Droplet Contact Analysis

Tutorial Created with Software Release: 2026-1
Topics: Consumer Packaged Goods, Pharmaceutical Formulations, Polymeric Materials
Methodology: All-Atom Molecular Dynamics
Products Used: Desmond, MS Maestro

Tutorial files

0.1 GB
This tutorial is written for use with a 3-button mouse with a scroll wheel.
Words found in the Glossary of Terms are shown like this: Workspacethe 3D display area in the center of the main window, where molecular structures are displayed

 

Tip: You can hover over a glossary term to display its definition. You can click on an image to expand it in the page.
Abstract:

 

This tutorial teaches how to use the Droplet Contact Analysis Calculation and Viewer panels to determine the contact angle of water droplets on polymer surfaces.

 

Tutorial Content

  1. Introduction to Droplet Contact Analysis

  1. Creating Projects 

  1. Importing Slabs for Droplet Contact Analysis

  1. Performing and Analyzing Droplet Contact Calculations

  1. Conclusions and References

  1. Glossary of Terms

1. Introduction to Droplet Contact Analysis

The contact angle (θ) is defined as the angle where a liquid-vapor interface meets a solid surface.

The contact angle is a useful parameter for studying solid or soft matter surface properties, such as adsorption and wettability. Moreover, determination of contact angle can be used as an indirect method for determining surface tension, which can be otherwise difficult to obtain for solids.

Herein, we will learn to use the Droplet Contact Analysis Calculation and Viewer panels to predict the contact angle for water on various polymer surfaces. As a first approximation, contact angles for water are a quantitative measure of surface hydrophilicity (or hydrophobicity), whereby a contact angle <90º indicates a hydrophilic surface and a contact angle >90º indicates a hydrophobic surface. 

In this tutorial, we will determine the water contact angle for two prototypical polymers, polypropylene (PP) and polyethylene terephthalate (PET).

Generally, computational prediction of contact angle for polymers (or other soft matter) or solids is useful for various applications, including packaging materials and consumer packaged goods, as well as polymeric materials, pharmaceutical formulations and organic electronic materials.

2. Creating Projects

At the start of the session, change the file path to your chosen Working Directorythe location where files are saved in MS Maestro to make file navigation easier. Each session in MS Maestro begins with a default Scratch Projecta temporary project in which work is not saved. Closing a scratch project removes all current work and begins a new scratch project, which is not saved. A MS Maestro project stores all your data and has a .prj extension. A project may contain numerous entries corresponding to imported structures, as well as the output of modeling-related tasks. Once a project is saved, the project is automatically saved each time a change is made.

Structures can be built in MS Maestro or can be imported using File > Import Structures (or drag-and-dropped), and are added to the Entry Lista simplified view of the Project Table that allows you to perform basic operations such as selection and inclusion and Project Tabledisplays the contents of a project and is also an interface for performing operations on selected entries, viewing properties, and organizing structures and data. The Entry Lista simplified view of the Project Table that allows you to perform basic operations such as selection and inclusion is located to the left of the Workspacethe 3D display area in the center of the main window, where molecular structures are displayed. The Project Tabledisplays the contents of a project and is also an interface for performing operations on selected entries, viewing properties, and organizing structures and data can be accessed by Ctrl+T (Cmd+T) or Window > Project Table if you would like to see an expanded view of your project data.

  1. Double-click the Materials Science icon

Figure 2-1. Change Working Directory option.

  1. Go to File > Change Working Directory
  2. Find your directory, and click Choose
  3. Pre-generated experimental files are included to be used in the analysis stage. Download the zip file here: schrodinger.com/sites/default/files/s3/release/current/Tutorials/zip/droplet_contact.zip
  4. After downloading the zip file, unzip the contents in your Working Directorythe location where files are saved for ease of access throughout the tutorial

Figure 2-2. Save Project panel.

  1. Go to File > Save Project As
  2. Change the File name to droplet_contact_tutorial, click Save
    • The project is now named droplet_contact_tutorial.prj

3. Importing Slabs for Droplet Contact Analysis

In this section, we will import two provided slabs for subsequent droplet contact analysis calculations.  

The input system for droplet contact analysis should be an equilibrated slab. The slab could be an amorphous surface (such as the polymer examples herein) or an ordered solid (such as a molecular crystalline slab).

In general, for performing a default water droplet calculation, the surface size should be relatively large (at least 100 Å x 100 Å in the a and b dimensions). This ensures that both the surface area is large enough for the water molecules to form a droplet and that periodic image effects in the a and b directions are avoided.

Similarly, the vacuum in the z-direction should also be large enough to avoid the water droplet interacting with the bottom of the surface due to periodicity.

In this example, we will import two provided polymer slabs (PP and PET).

Figure 3-1. Importing the provided slabs.

  1. Go to File > Import Structures
  2. Navigate to the tutorial files, presumably in your working directory, and choose both provided .cms files: amorphous_polypropylene_P1-001-surface_P1_system-out.cms and amorphous_polyethylene_terephthalate_P1-001-surface_system-out.cms
  3. Click Open

Figure 3-2. The provided slabs, tiled in the workspace. Note that tiling is purely for visualization, and is not necessary to proceed.

Two entries associated with the two polymer slabs are now available in the entry list. Feel free to visualize the entries and stylize however you prefer.

The two polymer slabs were constructed following similar procedures that can be referenced in the following tutorials:

In both cases, the a and b dimensions of the periodic system are >100 Å, and sufficient vacuum was generated to ensure enough space for the water droplet while also minimizing its periodic interactions with the bottom of the slab.

The bulk systems were built with the Polymer Builder panel and then equilibrated with the Compressive relaxation protocol in the Molecular Dynamics Multistage Workflow panel. The slabs were then generated using the Build Slabs and Interfaces panel with the Enforce normal C-axis option checked to minimize surface roughness. Finally, the slabs were equilibrated with a 1.2 ns NVT molecular dynamics simulation with the Molecular Dynamics Multistage Workflow panel.

For help on Desmond files, visit the reference documentation.

Feel free to practice constructing these, or similar, systems on your own.

4. Performing and Analyzing Droplet Contact Calculations

In this section we will use the Droplet Contact Analysis Calculation and Viewer panels to predict the contact angle for water on the two provided polymer surfaces.

Figure 4-1. Selecting the entry and opening the calculations panel.

  1. Select the entry associated with PP (amorphous poly(propylene) P1-001-surface P1) in the entry lista simplified view of the Project Table that allows you to perform basic operations such as selection and inclusion
  2. Go to Tasks > Materials > Classical Mechanics > Droplet Contact Analysis > Droplet Contact Analysis Calculations

Let’s familiarize ourselves with some of the key aspects of the Droplet Contact Analysis panel:

  • The excluded segment of the simulation is to ensure that the ‘dropping’ portion of the simulation is not used in the droplet analysis
  • The values associated with the Discretization bin width and Neighbor finding distance are associated with the clustering algorithm. These defaults are sufficient in most cases. 
  • In the Setup tab of the panel:
    • The Solvent can be selected. Water is the default, but many other solvents are available in the dropdown. Note that multi-solvent mixtures are not currently supported.
    • The Number of molecules defaults to 3000, which is a reasonable starting point. In the case of water, at least 1000 molecules is recommended for a reasonably sized droplet
    • The Initial droplet velocity can be customized, but should be kept reasonably small to avoid a momentum overflow or unphysical interaction of the droplet with the surface
    • The Substrate positional restraints section can be used to prevent movement of the slab during the simulation. For further discussion and an example of using similar restraints, see the Molecular Deposition tutorial
  • In the Simulation Protocols tab of the panel:

For a more comprehensive overview, see the help documentation on the panel.

Figure 4-2. Setting up the Droplet Contact Analysis panel.

  1. Retain the panel defaults associated with the Setup tab
  2. Go to the Simulation Protocols tab

Figure 4-3. Setting up the Droplet Contact Analysis panel, naming and running the job.

  1. Uncheck Save intermediate data
    • Saving data from the trajectory can be memory-intensive
  2. Change the Job name to droplet_contact_analysis_PP
  3. Adjust the job settings () as needed
    • This job requires a GPU host. The job can be completed in about 3 hours on a GPU host
  4. If you would like to run the job yourself, click Run. Otherwise, import the pre-generated Section_04 > droplet_contact_analysis_PP > droplet_contact_analysis_PP-out.cms file from the provided tutorial files via File > Import Structures
  5. Close the Droplet Contact Analysis panel

Figure 4-4. Visualizing the output.

When the job is finished or after importing, a new entry is added to the entry list.

The structure shown in the workspacethe 3D display area in the center of the main window, where molecular structures are displayed is the final frame from the end of the MD simulation.

 

Proceed to visualize the structure in the workspace. Feel free to stylize however you wish. Here we have represented the water molecules with the CPK style.

 

Note that because of periodicity, some water molecules may appear at the bottom of the box. Be aware that this is purely visual, but can be easily amended using the Periodic Structure Tool Window

Figure 4-5. Accessing the results panel via the WAM.

The Droplet Contact Analysis Results panel is used to view the quantitative results.

  1. Use the WAM (workflow action menu) button () to open the Droplet Contact Analysis Results panel
    • Alternatively, access the panel via Tasks > Materials > Classical Mechanics > Droplet Contact Analysis > Droplet Contact Analysis Results
    • The Droplet Contact Analysis Results panel opens


Figure 4-6. The default results panel after loading the output.

The Droplet Contact Analysis Results panel is used to quantify the Contact angle. Note that if you did not open the panel from the WAM, use the Load from Workspace button to import the results associated with an entry.

Be aware that the default fitting may not be optimal. In the next steps, we will fine-tune the fitting.


Figure 4-7. The results panel after fitting.

It is important to properly fit the water droplet in order to obtain a high quality contact angle value. Two key customizations are often made in order to obtain accurate results.

First, the Contact surface height can be adjusted. This is the height that will be considered the surface plane. By default, the water molecule in the droplet with the lowest Z-coordinate defines Z=0. The surface may have some degree of roughness, or some water molecules may modestly intercalate into the surface, therefore it is important to set this height carefully.

  1. Use the Contact surface height toggles to raise the surface height as shown in the Figure
    • The density of water is displayed based on the color. The region where water density is ~0.5 g/cm3, the orange-red color, is a reasonable target

 

Next, the Fit circle to points with Z values from option can be adjusted to carefully capture the curvature of the droplet. In some cases, individual points may not be representative of the overall droplet. Typically, these are at the top or bottom of the droplet. 

  1. Use the Fit circle toggles to remove some of the data points
    • Note that in this case, the data points fits the curve fairly well, so it is not necessary to tweak much, if at all

The blue line is tangent to the green curve at the point where the dashed line (based on the Contact surface height) intersects the green curve. The angle that the blue line makes with the dashed line is the contact angle.

The Contact angle, Contact area and Contact area to overall interfacial surface % are printed in the bottom of the panel, and will adjust accordingly with the fitting. In this case, a value of ~129º is obtained for the contact angle. This is expected given the hydrophobic nature of the surface.


Figure 4-8. The output of the calculation for PET.

Repeat the above steps for the PET surface, naming the job droplet_contact_analysis_PET. All of the default settings for the calculations panel can be used again. If you do not wish to run the job, import the pre-generated Section_04 > droplet_contact_analysis_PET > droplet_contact_analysis_PET-out.cms file from the provided tutorial files.

Figure 4-9. The output of the calculation for PET after manipulating the visualization.

In the Figure we have re-centered the cell and zoomed in to improve the visualization.

Figure 4-10. The viewer panel after adjusting the fitting parameters to the droplet.

Note that because PET is far more hydrophilic, the droplet is less well-defined. As a result, one should be very cautious with the fitting. In this case, we make significant adjustments to the Contact surface height and Fit circle in order to obtain a reasonable fitting.

A similar procedure to that which is demonstrated herein was replicated for several additional polymers for validation of the results. The comparison between the calculated and experimental contact angles are shown in the following plot:

Chart

5. Conclusion and References

In this tutorial, we learned to use the Droplet Contact Analysis Calculation and Viewer panels to predict the water contact angle for two prototypical polymers, polypropylene (PP) and polyethylene terephthalate (PET).

For further learning:

For introductory content, focused on navigating the Schrödinger Materials Science interface, an Introduction to Materials Science Maestro tutorial is available. Please visit the materials science training website for access to 100+ tutorials. For scientific inquiries or technical troubleshooting, submit a ticket to our Technical Support Scientists at help@schrodinger.com. 

For self-paced, asynchronous, online courses in Materials Science modeling, including access to Schrödinger software, please visit the Schrödinger Online Learning portal on our website.

For some related practice, proceed to explore other relevant tutorials:

For further reading:

6. Glossary of Terms

Entry List - a simplified view of the Project Table that allows you to perform basic operations such as selection and inclusion

Included - the entry is represented in the Workspace, the circle in the In column is blue

Project Table - displays the contents of a project and is also an interface for performing operations on selected entries, viewing properties, and organizing structures and data

Recent actions - This is a list of your recent actions, which you can use to reopen a panel, displayed below the Browse row. (Right-click to delete.)

Scratch Project - a temporary project in which work is not saved. Closing a scratch project removes all current work and begins a new scratch project

Selected - (1) the atoms are chosen in the Workspace. These atoms are referred to as "the selection" or "the atom selection". Workspace operations are performed on the selected atoms. (2) The entry is chosen in the Entry List (and Project Table) and the row for the entry is highlighted. Project operations are performed on all selected entries

Working Directory - the location where files are saved

Workspace - the 3D display area in the center of the main window, where molecular structures are displayed