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Article

How a water drop removes a particle from a hydrophobic surface

Abhinav Naga, Anke Kaltbeitzel, William SY Wong, Lukas Hauer, Hans-Jürgen Butt, Doris Vollmer

2021Soft Matter17(7)1746-1755

Keywordsdrop, wetting

DisciplinesFluid Mechanics, Physics, Surface Chemistry

CategoriesLaboratory experiments, Theoretical modelling

Scale1e-6 to 1e-3 m / 1e-3 to 1 s, 1 to 60 s

Non-expert summaryThe authors use confocal microscopy to directly image how drops (approx. 1 mm diameter) capture and remove particles (approx. 250 µm diameter) from flat hydrophobic surfaces at a range of speeds (10-500 µm/s). The force acting on the drop during the removal process is also measured by imaging the deflection of a flexible cantilever with a known spring constant. At low speeds (50 µm/s), the particle attaches to the drop/air interface due to surface tension, and the particle is successfully removed. At high speeds (500 µm/s), the particle crosses the interface because surface tension is insufficient to overcome the friction force acting on the particle as it rolls on the surface. Consequently, the particle detaches from the drop and remains on the surface. In both cases, the viscous stress acting on the particle is small compared to surface tension and can be neglected when the liquid is water. These results suggest that two main forces govern the removal of particles from surfaces by drops: surface tension of the drop and friction force between the particle and the surface. These findings will help guide the design of self-cleaning surfaces.

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Article

2. Non-chlorine detergent formulations as an alternative for unpasteurised milk removal from stainless steel surfaces

1. Joels Wilson-Nieuwenhuis, Jim Taylour, Luciana C. Gomes, Stephen Lynch, David Whitehead, Kathryn A. Whitehead

20243. Journal of Food Engineering374112031

DisciplineSurface Chemistry

CategoryLaboratory experiments

Non-expert summaryHygiene is a major concern in the dairy industry, and detergents based on hypochlorite have commonly been utilised for cleaning-in-place (CIP) regimes. However, due to concerns about chlorate residues entering the milk processing chain, new detergent alternatives that are free of chlorate sources are required. This work looked at the cleaning efficacy of two new formulations were developed based on ethylenediaminetetraacetic acid (EDTA) and wetting agents. Stainless steel surfaces were fouled with milk and cleaned once or 10 times using water, a caustic-EDTA product, a sodium hypochlorite product, an acid, or new cleaning formulations (Product A and Product B). The results demonstrated that the use of acid did not result in successful cleaning. The two new products performed well, with Product B performing equivalently during cleaning compared with the established hypochlorite or caustic-EDTA products. Product A exhibited better cleanliness than the other detergents tested. When allergen removal was considered, residual material was found to be retained on the surfaces, regardless of the cleaning type used. This study suggests that the new product formulations may be used to replace hypochlorite-based detergents to increase the hygienic status of a surface.

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Article

Experimental investigation of laminar and turbulent displacement of residual oil film

Zhang, Y., Barrouillet, B. and Skadsem, H.J.

2023Scientific Reports1321120

Keywordscleaning, flow displacement, shear flow, thin film

DisciplinesChemical Engineering, Fluid Mechanics, Mechanical Engineering

CategoryLaboratory experiments

Scale1e-6 to 1e-3 m / 1 to 60 s

Non-expert summaryThis paper reports an experimental study of the removal of thin oil films from the wall of a cylindrical tube by a flow of water. The removal of the oil film is measured by monitoring dye concentration in the flushing liquid leaving the system: local extents of removal were not measured. The authors investigated the effect of the flow regime of the displacing fluid (water Reynolds numbers ranged from 1480 to 12690), the duration of the displacing fluid flow, and the estimated wall shear stress imposed on the film. When the volume of fluid is limited, the largest displacement efficiency was observed at Re = 7000-8000 (named the intermediate turbulent regime). The largest Reynolds number flows were most effective for achieving complete cleaning over a long time period.

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Article

Mechanisms of photoresist dissolution

Hunek, B., Cussler, E.L.

2002AIChEJ48(4)661-672

Keywordscleaning, decontamination, mass transfer

DisciplinesChemical Engineering, Chemistry

CategoryLaboratory experiments

Scale1e-6 to 1e-3 m / 1e-3 to 1 s, 1 to 60 s, 1 to 60 min

Non-expert summaryPhotoresists are an example of long chain molecules which dissolve in the presence of alkali. Dissolution involves the transport of hydoxyl ions (OH-) into the layer: they react with the photoresist and, depending on the temperature (above or below the upper critical solution temperature, UCST), it will either be transported by diffusion through the fluid boundary layer (above UCST) or form a gel phase (below the UCST), introducing an additional diffusion step in the reaction and mass transfer processes. A physico-chemical model of the process is presented here, applied to novolak resins, which can be applied to other polymeric systems including printing inks and dairy protein foulants. Experiments are performed with a spinning disk apparatus, which gives well defined mass transfer conditions.

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Article

Self-cleaning of a hydrophobic surface by a rolling water droplet

Hassan, G., Yilbas, B.S., Al-Sharafi, A., Al-Qahtani, H.

2019Scientific Reports95744

Keywordscleaning, drop, flow displacement, wetting

DisciplinesMechanical Engineering, Surface Chemistry

CategoriesLaboratory experiments, Theoretical modelling

Scale1e-6 to 1e-3 m, 1e-3 m, 1e-3 to 1 m / 1e-3 to 1 s, 1 to 60 s

Non-expert summaryThis is an experimental study of removal of dust particles from a hydrophobic surface by a rolling/sliding water droplet. The effect of surface inclination angle on droplet dynamics and dust removal is analyses and compared with a model. Droplet rolling dominates over sliding. Removal is mainly due to the droplet liquid coating the particles as it passes over the particle, and the removal efficiency is determined by the inclination of the surface.

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Article

Transport enhancement mechanisms in open cavities

Horner, M., Metcalf, G., Wiggins, S., Ottino, J.M.

2002J. Fluid Mechanics452199-229

Keywordscavity flow, cleaning, hydrodynamics, mass transfer, shear flow

DisciplinesFluid Mechanics

CategoriesLaboratory experiments, Numerical simulations

Scale1e-3 to 1 m / 1 to 60 s, 1 to 60 min, 1 to 24 hours

Non-expert summaryWhen a steady shear flow passes over a long (2-D) cavity it sets up a circulation cell in the cavity, where the flow in the cavity is contained within the cavity (with a boundary called the separatrix) and mass transport into and out of the cavity is predominantly by diffusion. This is particularly true at small scales. The authors investigate numerically and experimentally two methods for enhancing mass transport from these 'cells' - by geometrically modifying the boundary driving the flow, and making the driving flow time-dependent. Both modifications destabilize one of the wall attachment points of the separatrix, allowing fluid exchange between the cavity and channel. The range of Reynolds and Reynolds-Strouhal numbers studied is 7.7 <= Re <= 46.5 and 0.52 <= ReSr <= 12,55 in the spatially dependent mode and 12 <= Re <= 93 and 0.26 <= ReSr <= 5.02 in the time-dependent mode. The transport is described theoretically via lobe dynamics, which characterizes the instability of the separatrix. They find that the resulting mass transfer between the cavity and the outer shear flow, through the distabilized separatrix is enhanced by several orders of magnitude compared to a diffusive mass transfer.

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Article

Movement of chemical warfare agent simulants through porous media

Jenkins, R.A., Buchanan, M.V., Merriweather, R., Ilgner, R.H., Gayle, T.M., Watson, A.P.

1994Journal of Hazardous Materials37(2)303-325

Keywordschemical warfare agent simulant, decontamination, permeation, porous material, risk

DisciplinesChemical Engineering, Chemistry, Fluid Mechanics

CategoryLaboratory experiments

Scale1e-3 to 1 m / 1 to 60 min, 1 to 24 hours

Non-expert summaryExperimental study of the permeation of CWA simulants through different layers of porous building materials: brick, cinder block, wall board, wood. The breakthrough time is measured in a specially designed cell. Anisotropy of the permeation is observed for anisotropic material such as wood. Surface decontamination is tested and shown effective.

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Article

Washing wedges: capillary instability in a gradient of confinement

Keiser, L., Herbaut, R., Bico, J., Reyssat, E.

2016Journal of Fluid Mechanics790619-633

Keywordscapillary instability, linear stability analysis, multiphase flow

DisciplinesApplied Mathematics, Fluid Mechanics

CategoryLaboratory experiments

Scale1e-3 m, 1e-3 to 1 m / 1 to 60 min

Non-expert summaryExperimental exploration of the exchange of two immiscible fluids in a wedge. Due to antagonist principal curvature, the more wetting fluid exchange with the less wetting fluid in the wedge, through a capillary instability. The instability develops in the form of fingers pinching into droplets. The less wetting fluid is thus displaced out of the wedge. Measurements are performed for varying geometry and viscosity. Discussion but no conclusion of the physical mechanisms are proposed in light of the more well known viscous fingering instability.

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Article

Modelling the cleaning of viscoplastic layers by impinging coherent turbulent water jets

Fernandes, R.R. and Wilson, D.I.

2020Journal of Non-Newtonian Fluid Mechanics282104314

Keywordscleaning, flow displacement, immiscible flow, impinging turbulent jet, viscoplastic fluid

DisciplinesChemical Engineering, Fluid Mechanics

CategoriesLaboratory experiments, Theoretical modelling

Scale1e-6 to 1e-3 m / 1e-3 to 1 s, 1 to 60 s

Non-expert summaryImpinging liquid jets are widely used to clean unwanted soil layers from the walls of structures and vessels. This paper investigates what is observed when a coherent, turbulent, water jet impinges normally on a thin layer of an immiscible viscoplastic material. Removal involves the growth of a cleared area (which is circular for a jet impinging normally) bounded by a berm of displaced material. Previously Glover et al. [2016, J. Food Eng.. 178, 95-109] presented a semi-empirical model relating the rate of removal (location of the berm) to the momentum flow rate in the liquid film. The authors present a first-order model for cleaning thin layers of these materials based on the rate of viscous dissipation in a shallow wedge of material at the cleaning front. This yields a result of the form of the Glover et al. model, with expressions linking the kinetic parameters to measurable quantities including the rheology of the soil. The fully coupled problem is not solved: the wedge angle and residual layer thickness need to be specified and in this work they were obtained by fitting to the data. New and existing experimental results are compared with the model for three soft solids immiscible with water: two petroleum jellies and a soft paraffin, which exhibited Bingham plastic behaviour and creep, for jet Reynolds numbers between 10,000-37,000. The ratio of average film depth and layer thickness was in the range 0.1-1.5.

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Article

Mass transfer from a flat surface to an impinging turbulent jet

Rao, V.V., Trass, O.

1964Canadian Journal Chemical Engineering42(3)95-99

Keywordscleaning, decontamination, impinging turbulent jet, mass transfer

DisciplinesChemical Engineering, Fluid Mechanics

CategoriesLaboratory experiments, Theoretical modelling

Scale1e-6 to 1e-3 m, 1e-3 to 1 m / 1e-3 to 1 s, 1 to 60 s

Non-expert summaryThis is a relatively early experimental study of mass transfer of a sparingly soluble material from a flat, solid surface when it is exposed to a normally impinging turbulent jet of water. The nozzle Reynolds numbers ranged from 25,000 to 125,000. The surface was coated with trans-cinnamic acid, and thickness profiles were measure over time to determine the local rate of mass transfer. The mass transfer flux is used to calculate the local Sherwood number (dimensionless mass transfer coefficient). In the wall-jet region these were found to be independent of the nozzle to plate distance, and were correlated as Sh = 1.3*Re^0.84*(x/d)^- 1.27. The authors found reasonable agreement with published heat transfer data . The average Sherwood numbers in the impingement region were found to decrease rapidly beyond a transition zone of 6.5 diameters from the nozzle: mass transfer rates are thus weak beyond this zone.

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Article

Bubble-driven detachment of bacteria from confined geometries

Khodoparast, S., Kim, M.K., Silpe, J.E., Stone, H.A.

2017Environmental Science & Technology511340-1347

Keywordcleaning

DisciplineFluid Mechanics

CategoryLaboratory experiments

Scale1e-6 to 1e-3 m / 1e-3 to 1 s, 1 to 60 min

Non-expert summarySmall particles may be removed from a surface by the motion of a contact line over the particle or by the passage of a bubble along a channel, which generates a thin film of liquid at the wall of the channel. In both cases the capillary forces induced on the particle can exceed the adhesion force holding the particle on the wall. The origin of this cleaning mechanism is discussed and the removal of bacteria (S. aureus) from the walls of microchannels with square cross section by the passage of bubbles is investigated experimentally. The organisms are not left for long periods to form strong adhesive bonds to the wall.

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Article

Hydrodynamic cleaning of a viscous film from the inside of a long tube

Mickaily, E.S. and Middleman, S.

1993AIChE Journal39(5)885-893

Keywordscleaning, decontamination, immiscible flow, thin film

DisciplinesChemical Engineering, Fluid Mechanics

CategoriesLaboratory experiments, Theoretical modelling

Scale1e-6 to 1e-3 m / 1e-3 to 1 s, 1 to 60 s

Non-expert summaryA simple, dynamic model, supported by experiments, is presented for the thinning (removal) of a viscous liquid film from the inside of the smooth interior surface of a long cylindrical tube. The model is based on the motion of the film generated by the shear stress imposed on it by the turbulent flow of air through the tube. The model gives estimates of the mean thickness of the film (an olive oil and a castor oil). The authors extend the investigation to include removal from roughened surfaces, modelling these as regularly spaced triangular cavities.

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