1. Introduction
Bubbles floating on a liquid can be seen in various places, and an understanding of the related phenomena is important in the natural sciences and industrial fields. For example, the bursting of floating bubbles in the ocean affects atmospheric circulation and marine pollution [
1,
2,
3,
4]. In a nuclear power plant accident, the bursting of bubbles in the reactor acts as a source of radionuclides [
5,
6]. Volcanic eruptions are caused by bubbles with sizes measured in meters [
7]. In food engineering, bubbles are involved in the release of foams and flavors, e.g., champagne, espresso, and beer [
8,
9,
10,
11]. Recently, boiling heat transfer in a complex nanofluid is closely related to bubble nucleation and growth [
12,
13].
Bubbles generated in a liquid rise due to buoyancy, and then burst after reaching the liquid surface [
14]. Bubbles floating on a liquid can remain in quasi-equilibrium for some time before bursting. During this time, the thin liquid film that defines the surface of the bubble gradually becomes thinner and unstable due to gravity, external disturbance, and impurities [
15]. After bursting, the dynamics of the bubble cannot be simulated by the present numerical method, and the partial differential equations of the conservation laws must be solved using the computational fluid dynamics techniques such as the finite element method [
16], the arbitrary Lagrangian–Eulerian moving mesh method [
17], the two-fluid two-phase flow model [
18,
19], and the adaptive multi-phase method [
20]. As shown schematically in
Figure 1, the shape of the floating bubbles in the quasi-equilibrium state can be divided into three curves, namely the cap, the cavity, and the meniscus. These three curves meet at a junction point
. As the various dynamic phenomena that occur when a floating bubble bursts depend largely on its initial shape, the accurate prediction of this shape is important [
21,
22,
23,
24,
25,
26,
27]. If the thickness and drainage of the liquid film can be neglected, then the shape of the bubble in the quasi-equilibrium state is determined by the Young–Laplace equation representing the balance between the forces of hydrostatic pressure and capillary action [
28,
29,
30,
31,
32]. Floating bubbles vary in shape from spherical to hemispherical, depending on their size. Therefore, the Bond number,
, is used as a key parameter.
Bashforth and Adams [
28] presented a systematic theoretical equation for hydrostatic pressure and capillary forces and obtained a series solution of drops of fluids. Subsequently, Toba [
29] calculated the shapes of floating bubbles by using a semi-graphical method. At the same time, Princen [
30] used the Table of Bashforth and Adams [
28] to find the junction point and calculate the shapes of bubbles of various sizes. Medrow and Chao [
31] numerically calculated the bubble shape for a broader range of the bubble sizes (which corresponds to a range of
, computed based on the parameter
values of [
31]), and derived an analytic solution using a perturbation technique for tiny bubbles that are difficult to calculate numerically. Recently, Lhuissier and Villermaux [
23] obtained the bubble shape via a method similar to that of Toba [
29]; however, there is no detailed information on their numerical method and on the accuracy of their solution. Bartlett [
32] used a similar method to Princen [
30] and presented the shapes of bubbles for Bond numbers between 0.1 to 100. Cohen et al. [
33] developed a model considering the film weight of giant soap bubbles and compared the results of numerical solutions and experiments. Shaw and Deike [
34] studied the coalescence of floating bubbles for a wide range of Bond number, and characterized the evolution of the underwater neck and the surface bridge. Miguet et al. [
35] investigated the life of a floating bubble by exploring the role of drainage and evaporation on film thinning. Despite a long period of study, the bubble sizes used for analysis have been limited, and there is no rigorous evaluation of the accuracy and robustness of the numerical methods used.
As bubble shape parameters play an important role in the theory describing various dynamic phenomena related to floating bubbles [
36], accurate prediction of the bubble parameter is crucial, especially in a wide range of Bond numbers. For instance, the lifetimes of floating bubbles are related to the surface area of the bubble film [
15]. The jet velocity generated when the bubble bursts is related to the depth of the cavity, and the amount of aerosol emitted depends on the radius and height of the cap [
23,
27]. Koch et al. [
5] used the correlation between the dimensionless cap area and the dimensionless diameter to calculate the number of film droplets generated in the bubble bursting process, and the correlation between the dimensionless base radius and the dimensionless diameter to obtain the critical film thickness. Teixeira et al. [
37] employed a perturbation method for large bubbles to obtain analytical expressions for bubble parameters as a function of the Bond number, and compared their results with experimental data. Puthenveettil et al. [
36] also derived approximate analytical solutions for bubble parameters over a moderate range of Bond number (
) and compared these with their experimental results. Moreover, to the best of the present author’s knowledge, no study has yet been published detailing the correlations between the Bond number and various bubble parameters over an extensive range of Bond numbers.
In the present work, an improved numerical procedure is proposed to calculate the shapes of floating bubbles over a much wider range of Bond numbers than that examined in the previous studies. The proposed method is validated against the earlier numerical and experimental results to prove its accuracy and robustness. New correlations between the Bond number and several important bubble parameters, along with their asymptotic relations, are then provided.
The novelty of this study is as follows. A new method is developed by introducing specific functions for accurately finding the location of the connection points on the bubble surface regardless of the ODE solvers and their boundary points. An improved algorithm to find the junction point is developed, based on a novel function to determine the type of meniscus.
2. Theoretical Model
A slightly modified form of the theoretical model presented by Toba [
29] and Medrow and Chao [
31] is used in the present work. Thus, the shape of the floating bubble is described by the Young–Laplace equation:
where
is the pressure difference across the interface,
is the surface tension, and
and
are the principal radii of curvature.
A half-section of an axisymmetric bubble floating on a liquid is presented, along with its associated parameters, in
Figure 1, where
is the depth of the cavity below the undisturbed liquid surface,
is the point where the meniscus meets the undisturbed surface, and
and
are the tangent angles of the cavity and meniscus curves, respectively.
If the thickness of the cap film is ignored, then the cap has a spherical shape with a radius
, and Equation (1) can be written as
where
denotes the pressure difference across the cap film. Furthermore, if the weight of gas inside the bubble is neglected, then the cavity satisfies
where
and
denote the liquid density and gravitational acceleration, respectively,
Similarly, the meniscus shape is given by
As the bubble shape is a surface of revolution about the
z-axis, the principal radii of curvature are given as
By using the capillary length,
, the bubble parameters are nondimensionalized as given by
Substituting Equations (2) and (5)–(7) into Equation (3) yields the dimensionless ordinary differential equations (ODEs) for the cavity shape, given here as
where the parameter
represents the dimensionless pressure difference across the bottom of the bubble, as defined by
Equations (8) and (9) can be rewritten with an independent variable
as
Application of the L’Hospital’s rule, i.e.,
, then leads to the first initial condition of Equations (8) as follows:
The second initial condition of Equation (9) is simply given as
Similarly, the relation
is used to obtain the ODEs for the meniscus shape as follows:
The boundary conditions of Equations (15) and (16) at
are given as
At the junction point
, the geometrical relationship given in Equation (18) must be satisfied:
From Equations (10) and (18), the relation between
and
is given as
As the cap is spherical with a radius
, its shape is given by
The volume of the bubble is computed analytically via integration by parts [
38]. The volume of the cap is given by
where
.
The volume of the cavity is given by
The surface area of the cap is obtained as
However, there is no analytic form for the surface area of the cavity. It should be numerically integrated as follows:
where
and
respectively denote the
X and
Y positions on the cavity at
.
The Bond number is defined by
where
denotes the equivalent spherical radius of the bubble.
3. Numerical Procedure
Given the parameter , the shape of the floating bubble can be determined. Since the bubble is symmetric about the z-axis, only half of it is needed. First, the shape of the cavity shape between the bottom and the junction point is calculated, then the shape of the meniscus between the point and the boundary point is determined. Finally, the shape of the circular cap is quickly obtained. The junction point between the cavity and the meniscus is not given analytically and should be found via a numerical iterative method. The accuracy of the bubble shape is significantly affected by the accuracy with which is located.
The dimensionless height is used to determine whether the guessed junction point matches the exact point . Given the position of , is readily calculated from Equation (19). Furthermore, after computing the meniscus shape, another height, , can be obtained. Thus, the bubble shape is considered to have converged when the error falls within a defined level of tolerance that indicates the accuracy of the numerical method used.
3.1. Improving the Accuracy of the Cavity Shape
The cavity shape can be obtained by integrating Equations (8) and (9) or Equations (11) and (12) numerically. However, previous studies have shown that it is necessary to divide the cavity into two [
29,
32,
39] or three [
31] parts in order to avoid divergence of Equations (8) and (9) at
. In the present study, when the cavity was divided into two parts, the numerical error was found to increase rapidly as the bubble size decreased (see Figure 6). Hence, the cavity shape was divided into the following three parts or intervals: (I)
, (II)
, and (III)
, and Equations (8) and (9) were applied to intervals (I) and (III), while Equations (11) and (12) were applied to interval (II). However, because
is not an independent variable in these equations, the numerical integrations are not completely finished at
and
. Hence, a conventional method is to set an arbitrary far point
(or
) as a boundary condition of the ODEs and to finish the calculation when
is just greater than
(or
). Unfortunately, this leads to potential errors at the endpoint
(or
) of the divided cavity depending on ODE solvers. The conventional method works well on medium-sized bubbles, but produces a severe error in large bubbles, especially at the position of
, because the bottom of the cavity becomes substantially flat. This error is not only affected by the boundary points
(or
), but also by the tolerance of the ODE solver.
To remedy this problem, an improved method was proposed for accurately finding the location of the connection points
and
regardless of the ODE solvers and their boundary points. In this approach, a monotonically increasing function
is defined as follows:
function |
to to find at the endpoint |
return
|
end function |
An accurate position
is then found by solving
in the interval
using a root-finding technique. Here, the boundary point
is any value greater than
; usually,
is appropriate for most of the bubble sizes considered in this study. As shown in
Figure 2b,
does not affect the accuracy of the solution obtained using this method at all. Once
has been determined, Equations (8) and (9) are integrated again from
to
to obtain the first part of the cavity profile along with the remaining
and
. Similarly, the second part of the cavity and its endpoint
can be computed using the following function:
function |
to to find at the endpoint |
return
|
endfunction |
The computed bubble shapes according to the position of the boundary point
for a large bubble (
) are presented in
Figure 2. Here, the bubble shapes are seen to vary significantly depending on
when the conventional technique is used (
Figure 2a). When the root-finding method is used, however, the bubble shapes not affected by
(
Figure 2b).
Similarly, the effect of ODE solver tolerances for the same size of bubble are presented in
Figure 3. Here, the bubble shape changes slightly according to the tolerance using the improved method (
Figure 3b), but the results obtained using the conventional method are unacceptable (
Figure 3a). Therefore, it is essential to compute the connection point accurately for large bubbles.
3.2. Finding the Junction Point Based on the Type of Meniscus
There are two general approaches to finding the junction point
according to the direction of integration of the meniscus. The first approach is to integrate the meniscus from
to
in the negative
X-direction [
29,
31]. Here,
is sought in an iterative manner so that the derivatives of the meniscus and cavity profiles match at the junction point
. However, it is difficult to compute the shape of the meniscus using this method because the initial slope becomes zero when the exact initial condition,
at
, is applied. To circumvent this, Toba [
29] and Freud and Freud [
40] employed
at
as an initial condition. Medrow and Chao [
31] further improved this method by using an approximate analytical solution of
with the Bessel function. Nevertheless, as these methods do not give an exact initial condition at
, the accuracy of the solution is still limited.
The second approach is to integrate the meniscus from
to
in the positive
X-direction [
23,
30,
32]. Although this approach has the advantage of obtaining the
position more precisely, there is another difficulty in that the meniscus shape changes dramatically around the junction point
, as shown in
Figure 4.
Hence, a method for effectively locating the junction point
is proposed herein by using a specific function that distinguishes the type of meniscus shape. A conventional bisection method for finding
narrows the interval by comparing the sign of
at both ends of the solution interval. At each iteration step, it is necessary to find the height
, which is calculated differently depending on the meniscus type: if the meniscus profile is convex, its minimum value is chosen; if the meniscus profile is monotonically decreasing, an inflection point is selected. Interestingly, without calculating
, the bisection method can become simpler by considering the meniscus type only. Hence, the following function is defined to determine whether a meniscus is convex at a guessed junction point:
function | |
| Integrate Equations (15) and (16) from to to find , where is any large value
|
return the logical value of |
endfunction | |
This function is then used to modify the bisection method as Algorithm 1:
Algorithm 1 Modified bisection method |
Input: |
Output: |
for to |
|
if |
|
else |
|
end if |
end for |
where
represents the
or
value on the cavity. The maximum number of iterations is given by
. Here,
and
are the error tolerance and initial interval of the bisection method, respectively. If
is between 90
and 135
, the initial interval is
; if
is between 135
and 180
, the initial interval is
, where
. However, if
and
are not sufficiently accurate, this method may fail to find the junction point
in the case of very small or large bubbles. The exact positions of
and
can be obtained by solving the equations
and
, respectively. Once the junction point
is obtained,
can be found by using the following function
, and then the meniscus shape:
function |
Integrate Equations (15) and (16) from to x to find at the endpoint |
return |
endfunction |
This modified bisection method is more efficient and robust than the conventional one because there are no expensive routines to find the minimum or inflection point of the meniscus. Although a similar meniscus type to that used herein was previously used by Princen [
30], the method employed therein remained limited in terms of bubble size and accuracy because it relied on Bashforth and Adam’s table [
28].
The overall procedure for calculating a floating bubble shape is summarized in Algorithm 2.
Algorithm 2 The improved numerical procedure for calculating the shape of a floating bubble |
Input: |
Output:Floating bubble shape and parameters |
Calculate the first part of the cavity profile by integrating Equations (8) and (9) from to and by solving . |
Calculate a part of the cavity profile
by integrating Equations (11) and (12) from to and by solving . |
if |
Find
by solving . |
Find the junction point by using the modified bisection method in . |
Calculate the rest of the cavity profile
by integrating Equations (11) and (12) from to . |
else |
Find
by solving . |
Find the junction point by using the modified bisection method in . |
Calculate a part of the cavity profile by integrating Equations (8) and (9) from to . |
Set the rest of the cavity profile as . |
end if |
Calculate the meniscus profile
by integrating Equations (15) and (16) from to . |
Calculate the cap profile by computing Equations (20) and (21) from to . |
Combine the profiles to obtain the floating bubble shape: . |
4. Results and Discussion
In the present study, the ODE solver employs a variable step, the 4th and 5th-order Runge–Kutta algorithm using Dormand and Prince pairs [
41]. A range of the Bond numbers, i.e.,
, is applied to the improved numerical method. Accordingly, the results obtained using the conventional and improved methods are compared in the following subsection. Then the present results are compared with those of previous computational and experimental studies.
4.1. Comparison between the Conventional and Improved Methods
The conventional and improved methods are compared for large bubbles in
Figure 5. Here, it can be seen that a smaller value of
corresponds to a larger bubble size (
Figure 5a). Thus, when
1 × 10
−7, the bubble shapes obtained using the two different methods are almost identical. However, when
1 × 10
−14, there is a slight difference between the two results, and when
1 × 10
−22, the two methods show a significant discrepancy in the obtained bubble shapes. The cause of inaccuracy in the conventional method is demonstrated in
Figure 5b, where the dimensionless cap radius
is seen to increase abnormally as the parameter
decreases in the conventional method.
The conventional and improved methods are compared for small bubbles in
Figure 6. Here, when
, the bubble shapes obtained using both methods are almost identical (
Figure 6a). However, when
and
, the conventional method raises the bubble in a non-physical manner because the bisection method does not converge and gives a negative value of
. As shown in
Figure 6b, when
is small, the dimensionless absolute height
is the same in both methods; however, as
becomes larger,
fluctuates significantly in the conventional method. These results clearly demonstrate that the improved method should be used to calculate large or small bubbles robustly.
4.2. Comparison of the Present Results with the Earlier Numerical and Experimental Results
The present computational results are compared with the previously published numerical results in
Table 1. Various bubble parameters were previously calculated in the range of
,
, and
by Toba [
29], Princen [
30], and Medrow and Chao [
31], respectively. However, Princen [
30] used the parameter
in place of
, where the relationship between these is given by
where
denotes the dimensionless parameter of the present work, and
denotes the dimensionless parameters of Princen’s work. The computed parameters in
Table 1 agree well with one another, with the exception of Toba’s parameters. This is because Toba employed a semi-graphical technique of limited accuracy. The earlier methods used an approximate analytical solution for tiny bubbles [
31] or an interpolation method with a pre-calculated Table [
30], so the sizes considered were narrow. Medrow and Chao [
31] used the boundary conditions of
and
. Princen [
30] searched the junction point until both
and
were less than
. In contrast, the present calculations maintain an accuracy of
and
, thus providing much more precise results.
The experimental results of Teixeira et al. [
37] are compared with the present numerical results for selected bubble parameters in
Figure 7a, where the parameters are plotted with respect to the Bond number based on the cap radius, i.e.,
. Here, the present computations are seen to agree well with the experimental results, except for the
values. For this parameter, the numerical results obtained using Surface Evolver S/W [
42] have shown a similar trend to the present computations, as described in [
37]. Meanwhile, the experimental results of Puthenveettil et al. [
36] are compared with the present numerical results for selected parameters in
Figure 7b, where
denotes the height of the bubble top above the undisturbed surface. All parameters are normalized with respect to the equivalent bubble radius
. The experimental results agree well with the present computation, except that the present computation overestimates the experimental data for
. Notably, the numerical results of Puthenveettil et al. [
36] also overestimate
by a similar amount.
The presently computed profiles for bubbles of various sizes (red outlines) are compared with the experimental photographs obtained by Teixeira et al. [
37] and Puthenveettil et al. [
36] in
Figure 8. In addition, the present predictions for the angle of the junction point (
are compared with those measured by Teixeira et al. [
37] in
Table 2. Teixeira et al. [
37] used a soap solution (
), and the images were taken in the range of
to
(
Figure 8a). Here, the calculated bubble shapes are seen to match the experimental images well, although the results in
Table 2 indicate slightly different values for the angle of the junction point (
). Specifically, the present computational work predicts
to be about
degrees higher than that measured by Teixeira et al. [
37] Meanwhile, the experimental investigations of Puthenveettil et al. [
36] were performed on various liquids (e.g., water and ethanol), and the photographs were taken for
to
(
Figure 8b). However, when the Bond numbers measured by Puthenveettil et al. [
36] were used in the present computations, the resulting numerical profiles for large bubbles were quite different from the experimental photographs. Hence, the
values used in the present work were adjusted in order to provide a better match between the predicted and measured profiles. These adjusted Bond numbers are compared with those measured from
Figure 8b in
Table 3. Here, similar Bond numbers are predicted and measured for the two tiny bubbles, but the present computations predict much higher Bond numbers than those measured for the two large bubbles. While Teixeira et al. [
37] measured the bubble cap radius and the junction angles to obtain an alternative form of Bond number (
), Puthenveettil et al. [
36] calculated
by measuring the volume of a bubble from the experimental image. It is expected that obtaining
is more complicated than obtaining
because the lower part of the floating bubble is immersed in the liquid, whereas the upper part is not. Therefore, the discrepancy in
Table 3 is thought to be due to experimental errors or the limitations of the current theoretical model.
4.3. Important Bubble Parameters and Profiles for a Wide Range of the Bond Numbers
The present numerical results for important bubble parameters in a wide range of Bond numbers (
, corresponding to
) are listed in
Table 4. The relative tolerance of the ODE solver is set to
. For the range of
considered here, the present computations satisfy
and
. Thus, the present method computes the boundary condition of the meniscus more accurately and converges the position of the junction point more precisely than the earlier methods. As a result, the present method provides highly precise and robust numerical results for a wide range of bubble sizes.
The calculated bubble shapes for
are presented in
Figure 9, where each shape is drawn at equal intervals on a log scale of
. In the interval of
to
, the bubble exhibits an almost hemispherical shape, and the cavity is virtually flat (
Figure 9a). However, in the interval
, the shape becomes less hemispherical as the bottom of the bubble continues to descend into the liquid (
Figure 9b). In the interval
, the shape continues to change from hemispherical to that of a bisected rugby ball (
Figure 9c). Interestingly, the depth
is found to increase to a maximum value and subsequently decrease within this interval. In the interval
, the bubbles have acquired a nearly spherical shape and are seen to protrude only slightly above the undisturbed liquid surface (
Figure 9d). Finally, in the interval
, the bubbles are almost spherical and are almost entirely submerged in the liquid (
Figure 9e).
4.4. Correlations and Asymptotic Behaviors of the Bubble Parameters
The log–log plots of the various bubble parameters with respect to the Bond number for the interval of
are presented in
Figure 10. Each plot can be divided into the following three regions: (I)
, (II)
, and (III)
. The parameters for each interval are fitted with a fifth-order polynomial on the log-log scale according to Equation (28):
where
denotes the dimensionless bubble parameters. The coefficients
are presented in
Table 5.
Interestingly,
can be well expressed even with a single “bending function” for
, as given by Equation (29):
The curves fitted by the three fifth-order polynomials and by the single bending function are compared in
Figure 11, where both correlations are seen to fit the numerical data well.
Two asymptotes appear in
Figure 10 as the Bond number goes to zero or infinity. These were obtained numerically via the following simple power law:
where the coefficients
and
are shown in
Table 6.
It is noted that
can be rewritten in a more straightforward form as
Furthermore, the asymptotes can be expressed approximately as a power of :
As , , , , , , and .
As , , , , , , , , and .
The height of the undisturbed surface above the bubble bottom has a maximum value of
when
, as shown in
Figure 10.