Langmuir 2002, 18, 4979-4983
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Investigation of Deposition of Monodisperse Particles onto Fibers N. T. Pham,† G. McHale,*,† M. I. Newton,† B. J. Carroll,‡ and S. M. Rowan† Department of Chemistry and Physics, The Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, U.K., and Unilever Research Laboratory, Quarry Road East, Bebington, Merseyside L63 3JW, U.K. Received January 15, 2002 In this work the deposition of particles onto the surface of a textile-type fiber was investigated using a novel experimental system in which thin, bare monofilaments are used as light guides. This method was previously developed as a tool for investigating the deposition of oil droplets from an emulsion, the extent of deposition being measured via changes in the attenuation of light transmitted down the fiber. The present system studies the deposition and subsequent evaporation of a droplet of an aqueous suspension containing 1.9 µm tracer particles. The dynamics of the evaporation of such a droplet from a flat surface are known to involve a build up of deposited particles at the drop periphery to form a ring stain, the rate of deposition increasing during the evaporation process due to a diverging flux. In this report the deposition characteristics on a flat surface are confirmed and a systematic sequence of experiments for the evaporation of droplets of suspensions containing tracer particles from a polyester fiber surface is reported. The experiments use both video-microscopy and the change in attenuation along the fiber caused by droplet deposition which is related to the wetted area. An initial loss is followed by a period where only a slow change in attenuation occurs and finally by a rapid and large change as the droplet completes its evaporation and a large number of particles are deposited. The relationship between the initial loss and wetted area and the final loss and total number of particles deposited is discussed.
Introduction In the detergency process, interest is focused on the study of the removal of soil from textiles. Soil is defined as matter that is introduced unintentionally and may be solid or liquid.1 Its characteristics are that it differs in shape and composition from the fibers of the textile material but is of a comparable size. The origin of soils can be bodily (skin cells, oily secretions, sweat residues, etc.), environmental (sand/clays, carbon, lubricating oils, etc.), or food or drink stains. One difficulty in detergency is to produce standard and reproducible tests. Standard artificial soils and reproducible application techniques do exist, but these tend to involve fabric material rather than the single fibers of which fabrics are ultimately composed. In one commonly used test, standard soiled fabrics are cleaned by agitating them in aqueous solutions containing additives and the change in light reflectivity measured. 2-4 However, a fabric material can be a complex mixture of weaves and types of fibers and this test does not, therefore, produce detailed information on the microscopic process of soil removal, especially in respect of its kinetic aspect. Further, reproducibility can also be a problem. There is therefore a need for the study of the principles of removal of soil from single fibers. An obvious method of studying the single fiber case is to use drops of liquids along with video-microscopy.5,6 However, this requires optical ob* Corresponding author. E-mail:
[email protected]. Tel: +44 (0)115 8483383. Fax: +44 (0)115 8486636. † The Nottingham Trent University. ‡ Unilever Research Laboratory. (1) Carroll, B. J. Colloids Surf., A 1993, 74, 131-167. (2) Tamai, H.; Suzawa, T. Colloid Polym. Sci. 1981, 259, 1100-1104. (3) Tamai, H.; Suzawa, T. J. Colloid Interface Sci. 1982, 88, 372377. (4) Gotoh, K.; Takahashi, E.; Maekawa, M.; Tagawa, M. J. Adhes. Sci. Technol. 1994, 8, 211-222. (5) Song, B.; Bismarck A.; Tahhan R.; Springer J. J. Colloid Interface Sci. 1998, 197, 68-77. (6) Carroll, B. J. Langmuir 1986, 2, 248-250.
servation of the drop-on-fiber system when it is immersed in an aqueous solution and, in addition, the technique may not be sensitive enough as to record deposition processes occurring on the micron to submicron length scales. These considerations motivated Carroll7 to suggest a method of studying the detergency process by using textile fibers as light guides. Textile fibers have refractive indices higher than water, and examination of their cross sections by scanning electron microscope shows that some fibers, such as polyester and Tencel (a synthetic cellulosic material produced by Acordis plc.) (but not cotton), have a smooth uniform appearance and are therefore candidates for light guides. Carroll demonstrated that it was possible to monitor changes occurring at a thick (250 µm) PMMA fiber surface by measuring the attenuation of light guided down the fiber. This idea was developed in a recent report, and an experimental system for studying the soiling process using light guided down a quasi-textile fiber was described.8 One test of the system was to monitor the deposition of oil drops from an emulsion. However, a disadvantage of this study was the complexity that occurs in the deposition due to the range of drop sizes and the ability of deposited drops to coalesce and thereby change their wetted area. This work is an extension of the previous studies to embrace suspensions of solid particles, the size of which can be more readily controlled than that of emulsion droplets. The drying of suspensions containing dispersed solids is important in many other industrial processes, such as coating and printing.9,10 Despite the obvious interest (7) Carroll, B. J. Colloids Surf. 1991, 58, 303-313. (8) McHale, G.; Carroll, B. J.; Pham, N. T.; Newton, M. I.; Rowan, S. M. Measurement of droplet deposition on fibers. In Contact Angle, Wettability and Adhesion; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 2002; Vol. 2, in press. (9) Brinker, C. J.; Scherer, G. W. Sol-gel science: the physics and the chemistry of sol-gel processing; Academic Press: San Diego, CA, 1990; Chapter 1.
10.1021/la020050c CCC: $22.00 © 2002 American Chemical Society Published on Web 05/16/2002
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Pham et al.
Figure 2. Experimental apparatus for monitoring particle deposition onto a textile fiber using the attenuation of transmitted light.
Figure 1. Schematic illustration of the contact line pinning during evaporation. (a) If the contact line is not pinned, the contact line will move from A to B due to uniform evaporation. (b) In the case of contact line pinning, the motion from A to B must be prevented by an outward flow to replenish the liquid removed from the edge.
arising from this wide range of applications, there have been relatively few publications investigating the basic scientific processes of drying drops containing dispersed solid. However, recent work by Deegan et al.11 used tracer particles in an aqueous solution to explain how the evaporation of such drops leaves a ringlike deposit along the perimeter of the original drop. They suggested that such ring stains could have potential applications for writing or depositing a fine pattern onto a surface. Ring stains occur when the contact line between the liquid phase and the solid phase (substrate) becomes pinned for whatever reason (surface roughness, chemical heterogeneity, etc.).12 To maintain contact line pinning during evaporation there has to be in the drying drop an outward flow from the center to the edge to compensate the liquid that is removed from the edge (Figure 1). During such a process, the macroscopic contact angle must change (decrease): it can range between its advancing and receding values. As the contact angle alters, the flow increases in a divergent manner and eventually virtually all the dispersed material is carried to the edge.11,13 Parisse et al. developed a model that describes the shape changes of the suspension droplets during drying.14 Another paper due to Deegan further predicts the flow velocity, the rate of growth of the deposit, and the distribution of solutes within the drop.12 This theory predicts a distinctive powerlaw growth of the ring mass with time. The present experiments involve the deposition of particles of known characteristics (size, shape, and refractive index) and with a well-characterized rate of deposition; removal of soil can be considered as the inverse process to deposition. In this paper the evaporation from a fiber surface of water drops containing tracer particles is investigated. Initial work is focused on determining whether the curvature of the fiber significantly perturbs the mechanism of deposition reported in the literature for evaporation of such drops from flat surfaces. Next, a previously developed experimental system with the fiber as a light guide8 is used to monitor the effect on light transmission of particle deposition. Unlike a commercial optical fiber, this fiber has no cladding, and this makes its surface highly sensitive to change in its dielectric environment induced by change in either the wetted area or by any particle (10) Brinker, C. J.; Hurd, A. J.; Schunk, P. R.; Frye, G. C.; Ashley, C. S. J. Non-Cryst. Solids 1992, 147, 424-436. (11) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827-829. (12) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. Rev. E 2000, 62, 756-765. (13) Deegan, R. D. Phys. Rev. E 2000, 61, 475-485. (14) Parisse, F.; Allain, C. J. Phys. II 1996, 6, 1111-1119.
attachment. The relationship between initial droplet size, concentration of suspension, and the initial and subsequent attenuation in transmitted light is discussed. Experimental Section Charge-stabilized particle suspensions of fluorescent polystyrene latex particles of 1.9 µm diameter (Duke Scientific) were used. The suspension was bought with a concentration of 1% w/w, and lower concentrations were made by dilution with distilled water. Latex particles were used because their refractive index of 1.59 at 589 nm is very close to those of polyester fibers (1.55 radial and 1.71 axial, due to birefringence15). In this case, particles depositing on the surface of the fiber from water introduce large additional losses in the light transmission. The fiber consisted of single filament of polyester of 800 µm diameter and was about 10 cm long and had polished ends for the efficient coupling of light. The light source was a laser diode of 635 nm wavelength, mounted on an x-y translator, and collimating and focusing lenses were used to compensate the high divergence of the beam and to focus the beam onto one end of the fiber. The laser diode was used in pulse mode at a frequency of about 500 Hz using a pulse generator and had a maximum power of 5 mW. The experimental arrangement, shown in Figure 2, has previously been described.8 The light transmitted down the fiber was detected using a photodiode with an internal high-gain low-noise amplifier, and the output of the photodiode was fed to a lock-in amplifier. This type of detection allows small signals at a particular frequency to be identified in a signal containing large amounts of noise. The dc signal provided from the lock-in amplifier represents the light intensity, and this was recorded by a personal computer using an IEEE connection to a digital voltmeter. A second experimental setup was used to investigate the change in wetted area of suspension drops on fibers and flat surfaces during evaporation. This consisted of a vertically mounted microscope with video camera attachment mounted on an x-y-z translator and connected to a VCR and monitor. Deposition of droplets of small volume onto the fiber, mounted horizontally, was made using a 0.5 µL syringe. After the experimental recordings, images were captured onto a personal computer using a Data Translations DT3152 scientific framegrabber. For each magnification used, images of a scale were captured for calibration purposes and to check for optical distortions. This system was capable of recording 25 frames/s with image resolutions of 769 × 576 and a 1:1 aspect ratio. To measure the surface area of a fiber covered by a droplet, a computer program was written to include a scaling factor to compensate for the curvature of the fiber surface. The length of a curve across the fiber in a direction perpendicular to the fiber axis is a factor of [1 - (x/rf)2]-1/2 greater than the planar projected length from the image, x in this formula being the coordinate across the fiber centered along the fiber axis and rf the fiber radius. The buildup of ringlike deposits during evaporation could be followed from the images taken during any one experiment. A limitation of this system was that simultaneous video-microscopy and measurement of transmitted light intensity by the fiber was not possible. Estimates of initial wetted area for the experiment using the light guiding system were therefore based on the known drop volume deposited and the separate calibration using the video microscope system of the resulting wetted area for that particular fiber. (15) Greaves, P. H.; Saville, B. P. Microscopy of textile fibers, 1st ed.; BIOS Scientific Publishers Ltd: Oxford, U.K., 1995; Chapter 2.
Deposition of Monodisperse Particles onto Fibers
Figure 3. Creation of a ring stain by the evaporation of a droplet containing a suspension of particles.
Figure 4. Power-law growth of the width, wr, of the rim of a ring stain in numbers of particles: (a) 0.5 µL suspension drop on flat polyester surface; (b) 0.1 µL suspension drop on a polyester fiber.
Results and Discussion The aim of the first set of experiments was to find out whether the deposition of tracer particles onto fibers via the evaporation of a droplet shows characteristics similar to those seen when a droplet evaporates from a flat surface. A small (
