymeric Icephobic and Hydrate-Phobic Coatings. Soft Matter 2018, 14 (18), 3443-3454. (33) Van Oss, C. J.; Good, R. J.; Chaudhury, M. K. Additive and Nonadditive Surface Tension Components and the Interpretation of Contact Angles. Langmuir 1988, 4 (4), 884-891. (34) Good, R. J. Contact Angle, Wetting, and Adhesion: a Critical Review. J. of Adhe. Sci. and Tech. 1992, 6 (12), 1269-1302. 21 ACS Paragon Plus Environment


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(35) Nemani, S. K.; Annavarapu, R. K.; Mohammadian, B.; Raiyan, A.; Heil, J.; Haque, M. A.; Abdelaal, A.; Sojoudi, H. Surface Modification of Polymers: Methods and Applications. Adv. Mater. Inter. 2018, 5 (24), 1801247. (36) Koos, E.; Willenbacher, N. Capillary Forces in Suspension Rheology. Science 2011, 331 (6019), 897. (37) Kreder, M. J.; Alvarenga, J.; Kim, P.; Aizenberg, J. Design of Anti-Icing Surfaces: Smooth, Textured or Slippery? Nat. Rev. Mater. 2016, 1, 15003. (38) Zellander, A.; Zhao, C.; Kotecha, M.; Gemeinhart, R.; Wardlow, M.; Abiade, J.; Cho, M. Characterization of Pore Structure in Biologically Functional Poly(2-Hydroxyethyl Methacrylate) - Poly(Ethylene Glycol) Diacrylate (PHEMA-PEGDA). PLoS ONE 2014, 9 (5), e96709. (39) Azimi, G.; Cui, Y.; Sabanska, A.; Varanasi, K. K. Scale-Resistant Surfaces: Fundamental Studies of the Effect of Surface Energy on Reducing Scale Formation. Appl. Surf. Sci. 2014, 313, 591-599. (40) Lee, S.; Lee, C.-H. Effect of Operating Conditions on CaSO4 Scale Formation Mechanism in Nanofiltration for Water Softening. Water Research 2000, 34 (15), 3854-3866. (41) Chen, N.; Kim, D. H.; Kovacik, P.; Sojoudi, H.; Wang, M.; Gleason, K. K. Polymer Thin Films and Surface Modification by Chemical Vapor Deposition: Recent Progress. Annual Rev. of Chem. and Biomol. Eng. 2016, 7 (1), 373-393. (42) Chan, K.; Gleason, K. K. Initiated Chemical Vapor Deposition of Linear and Cross-linked Poly(2-hydroxyethyl methacrylate) for Use as Thin-Film Hydrogels. Langmuir 2005, 21 (19), 8930-8939. (43) Sojoudi, H.; Kim, S.; Zhao, H.; Annavarapu, R. K.; Mariappan, D.; Hart, A. J.; McKinley, G. H.; Gleason, K. K. Stable Wettability Control of Nanoporous Microstructures by iCVD Coating of Carbon Nanotubes. ACS Appl. Mater. Inter. 2017, 9 (49), 43287-43299. (44) Kim, S.; Sojoudi, H.; Zhao, H.; Mariappan, D.; McKinley, G. H.; Gleason, K. K.; Hart, A. J. Ultrathin High-Resolution Flexographic Printing Using Nanoporous Stamps. Sci. Adv. 2016, 2 (12), e1601660.


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Table 1: Measured contact angles of three probe liquids on test substrates

Substrate

DI Water

Diiodomethane (DIM)

Ethylene Glycol (EG)

 A (°)

 R (°)

 A (°)

 R (°)

 A (°)

 R (°)

pPFDA

133.2 ± 1.7

100.2 ± 3.1

101.1 ± 2.2

74.2 ± 3.0

104.1 ± 1.4

84.2 ± 3.0

pV4D4

81.3 ± 0.5

52.4 ± 1.0

50.4 ± 0.2

46.6 ± 0.3

56.0 ± 1.0

35.2 ± 1.5

pV3D3

82.7± 0.5

52.1 ± 1.0

50.5± 0.1

44.6 ± 0.4

54.2 ± 0.1

30.2 ± 1.5

pDVB

97.3 ± 0.1

68.7 ± 0.2

49.0 ± 0.8

39.3 ± 0.1

65.0 ± 0.8

50.3 ± 0.6

pHEMA-co-pEGDA

16.2 ± 0.9

NA

61.5 ± 0.9

47.8 ± 0.1

19.7 ± 0.1

NA

Bare Si

35.2 ± 0.7

18.5 ± 4.0

41.8 ± 1.2

28.3 ± 1.5

18.1 ± 2.0

0

Table 2: Surface energy calculated using Van Oss-Chaudhury-Good approach and its components (apolar, polar)33 and RMS roughness of the polymer samples

Substrate

Surface energy (mN/m)

Roughness-Rq (nm)

 total

 LW

 AB

pPFDA

8.3

8.2

0.1

28.3 ± 3.8

pV4D4

34.9

33.0

1.9

1.9 ± 0.7

pV3D3

37.0

33.8

3.2

1.4 ± 0.4

pDVB

35.1

34.6

0.5

0.9 ± 0.2

pHEMA-co-pEGDA

46.6

27.9

18.7

3.2 ± 1.4

Bare Si

48.8

38.3

10.5

NA



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Figure 1

Figure 1: Schematic representation of salt deposit layer (scale) initiation and accretion on solid surfaces. The calcium sulfate-dihydrate crystals accumulate in the flow stream or adhere around heterogenous nuclei (impurities) either in-fluid or on the surface due to the surface roughness. Subsequent mass-gain leads to greater interfacial adhesive forces causing the scales to precipitate and stick to the surface. Both mechanisms can simultaneously contribute to the scale formation. 25 ACS Paragon Plus Environment


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Figure 2

Figure 2: Schematic of a micromanipulator set up used for adhering salt microparticles of calcium sulfate dihydrate onto tip-less cantilevers. The salt particle was adhered with glue to the tip-less cantilever of a MLCT-010 AFM probe mounted on a revolving rail with the help of micromanipulator arms under a high resolution microscopic lens. The position of the salt grain was fine adjusted manually. 26 ACS Paragon Plus Environment

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Figure 3

(a) i

(b)

ii

ii

Figure 3: (a) SEM image of a calcium sulfate dihydrate salt microparticle adhered on to a tip-less cantilever using a micromanipulator set up. The SEM image shows a non-uniform salt microparticle due to random nature of dispersed salt particles resulting in an uneven surface roughness with high peaks and valleys. (b) SEM image acquired after cutting the sides and top face of the microparticle using a Focused Ion Beam (FIB), resulting in a relatively flat salt microparticle on the cantilever enabling greater contact area between the salt particle and the test substrate. 27 ACS Paragon Plus Environment


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Figure 4

(b)

(c)


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Figure 4: Plots for adhesion force vs a) total surface energy and b) ratio of polar and apolar components of surface energy obtained from the molecular force probe (MFP) measurements in wet and dry conditions, averaged on sixteen different spots, on the bare, and the pHEMAco-pEGDA, the pV3D3, the pV4D4, the pDVB, and the pPFDA-coated silicon substrates. c) Schematic of the proposed capillary bridging model for the MFP measurements in wet condition.



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Figure 5

(a)

(b)

(c)

Figure 5: (a) Schematic shows monitoring of mass accumulation due to salt formation/deposition on silicon and polymer-coated quartz crystals using quartz crystal microbalance with dissipation monitoring (QCM-D) set up. (b) Mass gain per unit area over 30 ACS Paragon Plus Environment

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time obtained from the QCM-D measurements. A considerable decrease in rate of mass gain per unit area is observed due to low surface energy polymer coatings on the silicon quartz crystals. (c) Mass gain as a fraction of scale formation per unit area on bare silicon substrates; close to 60 % reduction in scale formation is observed due to smooth and low surface energy polymer coatings.



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Nanoscale Approach for Studying Scale Formation and

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