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Invited Feature Article
Surface Forces and Interaction Mechanisms of Emulsion Drops and Gas Bubbles in Complex Fluids Lei Xie, Chen Shi, Xin Cui, and Hongbo Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04669 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017
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Invited Feature Article
Surface Forces and Interaction Mechanisms of Emulsion Drops and Gas Bubbles in Complex Fluids Lei Xie, Chen Shi, Xin Cui, Hongbo Zeng* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada *Email:
[email protected], Phone: +1-780-492-1044, Fax: +1-780-492-2881
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ABSTRACT The interactions of emulsion drops and gas bubbles in complex fluids play important roles in a wide range of biological and technological applications, such as programmable drug and gene delivery, formation of emulsions and foams, and froth flotation of mineral particles. In this feature article, we have reviewed our recent progress on the quantification of surface forces and interaction mechanisms of gas bubbles and emulsion drops in different material systems by using several complementary techniques, including drop/bubble probe atomic force microscope (AFM), surface forces apparatus (SFA) and four-roll mill fluidic device. These material systems include bubble-self-assembled monolayer (SAM), bubble-polymer, bubble-superhydrophobic surface, bubble-mineral, water-in-oil and oil-in-water emulsions with interface-active components in oil production, as well as oil/water wetting on polyelectrolyte surfaces. The bubble probe AFM combined with reflection interference contrast microscopy (RICM) was applied for the first time to simultaneously quantify the interaction forces and spatiotemporal evolution of confined thin liquid film between gas bubbles and solid surfaces with varying hydrophobicity. The nanomechanical results have provided useful insights into the fundamental interaction mechanisms (e.g. hydrophobic interaction in aqueous media) at gas/water/solid interfaces, the stabilization/destabilization mechanisms of emulsion drops, as well as oil/water wetting mechanisms on solid surfaces. A long-range hydrophilic attraction was found between water and polyelectrolyte surfaces in oil, with the strongest attraction for polyzwitterions, contributing to their superior water wettability in oil and self-cleaning capability of oil contamination. Some remaining challenges and future research directions are discussed and provided.
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1. INTRODUCTION Liquid drops and gas bubbles are essential components in a wide range of biological and technological applications such as microfluidic devices,1-2 programmable drug and gene delivery,3-4 and formation of emulsions and foams.5-8 In these applications, the interactions of deformable drops and bubbles play critical roles in achieving desired characteristics and functionalities, attracting tremendous research interest from both fundamental and practical viewpoints. For example, the stimuli-responsive modulation of interactions between gas bubbles and loaded drug/gene is a critical determinant of effective delivery and release of molecular cargo from the delivery vehicle (i.e. gas bubbles). In emulsion systems, interface-active components (e.g. surfactants, proteins, polymers, nanoparticles) are commonly applied to modify the interfacial properties and interactions of emulsion drops, which ultimately determine the stability and behaviors of the emulsions. Therefore, it is of both fundamental and practical importance to understand the essential physics associated with the surface interactions involving drops and bubbles. Nevertheless, the drops and bubbles can readily change their shapes in response to external forces or pressures, which causes experimental and theoretical challenges to precisely and synchronously quantify the forces and thin film drainage process during their interactions. Over the last few decades, considerable effort has been made on characterizing the interactions of emulsion drops through macroscopic and microscopic visualization using techniques such as micropipette and four-roll mill fluidic device.9-13 The thin film drainage process for a millimeterscale bubble approaching a transparent solid substrate in aqueous medium was monitored by visualizing the interference patterns using optical interferometry to obtain the profiles of the confined water film.14-15 A theoretical model considering bubble deformation was applied to reveal
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the dynamic drainage process of thin water film.16 The drainage dynamics of thin liquid film confined between two drops were also measured by similar method.17 In another similar approach, thin film pressure balance was developed to simultaneously measure the film thickness profile and disjoining pressure between two gas bubbles.18-19 Surface forces apparatus (SFA), widely used for measuring physical forces as a function of absolute separation between two curved surfaces in complex fluids,20-23 was modified to directly study the evolution of thin film profiles between flat solid surfaces and deformable objects including mercury drop, oil drop and gas bubble.24-26 These reported methodologies and studies have significantly advanced the understanding of thin film drainage process during interactions of deformable drops and bubbles. Atomic force microscope (AFM) as a very useful nanomechanical technique has been extensively used to quantitatively measure the molecular and surface forces.27-31 The measurements of deformable drops and bubbles were far more complicated than that of solid surfaces due to the practical challenge of manipulating deformable objects and precise interpretation of the force results.32-33 The interactions between a drop/bubble and a solid surface have been measured using AFM colloidal probe and drop/bubble probe techniques.32-39 It was found that repulsive van der Waals (VDW) interaction could sustain thin water films between bubbles and hydrophilic substrates (e.g. mica, gold, silica), while attractive hydrophobic interaction could induce film rupture and bubble attachment on hydrophobic surfaces.32-39 The addition of surfactants was also found to effectively impede the thinning process of confined water film.32-35 For the colloidal probe technique, interaction forces were measured by driving a cantilever-attached solid particle toward an immobilized drop/bubble, which in certain cases limited its application due to the nonspherical feature of many natural materials (e.g. mica, molybdenite, graphite) and the infeasibility of force measurements between two deformable drops or bubbles.32-36 On the other
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hand, the drop/bubble probe technique, in which a drop/bubble was anchored on a tipless cantilever, enabled the measurements of interactions involving almost all kinds of drops/bubbles in different configurations.37-42 For all the AFM approaches, the absolute separation between the interacting surfaces was predicted through theoretical calculations.32-42 Simultaneous measurements of the interaction force and spatiotemporal evolution of confined thin liquid film remained a particular challenge. In this work, we have reviewed our recent progress in understanding the surface interaction mechanisms of various material systems involving deformable emulsion drops and gas bubbles by using several complementary techniques including drop/bubble probe AFM, SFA and fourroll mill fluidic device. We first review the studies on the hydrophobic interaction in asymmetric gas/water/solid systems. The bubble probe AFM combined with reflection interference contrast microscopy (RICM) was applied for the first time to simultaneously measure the interaction forces and thin film profiles between gas bubbles and solid surfaces of varying hydrophobicity. Next, the interactions between gas bubbles and other hydrophobic surfaces such as polymer films and superhydrophobic surfaces, and bubble-mineral interaction mechanisms in mineral flotation systems are discussed. The influence of the presence of nanobubbles on interactions between hydrophobic solid surfaces in aqueous solutions is also shown. Afterwards, the behaviors and interactions (e.g. stabilization mechanism) of water-in-oil and oil-in-water emulsion drops, in the presence and absence interface-active chemicals, and their interactions with various solid surfaces are reviewed. Our very recent findings on an unexpected long-range hydrophilic attraction between water and polyelectrolyte surfaces in oil are reviewed and the wetting mechanisms of polyelectrolyte surfaces are discussed. Some existing challenges and future perspectives are also discussed and provided. Our results provide useful insights into the basic
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understanding of surface forces and interaction mechanisms of deformable emulsion drops and gas bubbles in complex fluids, with implications in a variety of biological and engineering systems.
2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Drop/Bubble Probe AFM. The force measurements were conducted using an MFP-3D AFM (Asylum Research, Santa Barbara, CA) mounted on an inverted optical microscope (Nikon Ti-U). Custom-made rectangular AFM tipless cantilevers (400 × 70 × 2 μm) with a circular gold patch (diameter ~65 μm, thickness ~30 nm) at one end were used. Prior to the force measurements, the gold patch on the AFM cantilever was strongly hydrophobized by immersing in 10 mM dodecanethiol in absolute ethanol overnight for stable anchoring of hydrophobic oil drop or gas bubble. The glass substrate of an AFM fluid cell (diameter ~70 mm) was mildly hydrophobized by immersing in 10 mM octadecyltrichlorosilane (OTS) in toluene for ~10 s to give a water contact angle of 40-60°for immobilization of oil drop or gas bubble. Oil drops were generated on the glass disk of the fluid cell through a controlled de-wetting method,40 and gas bubbles were immobilized by carefully purging air in aqueous solution through a custom-made ultra-sharp glass pipette.39 For the preparation of water drop probe, water drops were injected into organic solvent and spontaneously settled down on the glass substrate. The glass substrate was strongly hydrophobized to facilitate easy lifting of water drop, while AFM cantilever was mildly hydrophobized for picking up the water drop to generate a water drop probe.43 The spring constant of the cantilever was determined using the Hutter and Bechhoefer method before loading, with a typical value to be 0.3-0.4 N/m.44
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A schematic of preparing a bubble probe and typical experimental setup for force measurements using the bubble probe is illustrated in Figure 1. A bubble probe was prepared by lowering the cantilever toward a gas bubble of suitable size (typical radius R0 = 60-90 μm) (Figure 1A) until bubble attachment and then lifting the cantilever to detach the bubble from the substrate (Figure 1B). The bubble probe was then moved laterally and positioned over a selected solid surface (Figure 1C) or carefully aligned with another bubble (Figure 1D). During the force measurements, the bubble probe was driven downward until a desired cantilever deflection was reached or bubble attachment occurred, whereupon it was then driven away from the surface. The force measurements for oil or water drops were conducted by following the same procedure above. The displacement of the cantilever and measured force data were recorded as a function of time by AFM software, which were used in the theoretical model for further analysis.
A
B
C
D
Figure 1. Schematic of preparing a bubble probe and typical experimental setup for force measurements using the bubble probe. (A) Lower the cantilever toward a gas bubble of suitable
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size until bubble attachment. (B) Lift the bubble probe from the substrate. (C) Move the bubble probe laterally and position it over a selected solid surface. (D) Align the bubble probe with another bubble.
2.2. Theoretical Model. Figure 2 illustrates the schematic of the geometry between two bubbles and between a bubble and a solid surface. The measured forces were analyzed using a Stokes-Reynolds-Young-Laplace model.37, 45 The tangentially immobile boundary conditions are assumed at the gas/water, oil/water and solid/water interfaces during the interactions, and thus the drainage process of the thin water film confined between the probe bubble and the immobilized bubble or surface can be described by Reynolds lubrication theory, as shown in equation 1.
h(r , t ) 1 p(r , t ) (rh3 (r , t ) ) t 12 r r
(1)
where h(r,t) is the water film thickness, μ is the viscosity of water, r is the radical coordinate, and p(r,t) is the excessive hydrodynamic pressure in the thin water film relative to the bulk liquid.
Figure 2. Schematic of force measurements between (left) two bubbles or (right) a bubble and a solid surface.
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Different from rigid particles, drops/bubbles can readily change their shapes in response to external forces. The deformation of the bubble during the interactions can be described by the augmented Young-Laplace equation (equation 2). n h(r , t ) 2 p(r , t ) [h(r , t )] r 2r r r R
(2)
where 𝜎 is the interfacial tension, R is the equivalent radius of the interacting bubble ( R 2( R011 R02 1 )1 for bubble-bubble interaction and R R0 for bubble-surface interaction), and the parameter n = 1 for bubble-bubble interaction and n = 2 for bubble-surface interaction. Π[h(r,t)] denotes the disjoining pressure arising from surface interactions such as van der Waals (VDW), electrical double layer (EDL), and hydrophobic interactions. The contribution of VDW interaction to the overall disjoining pressure could be calculated by equation 3.
VDW [h(r , t )]
AH 6 h3 (r , t )
(3)
where AH is the non-retarded Hamaker constant. The EDL interactions in the symmetric bubblebubble and asymmetric bubble-surface systems can be calculated using Equations 4 and 5, respectively. eb ) exp[ h(r , t )] 4kBT
(4)
2 0 2 [(e h e hbs (b2 s2 )] EDL [h(r , t )] (e h e h ) 2
(5)
EDL [h(r , t )] 64kBT 0 tanh 2 (
where kB is the Boltzmann constant, φb and φs are the surface potentials of the bubble and the solid surface, respectively, ρ0 is the number density of ions in aqueous solution, e is the electron charge, ε0 is the vacuum permittivity, ε is the dielectric constant of the medium, and κ, the reciprocal value of Debye length, can be given as (20e2 / 0 kBT )1/2 .
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The overall interaction force F(t) can be calculated as the integral of hydrodynamic pressure p(r,t) and disjoining pressure Π(h(r,t)) using the Derjaguin approximation as described in equation 6.
F (t ) 2 [ p(r , t ) (h(r , t ))]rdr 0
(6)
Due to the spherical geometry of the bubble, the initial thickness of the confined water film can be described by equation 7.
r2 h (0, t ) , 0 R h( r , t0 ) 2 h(0, t ) r , 0 2R
bubble-bubble interaction (7)
bubble-surface interaction
Other boundary conditions include h / r 0, p / r 0 at r 0 due to the axial symmetry, and r (p / r ) 4 p 0 at r rmax because p(r,t) decays as r-4 at r . By incorporating the position of the cantilever X(t) and assuming the constant bubble volume during the interactions, equations 8 and 9 elucidate the boundary conditions regarding the movement and deformation of the bubble with respect to time for bubble-bubble and bubble-surface interactions, respectively, where θ is the contact angle of the immobilized bubble, and K is the spring constant of the tipless cantilever.
rmax 1 1 cos 2 rmax h(rmax , t ) dX (t ) dF (t ) 2 1 1 cos 1 2 ln ln (8) ln ln t dt 2 dt K 2 1 cos 1 2 R01 2 1 cos 2 2 R02 rmax h(rmax , t ) dX (t ) dF (t ) 2 1 1 cos 1 ln ln t dt 2 dt K 2 1 cos 2 R0
(9)
The equations mentioned above are non-dimensionalized with the scaling parameters hc R0Ca1/2 , rc R0Ca1/4 , pc / R0 , tc Ca 1/2 / pc , and Ca V / , and then solved
numerically using MATLAB software.
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3. HYDROPHOBIC INTERACTIONS AT GAS/WATER/SOLID INTERFACES Hydrophobic interaction generally refers to the unusually strong attraction between hydrophobic objects in water, which is ubiquitous in water-based biological and technological systems, such as self-assembly of biomolecules,46-47 self-cleaning of superhydrophobic surfaces,48-49 and froth flotation of mineral particles.50-51 Hydrophobic interaction is commonly considered to be originated from the re-orientation of water molecules adjacent to the hydrophobic moieties in compensation for the loss of hydrogen bonding network.47 Despite much research on the range and magnitude of hydrophobic interaction between various solid surfaces,23, 52-53 the quantitative understanding of that at oil/water and gas/water interfaces still remains incomplete. In the approach using colloidal probe AFM, hydrophobic particles were found to readily attach onto the substrate-supported bubble, attributed to the attractive hydrophobic interaction.34-35 However, quantifying the force-separation profile of the involved hydrophobic interaction was elusive since the bubble deformation rendered it challenging to determine the absolute separation between the interfaces. 3.1 Bubble-Self-Assembled Monolayer (SAM) Interaction. To help quantitatively understand the hydrophobic interaction between gas bubbles and solid surfaces, we have examined the OTS SAM on mica surfaces with systematically varied hydrophobicity, as reported previously.54 The bubble/drop probe AFM coupled with RICM (Figure 3A) enabled simultaneous measurements of the interaction forces and spatiotemporal evolution of thin film drainage process involving deformable surfaces. The interaction of an air bubble on a bare hydrophilic mica in 500 mM NaCl was the simplest case to start with since the repulsive VDW interaction was the only surface force present in this system (note the EDL interaction was significantly suppressed under high salinity condition). The force curve in Figure 3B showed a
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typical approach-separation cycle, indicating the absence of bubble attachment. By analyzing the RICM patterns in Figure 3C, a stable thin water film with the minimum thickness of 7 nm was found to be confined within the flattened central region between the bubble and mica surface. For the hydrophobic surfaces, in contrast, different bubble attachment behaviors were observed, with stronger “jump-in” behavior for the surface with higher hydrophobicity (Figure 3D). The inverted optical microscopy and the evolution of interference patterns also confirmed the bubble attachment. The theoretically predicted results (red curve) were in excellent agreement with experimental results (open symbols) in Figure 3E, revealing the essential physics underlying the interaction mechanisms in gas/water/solid system were successfully elucidated. Based on thermodynamic considerations, we developed an exponential equation (equation 10) to describe the disjoining pressure contributed from hydrophobic interaction in the asymmetric gas/water/solid system.54
H [h(r, t )] [ (1 cos W ) / DH ]exp(h(r, t ) / DH )
(10)
The decay length DH was found to be 0.8-1.0 nm depending on the solid surface hydrophobicity for the gas/water/solid asymmetric case, close to that in solid/water/solid system but much higher than that reported for oil/water/oil system (DH ~0.3 nm).53-57 The discrepancy in the DH values could originate from the difference in the arrangement of water molecules at solid/water interface and oil/water or gas/water interface.53-56 The hydrophobic solid surface is incapable of forming hydrogen bonds with adjacent water molecules, thereby disrupting the hydrogen bonding network of interfacial water molecules. The deformable oil/water or gas/water interface undergoes continuous thermal fluctuations, which minimize the disturbing impact on the hydrogen bonding network of interfacial water molecules. Previous results on the structure of interfacial water molecules measured using sum frequency generation spectroscopy (SFG) have
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shown that the water molecules adjacent to hydrophobic solids exhibit ice-like structure, while the water molecules adjacent to hydrophobic oil or gas are less ordered.58-60 To elucidate the nature of hydrophobic interaction at different interfaces, further research could be conducted to characterize the interaction forces and corresponding water structure at a broad range of oil/water and gas/water interfaces by coupling nanomechanical techniques (e.g. AFM) with novel spectroscopic tools as well as accurate molecular simulations.
Figure 3. (A) Experimental setup of AFM coupled with RICM for synchronous measurements of interaction forces and spatiotemporal evolution of thin film drainage process. (B) Time variation of the force between an air bubble and a bare mica surface in 500 mM NaCl at driving velocity of v = 1 μm/s. (C) Water film profile at point D in panel B. (D and E) Time variations of the force and water film profile between a bubble and a hydrophobized mica surface (with water contact angle θW = 90°) in 500 mM NaCl at v = 1 μm/s. The open symbols are experimental results based on AFM-RICM measurements and the solid curves are theoretical calculations. Reproduced from Ref. 54. Copyright 2015 American Chemical Society.
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3.2 Bubble-Polymer Interaction. Inherently hydrophobic polymers have a broad range of industrial and biological applications.61-62 To date, as compared to other hydrophobic surfaces, less effort has been made to elucidate the hydrophobic interaction mechanism for the gas/water/polymer system.
Figure 4. Interaction forces between a bubble and a polystyrene surface in (A) 1000 mM (bubble radius R0=60 μm), (B) 500 mM (R0=68 μm), and (C) 100 mM NaCl (R0=77 μm), at v = 1 μm/s. The open symbols are experimental results and the solid curves are theoretical calculations. AFM images (1 × 1 μm2) of polystyrene surface in (D) 1000 mM, (E) 500 mM, and (F) 100 mM NaCl. Reproduced from Ref. 63. Copyright 2016 American Chemical Society.
We have prepared polydimethylsiloxane (PDMS) surfaces with water contact angle θW = 90° for the force measurements with bubble probe AFM. Bubble attachment was readily observed on PDMS surface, and the DH value of hydrophobic interaction was found to be ~1.1
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nm, in consistency with the mica-OTS system. The DH value and hydrophobic interaction were also found to be independent on salt concentration. While on the polystyrene surface (θW = 90°), it was interesting to observe different bubble attachment behaviors with varying salt concentration.63 The force curves measured in 1000 mM NaCl (Figure 4A) and 500 mM NaCl (Figure 4B) both showed the bubble attachment, and the DH value was fitted to be ~0.75 nm. Nevertheless, bubble attachment was surprisingly inhibited in 100 mM NaCl (Figure 4C). In situ AFM imaging in aqueous solutions demonstrated the presence of nanobubbles on PS surfaces, with lower coverage measured at higher salt concentration (Figure 4D-F). Since the interaction between two bubbles measured in 100 mM NaCl was found to be purely repulsive, the densely distributed nanobubbles observed under the same solution condition (Figure 4F) could prevent the direct contact of the AFM bubble probe with the pristine polystyrene surface, which stabilized the confined thin water film and inhibited the bubble attachment. The evolution of interfacial nanobubbles with salt concentration was likely due to the increased air solubility in water at lower salinity that facilitated the generation of nanobubbles.64 The hydrophobic interaction of gas/water/polymer systems exhibited typical DH values ranging from 0.75 to 1.1 nm, which is comparable to that for the cases of hydrophobized mica surfaces. With similar water contact angle in air for PDMS and polystyrene surfaces, different DH values have been found for interactions between bubble and the two polymers, revealing the strength of hydrophobic interaction cannot be simply characterized by surface wettability. Furthermore, the formation of nanobubbles on polystyrene surfaces and the associated ion effects on the bubble attachment suggested the in situ surface morphology could significantly influence the force measurements. Further investigation is needed to elucidate the correlation among
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hydrophobic interaction, gas solubility in water and hydrogen bonding network of water molecules at gas/water/solid interfaces under the influence of ion specificity. It is worth mentioning that our previous studies using SFA also demonstrated the influence of dissolved gases and presence of micro- and nanobubbles on the interactions between two polystyrene surfaces (see Figure 5A for the experimental setup).64 It was found that spontaneous cavitation of water confined between two interacting polystyrene surfaces could occur in degassed aqueous solution at a separation of ≤20 nm, as evident from the real-time fringes of equal chromatic order (FECO) using the multiple beam interferometry technique (Figure 5B).64 Based on the SFA results, a three-regime interaction model was proposed for interactions between hydrophobic surfaces (illustrated in Figure 5C): (I) a long-range interaction regime (tens of nm to hundreds of nm) due to the bridging of microscopic and submicroscopic bubbles (or the possible electrostatic attraction induced by the monolayer overturning), (II) an intermediate interaction regime (several nm to 20 nm) due to the bridging of nanobubbles or enhanced Hamaker constant associated with enhanced proton hopping in water (or spontaneous cavitation in degassed aqueous solution), and (III) a short-range interaction regime (
