A Novel Method of Flow Curve Measurement for Magnetic Fluid Based on Plane Poiseuille Flow

Round 1

Reviewer 1 Report

 The paper is interesting and suitable for the journal. I only recommend to rearrange some images.

1)     As it is mentioned throughout the text, this system is supposed to be mainly used for measuring of low-viscous MR suspensions. Could it be in the same way used for non-filled liquids? Are you able to predict the limit in the viscosity of the systems (the highest viscosity of the system, whose rheological parameters can be measured using the proposed device).

2)     Is it possible to use this setup also for measuring of static yield stress? It means measuring in controlled shear stress mode? If so, could you please discuss what would be the actual determined valued (regarding the position in the channel) of the static yield stress considering uneven shear stress distribution in plane Poiseuille flow.

3)     Figure 9a: Is it possible to show flow curves for both MR suspensions in the presence of magnetic field?

4)     Figure 8a,b: My premise is that the commercial MR suspensions does not behave as the Newtonian fluid. Subsequently, in my best opinion it is unapropriate to exhibit viscosity of the commercial MR suspension in the way it is, it means to take the value and present it as a Newtonian liquid. Could the authors try to rearrange the picture or to present the „factory value“ of the liquid in any other way? Is there any specification about conditions, under which the producer obtain this value?

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

In this work a new method of measuring viscosity of magnetic fluid is proposed, based on observation of the motion of a drop of magnetic fluid clamped in a channel between two transparent parallel surfaces. The surfaces are inclined at some angle to the horizon, which creates the motion of the drop under the action of gravity, and are placed between Helmholtz coils, which create a homogeneous magnetic field. The idea expressed in the paper is original and of some interest. However, determination of rheological behavior of liquid on the basis of presented observations raises a lot of questions.

1. The authors compare the viscosity of a magnetic fluid determined according to the proposed new method with the viscosity determined with a rotary viscometer and find a difference in the values determined by different methods. They explain this difference by the presence of the moment of inertia of the measuring system of the rotary viscometer. However, to the best of my knowledge, the limits of applicability of the measuring systems of commercial rotary viscometers are calculated on the basis of the possibility of excluding the influence of the system's own moment of inertia. Therefore, the authors are probably making an incorrect comparison, and in this case the measuring system of the rotary viscometer they use is simply not applicable for measuring low viscosities.

2.         In explaining the difference in the obtained viscosity values the authors also provide an incomprehensible phrase: 'With the increase of the shear rate, the internal structure of the magnetic fluid tends to be stable, the resistance torque increases, and the influence of the motor becomes smaller'. The correctness of this statement raises doubts.

3.         It is not clear why the authors do not conduct a comparison of measurement results by their proposed method with the results of other methods for non-magnetic fluids. In order to justify the efficiency and correctness of the proposed method, a wide class of traditional (homogeneous) molecular liquids with different properties should be analyzed. Only two samples of magnetic fluids were used in this work.

4.         The weakness of the method is the neglect of the effects associated with air being squeezed out during the movement of the droplet in a thin layer.

5.         The most significant disadvantage of the proposed method is the neglect of the wettability of the surface by the liquid, which may be different for different liquids and different surface materials, which may lead to a significant change in the dynamics of the motion of the liquid drop in a thin layer. Pre-wetting the surface in this case does not solve this problem.

6.         The influence of a magnetic field on the dynamics of a drop can be associated with a change in the shape of the meniscus of the drop under the action of the field, which is not taken into account in this study. In addition, the range of applied magnetic fields in the considered conditions is limited by the possibility of development of elliptical and labyrinth instability of the droplet under the action of the field, which is also not considered in this work.

7.         The paper contains many inaccuracies and incorrect formulations. For example, it is not clear how formula (1) appears in the text, the text before it does not precede its appearance.

Given all this, I believe that the submitted work has a large number of shortcomings and undeveloped problematic positions. As presented, the work cannot be recommended for publication.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

 

The manuscript written by J. Dong et al  (A novel method of flow curve measurement for magnetic fluid based on plane Poiseuille flow) is an interesting paper.  

 

The structure of the paper corresponds to the requirements of the Journal.  My notes in connection with the paper:

11)       The introduction part is too long and contains lot of information, which are not necessary in this paper.  

22)      The measuring system is well documented, and the description is fine. 

33)      In figure 2a the letter H denotes the distance between two plates, while in figure 7 letter H denotes the magnetic field, please clarify this.  

44)       I have further problems with H, as the magnetic field. The authors mention “Under 30 mT magnetic field intensity,  N75” and “ Under the magnetic flux density of 30 mT, N105”. I think is is not correct: the SI unit of magnetic flux density is Tesla, therefore mT is ok, but the SI unit of magnetic field intensity is A/m. It is problem in figure 7 as well. 

55)      According to SI we have to denote the units with standing letters as:  mT, Pa …. see in table 3, but authors using italic letters in table 1 and 2, which is not correct.

66)      Under Eq. 11 the relation between M and H is correct, and this means that if the unit of H is A/m then the unit of M is also A/m.  Against that the unit of H and M on fig 7 is mT, it is not correct.

77)      The unit of saturation magnetization is also A/m and not mT.

88)      I think the preparation of magnetite-based ferrofluid is well known in the literature, therefore figure 5 and the corresponding text are not necessary.

According to my notes I can propose this paper for publication after some revision.

 

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors did not take into account any of the comments made. The authors' responses are formal and do not provide any additional clarity. I believe that the earlier comments on the paper remain valid, as well as the evaluation of the paper as a whole.