3.1. Validation of numerical method

In figure 2, we compare our simulated coalescence events with the reported experimental results of Zhang et al. (Reference Zhang, Li and Thoroddsen2009). In their experiments, two vertically aligned water drops with parent ratio $\beta = {\bar{D}_f}/{\bar{D}_m} = 2.72$ are allowed to collide in an air medium in the absence of external field (Ca_E = 0) with a negligible approaching velocity. From their given data, the dimensionless parameters are obtained as Oh_i = 0.0058, Oh_e = 0.00011, Bo = 0.092 and A = 0.997. By comparing the drop contours at different times, it is confirmed that our numerical model predicts the progress of coalescence and pinch-off phenomena very well. The accuracy of the present numerical method is further examined by comparing the size of the daughter drop. From the present study, the diameter of the daughter droplet (or the generated droplet) is found to be ${\approx} 0.52{\bar{D}_m}$ which is approximately the same as the experimentally reported value of $0.48{\bar{D}_m}$. Note that Zhang et al. (Reference Zhang, Li and Thoroddsen2009), in their experiments, considered one supported drop (the bottom one) on a nozzle. The upper drop (the smaller one) was allowed to fall on it from a glass needle, for studying the satellite-drop formation in the coalescence event. They observed that pinch-off typically occurred much before the capillary wave propagation could reach the bottom nozzle. Thus, the presence of the bottom nozzle was inconsequential towards dictating the coalescence dynamics. Hence, two freely suspended drops in air could alternatively replicate the physical features of their experiments as well, as evidenced from figure 2.

Figure 2.

Advancement of coalescence of water droplets with time for β = 2.72, Ca_E = 0, Bo = 0.092, Oh_i = 0.0058, Oh_e = 0.00011 and A = 0.997. The upper panel shows the experimental results of Zhang et al. (Reference Zhang, Li and Thoroddsen2009) and the lower panel represents the present numerical results. The snapshots are taken at dimensional time (a) 0 ms, (b) 0.27 ms, (c) 0.67 ms, (d) 0.93 ms, (e) 1.2 ms and (f) 1.8 ms.

In an effort to establish the efficacy of our numerical approach in the scenarios where EHD interactions are present as well, we next present the interaction between two neutrally buoyant (Bo = 0 and A = 0) equal-sized drops in a uniform electric field, as shown in figure 3. In this study, the present results are compared with a reported numerical model of Baygents et al. (Reference Baygents, Rivette and Stone1998), where the parameters used for simulation were R = 2, S = 8, λ = 1 and $C{a_E} = 2$. They adhered to the Stokes flow limit by taking the Reynolds number $Re( = {\bar{\rho }_e}\bar{D}_m^2{\bar{\varepsilon }_e}\bar{E}_0^2/\bar{\mu }_e^2) \ll 1$. Since $O{h_e} = \sqrt {C{a_E}/Re}$, the representative choices of $C{a_E} = 2$ and Re = 0.125 yielded Oh_e = 4, conforming to the domain of applicability of our framework. From figure 3, it is evident that the numerically obtained shape and motion of the drop couple, as obtained from the present study, agree well with the previous findings of Baygents et al. (Reference Baygents, Rivette and Stone1998).

Figure 3.

Interaction of two equal-sized droplets in uniform electric field for Ca_E = 2, Oh_i = Oh_e = 4, Bo = 0 and A = 0. The solid lines represent the numerical results of Baygents et al. (Reference Baygents, Rivette and Stone1998) and the circles represent our numerical results.

4.1. Dynamical evolution without electric field

One of the primary hydrodynamic features, even without the action of an electric field, is the realm of two-drop interaction in the presence of fluid flow, which turns out to be significantly more involved as compared with the corresponding single-drop dynamics. On first physical contact as a prelude to possible coalescence, a liquid bridge connecting the two drops (herein termed as the ‘neck’) is formed. The interfacial tension tries to minimize the contact area during this course, so that the neck gradually widens. This results in liquid motion from the mother drop towards the father drop. At the onset of merging of the two drops, the curvature of the neck remains very high. As the neck attempts to expand, it generates a strong capillary wave, which during its propagation towards the ends of the drops, brings in shape distortions of the same, as shown in figure 2. Because of the size disparity between the parent drops (the mother drop is considered to be much smaller than the father drop), the capillary wave converges at the apex of the mother drop quite early, inducing a vertically upward capillary pull. This force is essentially a function of the initial neck curvature, which in turn is a function of the parent ratio, β (Deka et al. Reference Deka, Biswas, Chakraborty and Dalal2019). Increase in the neck curvature intensifies the capillary wave, strengthening the upward capillary pull that results in longer protrusions at the apex. The upward capillary pull, however, is resisted by the viscous effects and gravity.

For high Oh (referring to systems where either drop viscosity or outer fluid viscosity is high), the loss in capillary wave energy through viscous dissipation may be substantial (Blanchette & Bigioni Reference Blanchette and Bigioni2006). Thus, only a small fraction of the capillary momentum may be available at the top, weakening the net upward pull to an extent to preclude pinch-off. The gravitational force, which acts in the downward direction, is directly proportional to the specific weight of the fluid. For high Bo, thus, the capillary pull may not be strong enough to carry the fluid to a substantial height. In this case, vertical collapse may occur as against pinch-off. However, if the capillary-wave-induced upward pull overcomes the viscous- and the gravity-induced resistances, a cylindrical protrusion forms at the apex, as shown in figure 2(e). After achieving a threshold height, when the curvature at the peak of the cylindrical structure becomes sufficiently large, the same results in vertical downward pull (or vertical collapse) at the apex and a concomitant horizontal inward pull (or horizontal collapse) at the neck. Hydrodynamically driven pinch-off, thus, may emerge to be a competing consequence between the vertical and the horizontal collapse. If the vertical collapse prevails, pinch-off does not occur and vice versa (Blanchette & Bigioni Reference Blanchette and Bigioni2006).

4.2. Effect of electric field

Subjected to an electric field, instant charging of the drops occurs owing to the discontinuity in the relevant electrical properties at the interface. For R/S > 1, the upper face of a drop holds positive charge, whereas the lower face holds negative charge. A reverse charge distribution pattern can be observed for R/S < 1 (Salipante & Vlahovska Reference Salipante and Vlahovska2010; Mandal et al. Reference Mandal, Sinha, Bandopadhyay and Chakraborty2018). These distinctive scenarios, accordingly, alter the electrical stresses to an extent that results in concomitant variations in the viscous and the capillary stresses, so as to realize a dynamic force balance at the interface. Thus, the contrast in the relevant electro-physical properties plays the most non-intuitive role in altering the dynamic force balance and orchestrating topological alterations under electrical forcing.

To highlight some key distinctive features of drop–drop coalescence with and without electric fields, we portray the drop contour evolution for Ca_E = 0 and Ca_E = 0.1 in figure 4(a) and 4(b), respectively, considering parent ratio β = 1.6, electro-physical parameters (R, S) = (10, 2.3) and drop-side Ohnesorge number Oh_i = 0.005. In order to physically relate the above with an experimentally realizable system, we consider silicone oil (${\bar{\rho }_i}\sim 920\ \textrm{kg}\ {\textrm{m}^{ - 3}}$, ${\bar{\mu }_i}/{\bar{\rho }_i}\sim 1\ \textrm{cSt}$, ${\bar{\varepsilon }_e}\sim 2.3{\varepsilon _0}$) as the drop phase and air as the carrier phase with a surface tension value γ ~ 20 mN m⁻¹ (Ricci, Sangiorgi & Passerone Reference Ricci, Sangiorgi and Passerone1986). The conductivity of the air typically varies between $O({10^{ - 15}})$ and $O({10^{ - 14}})\ \textrm{S}\ {\textrm{m}^{ - 1}}$ (Kamsali, Prasad & Datta Reference Kamsali, Prasad and Datta2009), whereas that of the silicone oil varies between $O({10^{ - 13}})$ and $O({10^{ - 11}})\ \textrm{S}\ {\textrm{m}^{ - 1}}$ (Mhatre, Deshmukh & Thaokar Reference Mhatre, Deshmukh and Thaokar2015a; Abbasi et al. Reference Abbasi, Song, Kim and Lee2019). For such property ranges, the charge relaxation time scale may be comparable to the droplet coalescence time scale. However, in reported benchmark experiments, the conductivity of the silicone oil could be significantly enhanced (up to ~10⁻⁶ S m⁻¹) by using specific doping techniques (Yin & Zhao Reference Yin and Zhao2002), manifesting its leaky dielectric feature. Thus, for the silicone oil–air system, a wide range of conductivity ratios (R) can be realized in practice, covering both perfect dielectric and leaky dielectric behaviour. However, following a rich tradition of prior works (Ha & Yang Reference Ha and Yang2000; Lac & Homsy Reference Lac and Homsy2007; Mählmann & Papageorgiou Reference Mählmann and Papageorgiou2009; Poddar et al. Reference Poddar, Mandal, Bandopadhyay and Chakraborty2018), we considered R ~ O(10) for our reported results We have verified that while higher orders of magnitude of R indeed alter the quantitative results, the nature of physical forcing remains unaltered for the entire physically plausible regime of the pertinent electrical properties. The values of the Bond number, Atwood number and outer-side Ohnesorge number are taken as Bo = 0.32, A = 0.998 and Oh_e = 0.0002 (obtained for drop diameter of 0.8 mm), respectively, throughout this work, unless mentioned otherwise.

Figure 4.

Coalescence outcomes for (a) Ca_E = 0 and (b) Ca_E = 0.1. The other parameters considered are β = 1.6, R = 10, S = 2.3, Oh_i = 0.005, Oh_e = 0.0002, Bo = 0.32 and A = 0.998. (c) Schematic describing key forces acting at the top and the geometrical features during capillary pulling.

As shown in figure 4, the drop deformations for Ca_E = 0 and Ca_E = 0.1 are almost similar up to t = 0.6. However, at t = 0.8 the drop shape for Ca_E = 0.1 appears to be clearly more stretched. Eventually, significant topological variations start to occur, leading to pinching of the daughter drop, whereas for Ca_E = 0, no such phenomenon is observed over the same parametric space. Figure 4(c) shows the schematic representation of various forces acting on the drop apex that dictates topological variation during electric-field-mediated coalescence; the importance of these forces is discussed elaborately in Appendix B.

The role of electric field in triggering a bifurcation to the pinch-off behaviour can be best explained by analysing the vertical deformation of the drop with time. Figure 5(a) depicts the transient variation in the apex position of the mother drop (H_T) in the computational domain for Ca_E = 0 and 0.1, from which it can be observed that the variations due to electric field are not significant until t = 0.5. However, beyond this temporal limit, H_T increases dramatically for Ca_E = 0.1, resulting in the appearance of a significantly longer protrusion at t = 0.8. This delays the subsequent vertical collapse, leading to pinch-off.

Figure 5.

(a) Variation of drop apex position H_T with time. (b) Demonstration of electric force acting on drop and (c) electric field profile at t = 0.84. The parameters considered are β = 1.6, R = 10, S = 2.3, Oh_i = 0.005, Oh_e = 0.0002, Bo = 0.32 and A = 0.998.

The bifurcation from vertical collapse to pinch-off may be addressed from a mechanistic point of view, by considering the geometrical space at top occupied by the northern half of a polarized drop having originally a spherical shape. The jump $([{\cdot} ])$ in the normal component of the electric traction at the interface of the said entity can be derived as

(4.1)\begin{equation}\boldsymbol{n}\boldsymbol{\cdot }[{\boldsymbol{\tau }_M}\boldsymbol{\cdot }\boldsymbol{n}] = \frac{{{q_S}({E_{n,i}} + {E_{n,e}})}}{2} - \frac{{(1 - S)}}{2}({E_{n,i}}{E_{n,e}} - E_{t,i}^2),\end{equation}

where ${q_S}$ $( = {E_{n,e}} - S{E_{n,i}})$ is the surface charge density (Saville Reference Saville1997) and E_n and E_t are the normal and tangential electric field components, respectively. The first term on the right-hand side of (4.1) is the stress due to Coulombic force (${\tau _{coulomb}}$, generated due to interaction between surface charge and electric field) and the second term is the stress generated due to dielectrophoretic force (${\tau _{diel}}$, generated due to jump in electrical permittivity at the interface). By appealing to the expressions of the electric and the velocity field calculated by using Taylor's leaky-dielectric model (Reference Taylor1966) for the individual drops, the order of magnitude of the respective normalized stress components may be estimated as ${\bar{\tau }_{coulomb}}/(\gamma /{\bar{D}_m})\sim C{a_E}(9/2)(R - S)(R + 1)/{(R + 2)^2}$ and ${\bar{\tau }_{diel}}/(\gamma /{\bar{D}_m})\sim C{a_E}(9/2)R(S - 1)/{(R + 2)^2}$. Note that the Coulombic stress vanishes for R = S, which is the condition for a perfectly dielectric fluid system having zero net charge (Mandal, Bandopadhyay & Chakraborty Reference Mandal, Bandopadhyay and Chakraborty2016a). Similarly, the dielectrophoretic stress vanishes when there is no permittivity jump across the phases, i.e. S = 1.

The electric stress induces flow in the drop vicinity, the jump in which induces hydrodynamic stress at the interface $({\tau _{EHD}})$, where ${\bar{\tau }_{EHD}}/(\gamma /{\bar{D}_m})\sim ({\textstyle{9 \over 5}})C{a_E}((R - S)(3\lambda + 2)/{(R + 2)^2}(\lambda + 1))$. Thus, the total electrically induced stress created at the apex, in its normalized form, scales as

(4.2)\begin{align} {\tau _E} & = \dfrac{{{{\bar{\tau }}_{coulomb}} + {{\bar{\tau }}_{diel}} + {{\bar{\tau }}_{EHD}}}}{{(\gamma /{{\bar{D}}_m})}}\sim C{a_E}\left\{\begin{array}{@{}c@{}} {\underbrace{{\dfrac{9}{2}\dfrac{{({R^2} - S)}}{{{{(R + 2)}^2}}}}}_{{\textrm{due}\ \textrm{to}\ \textrm{electric}\ \textrm{force}}} + \underbrace{{\dfrac{9}{5}\dfrac{{(2 + 3\lambda )(R - S)}}{{(1 + \lambda ){{(R + 2)}^2}}}}}_{{\textrm{due}\ \textrm{to}\ \textrm{EHD}\ \textrm{flow}}}}\end{array} \right\}\nonumber \\ & = \dfrac{9}{{10}}C{a_E}\dfrac{{(5{R^2}(\lambda + 1) + 2R(3\lambda + 2) - S(11\lambda + 9))}}{{(\lambda + 1){{(R + 2)}^2}}}. \end{align}

While the gravitational force and capillary pull always attempt to pull the apex downwards and upwards, respectively (the corresponding normalized stresses respectively read as ${\bar{\tau }_g}/(\gamma /{\bar{D}_m})\sim Bo{H_T}R_n^2$ and ${\bar{\tau }_\gamma }/(\gamma /{\bar{D}_m})\sim 1/{R_n}$, where R_n is the dimensionless instantaneous neck radius as shown schematically in figure 4c), the electrically induced traction at the top can preferentially act either upwards or downwards, depending on the specific values of (R, S). Additionally, (4.2) suggests that there exists a critical characteristic demarcating boundary line in the (R, S) phase space, along which the net electric forcing effect turns out to be zero, recovering the hydrodynamically manipulative limit. The equation of this critical characteristic line is found as

(4.3)\begin{equation}S = R(5R\lambda + 5R + 6\lambda + 4)/(11\lambda + 9).\end{equation}

For the property combination values cited herein, i.e. (R, S) = (10, 2.3), the electric stress pulls the mother drop in the upward direction as suggested by the volumetric electric force $({\boldsymbol{F}_E})$ profile presented in figure 5(b) as well as the theoretically obtained sign of ${\tau _E}$ from (4.2). As the capillary pull gradually stretches the mother drop, the charges are pushed towards the apex region. With the continuation of vertical stretching, the lateral width of the protrusion gradually decreases, thereby decreasing the local surface area at the apex. Thus, the surface charge density ${q_S}$ becomes locally high. Since ${q_S}\boldsymbol{= }({\boldsymbol{E}_e} - S{\boldsymbol{E}_i})\boldsymbol{\cdot }\boldsymbol{n}$, increase in the charge density amplifies the jump in the normal component of the electric field across the interface, which results in a significantly high localized electric field distribution at the top surface as compared with the bottom, as shown in figure 5(c). Consequently, the electric force $({\boldsymbol{F}_E} \propto {E^2})$ becomes large at the top (as shown in figure 5b) and, thus, produces strong upward stretching, aiding the capillary pull. Their cooperative interplay, as further orchestrated by the simultaneous alteration in Ca_E and β, can result in decisive dynamical bifurcations over preferential parametric spaces, as discussed in the subsequent sections.

5.1. Pinch-off modes and topology of secondary (or daughter) drop

Post-identification of the critical limit of parent ratio, we probe the effect of the electric field on the daughter drop topology in the pinch-off regime. The electrical properties are kept unchanged in the current section, i.e. R = 10 and S = 2.3, for consistency in the discussion. Figure 8(a) illustrates the variation of daughter drop size (D_d) with Ca_E for Oh_i = 0.005 and β = 2. The drop contours for different Ca_E are compared at the onset of pinch-off for better physical understanding. The size of the daughter drop shows an increasing trend with increasing Ca_E. However, the increments are only prominent at the initial rise in Ca_E. Beyond a certain $C{a_E}$ $({\approx} 0.1)$, negligible variations are observed.

Figure. 8.

Effect of electric capillary number Ca_E on (a) daughter drop size, (b) transient variation in drop apex H_T, (c) transient variation in neck radius R_n and (d) centreline velocity u_z,c around the neck at t = 0.7. The parameters considered are β = 2, R = 10, S = 2.3, Oh_i = 0.005, Oh_e = 0.0002, Bo = 0.32 and A = 0.998.

We further analyse the transients in the drop apex position and neck radius for different Ca_E values in figures 8(b) and 8(c), respectively. In the H_T versus time characteristics shown in figure 8(b), the continuous rise in H_T refers to cylindrical structure formation which has been discussed earlier, whereas the fall in H_T corresponds to the vertical collapse phase. At the initial stage, a small increment in H_T is observed with the increase in Ca_E, attributed to the electric-field-induced deformations only (Saville Reference Saville1997; Vlahovska Reference Vlahovska2019). However, after the collapse of the capillary wave at the apex, the coupled effect of EHD–capillary pull comes into play, causing a noticeable increase in the elongation rate. As the deviation in maximum apex position $(d(H_T^{max})/d(C{a_E}))$ continuously increases with increasing Ca_E, it signifies that the coupled EHD–capillary pull is much stronger than the linear superposition of individual pulling forces. Looking at the augmented elongation rate, one may argue that the upward stretching directs the flow upwards inside the drop, causing an increase in the volume of the daughter drop. Whereas in reality, the flow always directs towards the downward direction (shown in figure 9), discarding the above possibility. In figure 8(d), on comparing the flow velocities along the centreline (u_z,c), we show that for Ca_E = 0.1, the fluid motion near the neck region is weaker as compared with the case for Ca_E = 0. As the flow drainage is mainly generated due to the pressure difference between the father drop and the mother drop, the alteration in the fluid velocity must be attributed to this observed finding. Comparing the results at t = 0.55 (during the elongation phase), we observe that on increasing Ca_E, the pressure at the exit of the neck decreases insignificantly (p = 1.75, 1.66 and 1.58 for Ca_E = 0, 0.1 and 0.2, respectively). In contrast, at the neck entrance, the pressure falls considerably (p = 5.2, 4.4 and 3.9 for Ca_E = 0, 0.1 and 0.2, respectively). The small pressure variations in the father drop may be due to its diminutive shape modulation under weaker electric forcing, as shown previously in figure 5(b). The above data show reduction in net pressure difference between neck inlet and outlet, reflected in weaker flow rate. Furthermore, the vertical collapse rate decreases with increasing Ca_E, as shown in figure 8(b). In conjunction, these phenomena resist the downward flow, causing entrapment of more fluid within the mother drop. Thus, larger daughter drops are produced.

Figure 9.

Flow patterns near the upper drop for (a–c) different Ca_E at t = 0.8. Other parameters considered are β = 2, R = 10, S = 2.3, Oh_i = 0.005, Oh_e = 0.0002, Bo = 0.32 and A = 0.998.

Fundamentally, the normal electric stress is responsible for the above-mentioned variations in drop deformation, while both the tangential and normal electric stress cause alterations in the local flow field (Lac & Homsy Reference Lac and Homsy2007; Esmaeeli & Sharifi Reference Esmaeeli and Sharifi2011; Lanauze, Walker & Khair Reference Lanauze, Walker and Khair2015; Behera & Chakraborty Reference Behera and Chakraborty2020). Figure 9 shows the flow patterns for different Ca_E at t = 0.8, considering Oh_i = 0.005 and β = 2. For Ca_E = 0, the flow is unidirectional throughout the mother drop, whereas for Ca_E = 0.1 circulations (or secondary flows) are observed. These circulations emerge to balance the tangential electric stress at the interface. Although an increase in Ca_E strengthens the circulations, the flow patterns in the neck remain mostly unaffected.

Even though the transient variation in drop apex looks qualitatively similar for all Ca_E, for Ca_E = 0.2 the downward displacement of the apex is appreciably small. This suggests that the electric stress, in this case, is comparable to the combined effect of the downward capillary stress and gravitational stress, i.e. τ_E = τ_γ + τ_g, at the onset of the vertical collapse phase. Importantly, increasing Ca_E from 0 to 0.2 reverses the direction of the outer flow from downward to upward, indicating that for ${\tau _E} \gg {\tau _\gamma } + {\tau _g}$, non-trivial interfacial deformations are most likely to be produced due to the strong electric pulling and the shearing effect of outer flow (as inferred from the flow patterns shown in figure 9 for $Ca_{E} = 0.1$ and 0.2). Accordingly, for Ca_E = 0.3, the apex rises greatly instead of falling, as demonstrated in figure 10(a). Accordingly, the H_T versus t characteristic shown here exhibits a different profile from that shown previously. While for Ca_E ≤ 0.2 the typical neck collapses are seen to occur around t = 1.1 as shown earlier, the same does not happen for Ca_E = 0.3, allowing prolonged elongation of the mother drop. At t = 1.2, a thin jet emerges from the mother drop, resulting in tip streaming. Such tip streaming is an electric-field-induced breakup mode reported in earlier numerical and experimental studies (Herrada et al. Reference Herrada, López-Herrera, Gañán-Calvo, Vega, Montanero and Popinet2012; Karyappa, Deshmukh & Thaokar Reference Karyappa, Deshmukh and Thaokar2014; Casas et al. Reference Casas, Garzon, Gray and Sethian2019; Gawande, Mayya & Thaokar Reference Gawande, Mayya and Thaokar2020). Owing to substantial elongation, the curvature at the top end becomes very high, resulting in high charge concentration at the top, which is the reason behind this ejecting mode. Note that there is a slight fall in the drop apex at t ≈ 1.2, which can be due to the instant fluid loss by tip streaming. The overall length starts to increase for t > 1.5, as the mother drop continues to stretch. With time, the downward capillary propagation eventually converges at the bottom of the father drop, creating a similar EHD–capillary pull as shown earlier in the case of the mother drop. At this point, the drop phase behaves as a single entity experiencing tensile force, simultaneously from both ends. The effective capillary number in such a scenario can be calculated as $Ca_E^{eff} = \bar{\epsilon}_e\bar{E}_0^2{(\bar{D}_m^3 + \bar{D}_f^3)^{1/3}}/\gamma = C{a_E}{(1 + \beta )^{1/3}}$. For the parent ratio of β = 2 as considered herein, the effective capillary number is calculated as $Ca_E^{eff} = {3^{1/3}}C{a_E} \approx 0.43$, which is larger than the critical limit required to induce unstable elongations. Consequently, the instability grows as the elongation continues, finally leading to asymmetric breakup via end-pinching mode, resulting in the generation of small droplets at both ends.

Figure. 10.

(a) Transient variation in drop apex position and (b) evolution of drop for Ca_E = 0.3. Other parameters considered are β = 2, R = 10, S = 2.3, Oh_i = 0.005, Oh_e = 0.0002, Bo = 0.32 and A = 0.998.

While β and Ca_E can influence pinch-off as well as the daughter drop size, a comprehensive picture of the coalescence subjected to the variations in both can be intriguing, as shown in figure 11(a). Remarkable alterations in the pinch-off modes can be observed with an increase in Ca_E. For Ca_E ≤ 0.1, in all cases, a dimple (or crater) forms on the father drop at the onset of pinch-off. Such pinch-off events have been observed to be common in most of the previously reported studies (Blanchette & Bigioni Reference Blanchette and Bigioni2009; Ray et al. Reference Ray, Biswas and Sharma2010; Garzon et al. Reference Garzon, Gray and Sethian2018). Counterintuitively, the pinch-off mode shifts to a cusping-like mode when Ca_E is increased to 0.2. In such cases, the top portion of the father drop elevates, transforming into conical ends prior to neck collapse. Note that the angle of these conical ends increases with increasing β. Another peculiarity can be found in terms of the time taken for pinch-off (Δt) (see figure 11b). For the former pinch-offs (i.e. for Ca_E ≤ 0.1), while Δt ≈ 0.87 for all β, for the cusping modes Δt is much longer and monotonically increases with β. Following the above discussion, it can be said that the mechanism responsible for cusping is similar to that inducing end-pinching (see figure 10), except that the latter is most likely to occur at a much higher Ca_E as it requires substantial elongations in the father drop also, as discussed before. From figure 11(a), one important feature that can be noted is that, for higher β (e.g. β = 5), increase in Ca_E produces noticeably longer daughter drops, which are highly unstable and may undergo capillary-driven breakup before or after pinch-off.

Figure. 11.

(a) Pinch-off events for various pairs of β and Ca_E. (b) Coalescence time Δt and (c) daughter drop size as functions of β for different Ca_E. The parameters considered are R = 10, S = 2.3, Oh_i = 0.005, Oh_e = 0.0002, Bo = 0.32 and A = 0.998.

Figure 11(c) depicts the effect of β on non-dimensional daughter drop size $({\bar{D}_d}/{\bar{D}_m})$ for different Ca_E. For all Ca_E, increase in β leads to bigger daughter droplets. Deka et al. (Reference Deka, Biswas, Chakraborty and Dalal2019), in their numerical analysis, demonstrated that increase in β lessens the neck expansion, thus decreasing the fluid drainage to the father drop. To this, as we have already discussed, increase in Ca_E further restricts the flow rate and thus increases ${\bar{D}_d}/{\bar{D}_m}$. However, altering Ca_E from 0.1 to 0.2 leads to negligible variation in ${\bar{D}_d}/{\bar{D}_m}$. This is for the reason that, as compared with Ca_E = 0 and 0.1, the time needed for pinch-off for $Ca_{E} = 0.2$ is much longer. Accordingly, more fluid is transported to the father drop, even if the flow rate is low.

5.2. Influence of the relative electrical properties

As the electric stress is characterized by (R, S), it is imperative to investigate the effect of these parameters on the observed coalescence dynamics. In figure 12(a), we delineate the coalescence dynamics in terms of the temporal variation of the drop apex for different sets of electrical properties (R, S) and its consequence for pinch-off, considering β = 2, Ca_E = 0.2 and Oh_i = 0.01. It is pointed out that as the deviation of R/S from unity increases, the temporal characteristic of H_T shows higher deviation from the same obtained for Ca_E = 0. For R/S > 1, while this deviation in H_T is positive, for R/S < 1 it is negative. For the systems (R, S) = (5, 2.3), (R, S) = (10, 2.3) and (R, S) = (25, 2.3), using (4.2), the values of normal electric stresses (τ_E) at the apex are obtained as 0.476, 0.668 and 0.80, respectively. This explains the rise in H_T with increasing R/S. Conversely, for cases with R/S < 1, decrease in R/S causes larger reduction in H_T as the dimensionless τ_E for (R, S) = (0.5, 2) and (R, S) = (0.5, 4) are found to be 0.511 and −1.144, respectively. By looking into the locations of these said systems in phase space with reference to the zero net force line (see (4.3)), as depicted in figure 12(b), it can be inferred that as we move far from the said line, the electric-field-induced modulations become more important. The sign and the relative magnitude of the various electric stresses that define the resulting interplay have already been discussed in § 4.

Figure. 12.

(a) Temporal variation of the drop apex (H_T) under electric field for different electrical properties (R, S). The insets display the outcomes of coalescence for different (R, S). The H_T versus t curves marked with crosses indicate non-pinching modes. The parameters considered are β = 2, Ca_E = 0.2, Oh_i = 0.01, Oh_e = 0.0002, Bo = 0.32 and A = 0.998. (b) Location of the systems considered in (a) with reference to the zero net force line in the (R, S) phase plot.

From the insets provided in figure 12(a), it can be observed that for the considered parameters, the systems with R/S > 1 exhibit pinch-off, unlike for the case without electric field (non-pinching modes are marked with a cross). For (R, S) = (5, 2.3), while the daughter drop is close to spherical shape, for (R, S) = (25, 2.3), pinch-off generates a long drop with a tail-like structure. This tail eventually becomes detached and breaks into tiny droplets as shown in the inset. In figure 11(a), we have presented a similar daughter drop topology for lower R/S values (i.e. R = 10 and S = 2.3) and higher values of β. A comparison between figures 11(a) and 12(a) signifies that EHD–capillary pulls of comparable magnitude, creating long unstable daughter droplets, can be generated even for lower values of β, by increasing R/S.

For the cases with R/S < 1, no pinch-offs are obtained (marked with a cross), owing to weak upward pull. In fact, the system (R, S) = (0.5, 4) exhibits complete coalescence. For this system, the electric stress significantly suppresses the capillary pull, not allowing the apex to move upwards. As a result, the mother drop entirely merges with the father drop within a much shorter time (at $t \approx 0.8$) without showing any sign of partial coalescence. The drop evolution leading to complete coalescence is provided in the next section while discussing the coalescence cascade.

The electric-field-mediated coalescence for the cases of R/S < 1 is elucidated in more detail in figure 13, considering the system (R, S) = (0.5, 2) and Oh_i = 0.01. Figure 13(a) depicts the phase diagram in $C{a_E}$–$\beta$ parametric space demarking the pinch-off regimes. This demonstrates that the increase in Ca_E remarkably increases the minimum (or critical) value of the parent ratio (β_c) required for pinch-off, owing to the additional capillary energy required for overcoming the effect of electrically induced perturbation. This qualitative relation between β_c and Ca_E is opposite to that obtained for R/S > 1, where β_c is directly proportional to the negative exponent of Ca_E. Consequently, on increasing Ca_E, while for R/S > 1, β_c tends to unity (see figure 7), for R/S < 1, β_c can deviate up to O(10), where the drop–drop system can be typically considered as a drop–pool system. It is worth mentioning that an increase in Oh_i can further increase β_c by providing higher viscous resistance to the capillary pull.

Figure. 13.

(a) The $C{a_E}$–$\beta$ phase diagram indicating no-pinch-off and pinch-off regimes. (b) Effect of electric capillary number Ca_E on daughter drop size for β = 4. The inset compares the shapes of the drop for different Ca_E at Δt = 0.75. The parameters considered are R = 0.5, S = 2, Oh_i = 0.01, Oh_e = 0.0002, Bo = 0.32 and A = 0.998.

Figure 13(b) shows the effect of Ca_E on the non-dimensional daughter drop size $({\bar{D}_d}/{\bar{D}_m})$ in the pinch-off regime for β = 4. Unlike the case of R/S > 1, here we obtain a decrease in ${\bar{D}_d}/{\bar{D}_m}$ with an increase in Ca_E. As the downward electric pull lowers the maximum limit of the drop apex, it causes an early vertical collapse, as shown in figure 12. Moreover, on analysing the contours at t = 0.75 (during the vertical collapse phase), we observe that the increase in Ca_E results in wider necks, as opposed to the thinner necks obtained for the case of R/S > 1. As the neck becomes wider, the local pressure (p_neck) reduces as discussed before, resulting in slower horizontal collapse. For example, for Ca_E = 0, 0.1 and 0.2, pinch-off occurs at Δt = 0.86, 0.9 and 0.96, respectively. The augmented cross-sectional area of the neck and higher pre-pinch-off time enhance the fluid displacement from the mother drop, leading to reduction in ${\bar{D}_d}/{\bar{D}_m}$.

5.3. Coalescence cascade

The smaller drops (or daughter drops) generated from partial coalescence fall onto the bigger drop to initiate a self-similar coalescence event. This process may continue several times until the formation of a single entity, thus generally termed as a coalescence cascade. Here we compare the solely capillary-driven coalescence cascade (when Ca_E = 0) with the electrically mediated one considering R/S > 1 and R/S < 1.

Figure 14 depicts the results for Ca_E = 0, β = 1.6 and Oh_i = 0.005. In this case, the first pinch-off does not occur, and the neck starts to widen again, similar to that observed in the initial stage. This initiates the second-stage coalescence, where the capillary wave deforms the drop again into a cylindrical protrusion, followed by the vertical collapse phase. Interestingly, the second pinch-off successfully takes place, generating a droplet of size 0.19. Similar mid-stage pinch-off modes were previously reported by Zhang et al. (Reference Zhang, Li and Thoroddsen2009), which can be attributed to the fact that in the second stage, the attached daughter drop (that acts like the mother drop) maintains a higher parent ratio, β > β_c, with the father drop. The third stage of coalescence starts at time t = 1.315 from which a tertiary droplet is produced at $t \approx 1.39$ with ${\bar{D}_d}/{\bar{D}_m}$ $( = 0.077)$ almost 0.4 times the preceding mother drop size. As this drop's size is very small, it completely merges with the bigger drop in the next stage. In the current case, a four-stage coalescence is observed with two pinch-offs; however, by increasing β, more stages can be obtained with more pinch-offs. The maximum number of coalescence stages reported in the literature, in solely capillary-driven systems, is six, which was observed by Thoroddsen & Takehara (Reference Thoroddsen and Takehara2000) for a drop–pool system.

Figure. 14.

Coalescence cascade of drops for parent ratio β = 1.6 in the absence of electric field. The parameters considered are Oh_i = 0.005, Oh_e = 0.0002, Bo = 0.32 and A = 0.998. The panels capturing the occurrence of pinch-off are marked with dashed lines in red colour for better visualization.

Figure 15 shows the influence of electric field on a cascade for R = 10, S = 2.3 and Ca_E = 0.2, with other parameters remaining the same. Here, significant alterations in the cascade process are found. A five-stage coalescence is obtained, including pinch-off in the first four stages. The sizes of the daughter drops found in successive stages are 0.53, 0.32, 0.16 and 0.076. For R/S > 1, as the electric forcing lowers the critical parent ratio, it leads to the transition from no pinch-off to pinch-off (see figures 7 and 8a) as well as producing daughter drops larger than that obtained for Ca_E = 0 in the respective stages. As a post-pinch-off event, the daughter drop exhibits movement in the upward direction while undergoing prolate–oblate oscillatory deformations until it falls back to meet the father drop. Interestingly, as the electric field's presence results in a larger daughter drop, it displaces to a greater distance due to higher inertia. As a consequence, for Ca_E = 0.2 the time gap between the first pinch-off and the initiation of the second stage is found to be very high, i.e. almost 1.35, which makes the total coalescence process highly time-consuming. It also can be noticed that the values of daughter to mother droplet size ratios in each stage fall in the range of 0.55 + 0.05. From the times mentioned in each panel of figure 15, it is apparent that the higher stages of cascade occur over progressively shorter times. The reason is that the time scale for the completion of the nth stage (Δt_N) is typically of the order of ${t_{\gamma ,N}} = \sqrt {{{\bar{\rho }}_m}\bar{D}_{m,N}^3/\gamma }$, where ${\bar{D}_{m,N}}$ is the size of the mother drop in the Nth stage or the size of the daughter drop in the (N − 1)th stage. Thus, Δt_N can be calculated as $\varDelta {t_N} \approx {({\bar{D}_{m,N}}/{\bar{D}_{m,1}})^{3/2}}\varDelta {t_1}$, where ${\bar{D}_{m,1}}$ is the size of the mother drop at the initial stage. Thus, as ${\bar{D}_{m,N}}/{\bar{D}_{m,1}}$ decreases at each stage as mentioned above, the time needed for the completion of the cascade phenomenon also decreases perceptibly.

Figure. 15.

Coalescence cascade of drops for the system (R, S) = (10, 2.3) and parent ratio β = 1.6. Other parameters considered are Ca_E = 0.2, Oh_i = 0.005, Oh_e = 0.0002, Bo = 0.32 and A = 0.998. The panels capturing the occurrence of pinch-off are marked with dashed lines in red colour for better visualization.

Although figures 14 and 15 suggest an increase in the number of coalescence cascade stages under an electric field, in reality the coalescence cascade for the different parent ratios may emerge to be contrary to that intuitive perception, as demonstrated in figure 16(a). From the figure, it is evident that for Ca_E = 0, the number of pinch-offs (N) can be increased by increasing the value of β. As the increase in β produces larger daughter droplets after the first stage, it reduces the effective Ohneseorge number, i.e. $Oh_i^{eff} = {\bar{\mu }_i}/\sqrt {{{\bar{\rho }}_m}{{\bar{D}}_d}\gamma }$, reducing the viscous resistance in the second stge. Owing to self-similarity, the viscous resistance in the successive stages is also relatively less, favouring more pinch-offs. However, the discontinuous variation in N indicates the existence of a critical value of β for each N. Additionally, between the two successive critical parent ratios, there exists a range of β producing the same number of pinches. Note that a maximum of four pinch-offs is observed for β = 5 and Ca_E = 0.

Figure. 16.

(a) Number of pinch-offs before complete coalescence for different parent ratios β and electric capillary numbers Ca_E. The system (R, S) = (10, 2.3) is considered along with Oh_i = 0.005, Oh_e = 0.0002, Bo = 0.32 and A = 0.998. (b) Second-stage coalescence for β = 3 and Ca_E = 0.2. Schematic representation is provided as the inset to explain the reason behind the non-pinching mode.

On increasing Ca_E to 0.1, N increases considerably for all β up to β = 4. It is noteworthy that a maximum of six pinch-offs or alternatively a seven-stage coalescence is observed under electric field, for β = 4 and Ca_E = 0.1 (see the supplementary movie available at https://doi.org/10.1017/jfm.2022.1096), beyond the maximum numbers of stages observed previously by Thoroddsen & Takehara (Reference Thoroddsen and Takehara2000) for drop–pool systems. Surprisingly, when β is increased to 5, we observe fewer pinch-offs (N = 3), which is even fewer than that found for the corresponding case in the absence of electric field (N = 4). The non-monotonic trend becomes more evident at higher Ca_E. For $\beta \le 2$, the pinch-offs obtained for Ca_E = 0.2 are more in number as compared with that obtained for Ca_E = 0. However, for β > 2, N dramatically decreases to unity, signifying the occurrence of complete coalescence in the second stage.

To explain the distinctive and apparently non-intuitive cascading trends, we illustrate the drop evolution in the second stage for Ca_E = 0.2 and β = 3 in figure 16(b). Unlike the first stage, the drops at the initiation of the second stage are prolate-shaped owing to electric-field-induced deformations for systems with R > S (Taylor Reference Taylor1966; Lac & Homsy Reference Lac and Homsy2007; Behera, Mandal & Chakraborty Reference Behera, Mandal and Chakraborty2019). Accordingly, the initial neck curvature at the onset of second stage reduces, leading to weakening of capillary-wave propagation as demonstrated schematically in the inset. Importantly, as the size of the mother drop reduces by almost (0.5)^N times in the Nth stage, the effective electric capillary number $Ca_E^{eff} = (\bar{D}_m^N{\bar{\varepsilon }_e}\bar{E}_0^2/\gamma )$ reduces by the same fraction. Thus, the electric-field-induced deformation of the mother drop in the second stage is significantly small, contributing less to the variation in neck curvature. Conversely, the father drop, which is of larger size, deforms more ($Ca_E^{eff}$ for the father drop is ${\bar{D}_f}{\bar{\varepsilon }_e}\bar{E}_0^2/\gamma$), as evidenced from figure 16(b), which thus plays the key role in reducing the neck curvature. Moreover, the fluid transport from the mother to the father drop leads to increase in D_f and thereby $Ca_E^{eff}$ for the father drop. At a certain stage if $Ca_E^{eff}$ crosses the threshold limit due to either an increase in electric field strength or size of the father drop, the father drop may exhibit unstable EHD elongations, inhibiting pinch-off as shown earlier in figure 10(b).

Figure 17 reveals the cascading trends for the system with R/S < 1 considering R = 0.5, S = 2, β = 1.6, Ca_E = 0.2 and Oh_i = 0.005. Surprisingly, here no cascading is observed. Instead, the mother drop completely merges with the father drop. The contour analysis shows that at t = 0.7, the height of the protrusion above the father drop is much shorter compared with the previous two cases. Owing to the shorter length, the protrusion becomes highly stable resulting in insignificant horizontal collapse. In fact, the neck cannot be identified after t = 0.7. As a result, the first pinch-off does not occur. The top of the protrusion, where the curvature is maximum, continuously falls, pushing the fluid into the father drop and leading to complete coalescence without going to the further stages.

Figure. 17.

Coalescence cascade of drops for the system (R, S) = (0.5, 2) and parent ratio β = 1.6. The other parameters considered are Ca_E = 0.2, Oh_i = 0.005, Oh_e = 0.0002, Bo = 0.32 and A = 0.998.