I have solved the problem. The code is as below:
crownAndSecondaryDroplet=imread('inputImage.png');
%get the row number of the lowest white pixels
lowestRow = find(sum(crownAndSecondaryDroplet, 2)>0, 1, 'last'); % Bottom white row of the image
hp = impixelinfo(); %to check the output of the lowest row
%find the column numbers of the lowest row
colummsOfLowestRow= find(crownAndSecondaryDroplet(lowestRow,:)==1);
%find the x value for the leftmost and the rightmost pixels of the lowest row
lowestLeftXvalue = min(colummsOfLowestRow);
lowestRightXvalue = max(colummsOfLowestRow);
%get the boundary of the contour
boundary = bwtraceboundary(crownAndSecondaryDroplet,[lowestRow, lowestLeftXvalue],'N');
%display the image
figure, imshow(crownAndSecondaryDroplet, []);
title('Image with Rim Contour', 'FontSize', fontSize, 'Interpreter', 'None');
axis on;
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
hold on;
%plot the contour and lowest end points
plot(boundary(:,2),boundary(:,1),'g','LineWidth',2);
plot(lowestLeftXvalue , lowestRow, 'r+', 'LineWidth', 3, 'MarkerSize', 20 );
plot(lowestRightXvalue , lowestRow, 'r+', 'LineWidth', 3, 'MarkerSize', 20 );
hold off;
