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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Attachment and Detachment of Particles from a Surface under Shear Flow Jana Schwarze Analytical ChemistryBiointerfaces, Ruhr-University Bochum, 44801 Bochum, Germany
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Max-Planck-Institut für medizinische Forschung, 69120 Heidelberg, Germany
Markus Karahka and H.J. Kreuzer* Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada ABSTRACT: In a recent paper, we described a quantitative analysis of the detachment kinetics of beads adsorbed in a microfluidic system under laminar shear flow. In this paper, we extend this formalism to the kinetics of attachment with concurrent detachment under increasing shear flow, ramped up in a controlled time dependence. The motivation for this work is to provide a theoretical framework or tool that helps experimentalists to extract relevant parameters such as attachment and detachment rates. We also present a graphical method to simplify the extraction of the pre-exponential and activation energies for the attachment and detachment process.
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INTRODUCTION
an atomic force microscope. The normal force scales with the contact area and depends on surface roughness (which may reduce the contact area) as found in lift-off experiments of micrometer-sized beads by Schaefer et al.3 However, under shear flow detachment (desorption) of attached spherical (or close to spherical) particles or cells into the fluid is normally preceded by rolling along the surface,4−6 and for flat or spread individual cells or cell assemblies by a peeling process,7 i.e. as applied to remove scotch tape from a surface. Sliding of the object along the surface is observed when the forces acting over the projected area are homogeneous (or the object is pushed with a force probe). Rolling of micrometer-sized spheres, as is the case in this study, is induced by the parabolic flow profile8 near the walls in the microfluidic channel. This laminar flow profile in the direct vicinity of the surface creates a torque on round objects of sufficiently large size. We note that in the rolling and peeling detachment scenarios, only a fraction of bonds between the cell and the surface have to
Many experiments in biomedical research aim to determine the number of cells or organisms adsorbing on a surface. To discriminate between physisorbed (reversibly adsorbed) and settled cells (irreversible attached), a rinsing step is applied when the sample is removed from solution to count the strongly adhering cells.1 However, even when a calibrated shear flow is applied in the rinsing step, no information is obtained about the threshold energy needed to overcome the activation barrier for detachment, nor the rate of removal. The question about the energy barrier and kinetics for removal of cells and organisms arises in biomedical experiments, but even more so when investigating and optimizing surface coatings to reduce biofouling in marine environments.2 In this work, we discuss the formal kinetic background and present a method to extract the relevant kinetic parameters (activation energies and pre-exponentials) for the attachment and detachment process from experiments. The focus of our work is to develop a protocol to quantitatively compare different surfaces for their biofouling properties by formulating the kinetic equations for the rate-determining step of attachment and detachment. The adhesion energy of particles, cells, and organisms is related to the normal force needed to break all bonds to the substrate, which can be measured e.g. by a force probe such as © XXXX American Chemical Society
Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift Received: August 23, 2018 Revised: December 5, 2018
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DOI: 10.1021/acs.jpcc.8b08215 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C break at a given time, i.e. the initial activation barrier to initiate detachment is much lower than the adhesion energy which has to be overcome for removal normal to the surface.9 In our previous paper,9 we developed the theory to determine the activation energy of detachment of carboxylate coated styrene beads (diameter 4.5 μm) from chemically different (hydrophilic, hydrophobic, and charged) selfassembled monolayers of different thickness (i.e., different hydrocarbon chain length) adsorbed on gold surfaces. These previously reported experiments were done by saturating the surface with beads and then increasing the shear flow using water or buffer to measure the onset and then rate of detachment until all beads were removed. The important conclusion of this work was that characterizing bead detachment by the shear stress at which half the beads have been removed (i.e., the maximum in the integrated detachment curve) has no relation to the activation energy of detachment. It can only be a measure of the relative difference in the effect of surface chemistry on the detachment process. We further showed that a statistical data analysis is required to extract physically meaningful parameters. Our results confirmed that detachment of the beads occurs via a rolling motion, where the activation barrier to initiate rolling is due to dry static friction. In this paper, we describe and analyze experiments under increasing and decreasing shear flow, i.e. over a certain range of shear force where the actual surface coverage is determined by both attachment and detachment rates. This scenario, adsorption and removal in a flowing solution of cells and bacteria, is encountered in a real biofouling environment, i.e. on the hull of a moving ship. As compared to our previous work, we here include the attachment rate from the solution to determine the time and shear force dependent coverage. To delineate the scope of this work, we stress again that the aim is not to understand the fluid dynamics of beads in a shear flow but to provide a tool to analyze and compare the rate determining processes in biofouling.
SAM was determined by fitting the data as a single organic layer with a wavelength depending refractive index described by a Cauchy model (A = 1.45, B = 0.01, C = 0). Thicknesses were measured on 9 spots on the surface, and through use of a custom-built contact angle goniometer, a water contact angle of 26 ± 4° was obtained. Polystyrene Microsphere Solution. For the experiments, Polybead carboxylate coated microspheres of 4.5 μm diameter (2.5% (w/v) aqueous suspension) from Polysciences Inc. (Mannheim, Germany) were used as in our previous experiments. Twelve milligrams of the polystyrene microsphere suspension was dispersed in 10 mL of PBS buffer (pH 7.4) (Sigma-Aldrich, Munich, Germany) solution. The polystyrene microspheres were washed three times with PBS buffer by centrifugation (4.5 min at a rcf of 1920, Hettich Zentrifugen, Lauenau, Germany) and resuspended again in PBS buffer. The microsphere concentration was adjusted to 1 × 107 beads mL−1 using OD250 in a Cary 60 spectrophotometer (Agilent Technologies, Santa Clara, California, United States). Microfluidic Experiments. The setup we used in this study is the same as described previously.9 The microfluidic channels were constructed by attaching glue-on channels (IBIDI sticky slides 0.1, IBIDI, Planegg/Martinsried, Germany) onto the SAMs, resulting in a channel height of approximately 150 μm and a channel width of 5 mm. To improve sealing, a custom-built metal bracket was used. A broad flat velocity profile is formed across the channel due to the Purday approximation (channel height h ≪ channel width w), while a Poiseuille profile is established in the vertical direction. The wall shear stress and the Reynolds number can be calculated as12
MATERIALS AND METHODS Surface Preparation of the Shear Channel Substrate. 11-Hydroxyundecanethiol (HUDT HS-(CH2)11-OH) was used for self-assembling monolayer (SAM) formation and purchased from ProChimia Surfaces (Sopot, Poland). Extra smooth float glass slides (Nexterion BR, Schott, Germany) coated with a 5 nm titanium adhesion promoter followed by 30 nm gold film (99.9% purity) (Georg Albert, PVD-Beschichtungen, Silz, Germany) were used for SAM preparation. The samples were stored under argon until use. Before monolayer assembly, the gold surfaces were cleaned under UV light for 1 h, rinsed with ethanol p.a. for 3 min, followed by drying in a nitrogen stream. For SAM preparation, the gold substrates were immersed in 1 mM solution of HUDT in ethanol for 24 h. After immersion, the SAM surfaces were rinsed with ethanol, sonicated for 3 min, and blown dry with nitrogen. Storage took place under argon until use.10 Surface Characterization. Thickness determination of the SAM was done by using a M-2000 multiple wavelength (200− 1000 nm) spectroscopic ellipsometer (J. A. Woollam Co. by LOT Quantum Design GmbH, Darmstadt, Germany). Measurements were done at three angles: 65°, 70°, and 75° relative to the surface normal. The thickness of the gold substrates used as reference before SAM assembling were analyzed with a B-spline model.11 The layer thickness of the
where τw is the wall shear stress, h is the channel height, w the channel width, and Qpp is the volumetric flow of a liquid with a density ρ and a dynamic viscosity η.7 For Reynolds numbers
