response to “comments on the use of rheometers with rough surfaces or surfaces with protrusions”
TRANSCRIPT
Response to “Comments on the use of rheometers with rough surfaces or surfaceswith protrusions”Charles S. Nickerson and Julia A. Kornfield
Citation: Journal of Rheology (1978-present) 49, 1157 (2005); doi: 10.1122/1.1993597 View online: http://dx.doi.org/10.1122/1.1993597 View Table of Contents: http://scitation.aip.org/content/sor/journal/jor2/49/5?ver=pdfcov Published by the The Society of Rheology Articles you may be interested in Contact line dynamics on heterogeneous surfaces Phys. Fluids 23, 072103 (2011); 10.1063/1.3609817 Numerical investigation of the reduction of wall-slip effects for yield stress fluids in a double concentric cylinderrheometer with slotted rotor J. Rheol. 54, 1267 (2010); 10.1122/1.3484955 The effective slip length and vortex formation in laminar flow over a rough surface Phys. Fluids 21, 052105 (2009); 10.1063/1.3121305 Rheological study of polymer flow past rough surfaces with slip boundary conditions J. Chem. Phys. 129, 144902 (2008); 10.1063/1.2988496 Letter to the editor: Comments on the use of rheometers with rough surfaces or surfaces with protrusions J. Rheol. 49, 1153 (2005); 10.1122/1.1993596
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Response to “Comments on the use of rheometers withrough surfaces or surfaces with protrusions”
�DOI: 10.1122/1.1993597�
We would like to thank Dr. Kalyon for his thoughtful letter to the editor, “Commentson the use of rheometers with rough surfaces or surfaces with protrusions,” written inresponse to our “cleated” tool geometry. We fully agree with him regarding the impor-tance of understanding wall slip. As explained in Dr. Kalyon’s letter, wall slip is offundamental and practical interest because it is the result of complex molecular-levelinteractions and is often manifest in processing polymers, suspensions, and emulsions.The motivation for the cleated tool is complementary: our aim is to enhance character-ization of continuum properties, which must be known together with boundary conditionsto analyze any given flow situation.
We respectfully disagree that the use of tools engineered to overcome wall slip, suchas the cleat geometry, “should … be discouraged;” such tools provide access to bulkproperties that might otherwise be burdensome or even impossible to obtain. The prob-lems Dr. Kalyon cites in support of his position �complicated flow fields and multipleshear rates� are significant issues with the tools he used and those described in priorliterature. However, as we have demonstrated in Nickerson and Kornfield �2005�, wehave overcome these obstacles through the specific geometrical parameters of the cleatgeometry, which creates a planar surface, near the tips of the cleats and parallel to the toolsurface, at which the velocity of the fluid is zero. In the cleat geometry the array of cleatsis dense enough that the penetration distance of the flow is small compared to the lengthof the cleats. The array of cleats is also uniform so that the penetration distance is uniform�otherwise the position at which the velocity falls to zero would not be a constant dis-tance from the tips of the cleats�. Tools with broad, widely spaced protrusions �e.g., vanedtools, as shown in the photographs by Dr. Kalyon� do not provide a planar surface atwhich the velocity matches that of the plate; instead an irregular sample boundary andflow field are induced.
Clearly, it is best to use smooth tools wherever feasible; however, the ability to obtainquantitative rheological measurements in slip-prone systems makes the cleat geometryextremely useful for appropriate samples. For example, in our research on biologicaltissues we have found that slip can be so extreme that the torque is below instrumentresolution, and the slip layer varies from sample to sample and over time. Therefore, slipmust be suppressed to obtain the information that is relevant to biomechanics: in theorganism the tissue is attached to adjacent structures—slip does not occur. Also, analysiswith the cleat geometry does not preclude separate in-depth analysis of wall slip. Thus,for the benefit of others who are faced with limited quantities of delicate, slip-pronesamples, we present a method for obtaining quantitative rheological properties.
References
Nickerson, C. S., and J. A. Kornfield, “A novel“cleat” geometry for suppressing wall slip,” J. Rheol. 49,
865–874 �2005�.
© 2005 by The Society of Rheology, Inc.1157J. Rheol. 49�5�, 1157-1158 September/October �2005� 0148-6055/2005/49�5�/1157/2/$25.00
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1158 C. S. NICKERSON AND J. A. KORNFIELD
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Charles S. Nickerson and Julia A. Kornfield
Division of Chemistry and Chemical Engineering,California Institute of Technology
Electronic mail: [email protected]
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